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What's the name of this flower?

What's the name of this flower?



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I photographed this flower in Central Europe. I looked in online databases but can't really identify the actual flower, since there are so many that look quite similar. Can you help me? What's the name of this flower?


It looks like the common Geranium sylvaticum (also called wood cranesbill or Mayflower), and it is at least a close relative (member of the Geranium genus). This plant is commonly found across Europe and in parts of Asia (see map below), and it is sometimes planted in gardens. It is a perennial herb that grows in many types of habitats (woods, meadows, road sides, mountain areas), and the flowers are most commonly violet to blue, but can also be white. Geranium pratense is very similar, but usually has more narrow leaf lobes and bent flower stalks. However, it might not be possible to separate these two species based on your picture.

There are also similar relatives found in North America (e.g. Geranium maculatum) and Asia (e.g. Geranium himalayense) and many hybrids of plants from within this genus are cultivated.


(picture from Swedish Wikipedia)


(distribution map from the Swedish Museum of Natural History: Den virtuella floran)


My guess would be the meadow cranesbill (Geranium pratense), see this image (from here, more images are available there):

The flower is quite common on meadows in europe, see here.


23.5: Anatomy of a Flower

Flowers are composed of sets of highly modified leaves arranged in whorls. The outermost whorl of a flower is called the calyx and is composed of sepals. Inside the calyx is the corolla, which is composed of petals. The sepals are often smaller and less colorful than the petals, but this general rule can be misleading. For example, lilies often have identical sepals and petals. The only way you can distinguish between them is by location: Which whorl is on the outside?

Together, the calyx and corolla are called the perianth (peri- meaning around, anth- meaning flower). Inside the perianth is the androecium (house of man), a whorl composed of stamens. Each stamen has a long filament holding up pollen sacs called anthers. Inside the androecium is the gynoecium (house of woman), which is composed of carpels. Each carpel has an ovary at the base where ovules are housed. The style emerges from the ovary and is topped by the stigma. Pollen grains land on the stigma and must grow a tube down the style to reach the ovule and complete fertilization.

All of these whorls attach to an area called the receptacle, which is at the end of the stem that leads to the flower. This stem is called the peduncle. In the case of an inflorescence, where multiple florets are produced in place of a single flower, the stems leading to the florets are called pedicels.

In the diagram of the flower below, add labels for all of the bolded terms above and assign each whorl a different color. Make a key for the colors and whorls.

Figure (PageIndex<1>): Floral Structure


The Biology of a Rose

Roses are one of the most popular flowers around the world. While most rose plants descend from European and Asian rose families, in 1986, Congress made the rose the national floral emblem of the United States. With various sizes and colors to choose from, certain roses have come to have specific meanings. A red rose is a symbol of love, while yellow roses signify friendship. Some roses are even named after celebrities, world leaders, and scientists. From the Agatha Christie rose to a Ronald Reagan rose, there are lots of different varieties. Roses are ornamental plants that add an aesthetic appeal to any garden however, they also serve several other purposes. They can also be cut and transformed into a floral bouquet. Some roses are even used to create the perfect perfume. And it might be hard to believe, but some parts of the rose are pressed into oils, mashed into jelly, or brewed as part of a tea! Could you imagine walking out to the garden and grabbing a snack of roses?

Despite looking different, all roses have the same general anatomy. The anatomy of a rose is made up of many parts all working together to produce a thriving plant. Before a rose blooms, the sepals serve to protect the bud. As the flower blooms, the sepals move back to provide enough space. Once the rose has bloomed, its petals become evident. Petals are just one way that people can distinguish one rose from another. Right in the middle of the petals lies the stigma, a lump that looks like it is covered in a yellow powder. It is here that the rose accepts pollination. The stigma sits on top of the style, where a small opening allows the pollen to move down into the rose's ovaries. Unlike other flowers, the rose has multiple ovaries. The scientific classification of a rose places it in the Plantae kingdom and groups it with vascular plants, seed plants, and flowering plants all the way down to the Rosaceae, or the rose family. The specific genus is called Rosa L. From there, roses are broken up into various species.

Roses have been around for quite a while. Scientists have found fossils that suggest that roses are around 35 million years old. That means that even the dinosaurs were able to stop and smell the roses! As time passed, more variations of roses have been discovered, and some plant-lovers even enjoy creating hybrid roses from the seed of one rose and the pollen of another. A group of rose breeders in Canada have even achieved an entire line of roses they call the Explorer series. Each one is named for a different Canadian explorer.

Planting roses takes time and dedication and starts with finding just the right location. It is important to take into consideration the area's weather and overall environment. Certain roses will thrive in certain locations. Just like other flowers, roses need a combination of sunlight, soil, water, and fertilizer. Because the seeds usually drop down in the fall, roses need a couple of months of cold in order to get the process started. This is called stratification. When the weather begins to warm up, seedlings sprout. Different species of roses grow at different rates, so it is possible to see some seedlings as early as eight weeks while others could take several months before making an appearance. It takes some patience to grow a rose from a seed, but the final results are well worth the wait. One rose even had the opportunity to grow in space!

Roses come in all shapes and sizes. The smallest variations are called micro-miniature roses, and they only stand a little more than a foot tall. When the blooms are fully opened, they are about the size of a dime. The largest roses fall into the category of "grandiflora." They stand about six feet tall and have large flowers on each stem. In addition to growing individually, some roses can climb as they grow, reaching more than 12 feet tall, while others grow in a shrub or a bush.

Because of these variations, it can be difficult to come up with an exact number of rose species. However, most people agree that there are around 100 species. With multiple uses and a wide range of aesthetics, it's no surprise that roses are held in such high regard around the world. From adding a pop of color to the outdoor landscape to brightening up someone's day, the rose delivers with beautiful colors, soft petals, and, in some cases, a fresh fragrance.


Parts of a Typical Flower (With Diagram)

Read this article to learn about Parts of a Typical Flower !

Flower develops on the mother axis (stem) in the form of floral bud.

A typical angiosperm flower has following parts:

It is a leaf like structure in whose axil a flower often develops.

It is the stalk of the flower which may be short, long or even absent.

They are scaly appendages present on pedicel.

4. Receptacle (= Thalamus or Torus):

It is the swollen or expanded tip of the pedicel which bears four whorls i.e. calyx, corolla, androecium and gynoecium. Of these, calyx and corolla are collectively called as helping or accessory whorls, while androecium and gynoecium are together known as essential or reproductive whorls.

It is the first or outermost protective whorl. Individual member of calyx is called a sepal which is generally green.

It is the second or attractive whorl present inner to calyx. Each member of corolla is called a petal.

It is the third or male whorl. It is a collection of male parts called stamens. Each stamen is a modified leaf or microsporophyll. Each stamen consists of 3 parts – filament, anther and connective. Each anther has two anther lobes and each lobe usually contains two pollen sacs or micro-sporangia filled with pollen grains or microspores.

(d) Gynoecium or Pistil:

It is the fourth or female whorl, arid its functional units are called carpels (= megasporophylls). A typical carel consists of ovary, style and stigma. Ovary is the swollen basal part of the carpel that contains one or more ovules. Each ovule connected to the ovary wall through a special tissue called palacenta.

Bracts are specialized leaves from the axil of which bracteate flowers arise.

Bracts vary in size, colour and duration and are of following kinds:

(i) Foliaceous or Leafy bracts:

Green, flat and leaf like, e.g., Acalypbn, Adhatuda Gynandropsis.

Large, boat-shaped and tightly coloured bract enclosing lowers, e.g., banana, palms, Coloscassia.

Brightly coloured bracts like petals, e.g Polnsettia (Euphorbia pulcherrima)

Group of bracts in one or more whorls around luster of flowers, e.g., sunflower.

Whorl of bracteoles arising at the base of tie calyx, e.g., cotton, lady’s finger, strawberry.

Small and dry scaly bracts found only in gasses and sedges.

Present at the base of each floret of members of compositae, e.g., sunflower.

The calyx is the outermost whorl which consists of sepals.

(a) Sepaloid: When sepals are green.

(b) Petaloid: When sepals are coloured, e.g., Mirabilis, Delphinium

(i) Polysepalous – When sepals are free, e.g., Mustard

(b) Gamosepalous – When sepals are united, e.g., Datura, Hibiscus

(a) Caducous (Fugacious):Sepals that fall-off early or prematurely, e.g., Argemone, Papaverine.

(b) Deciduous: Sepals fall-off along with the petals just after fertilization, e.g., Brassica.

(c) Persistent: They remain attached to the fruit, e.g., tomato, brinjal, Solatium, Datura etc.

(d) Marcescent: This is also a persistent calyx, but it takes shrivelled, and dried-up looks, e.g., Guava (Psidium guajava).

(e) Accrescent: Again a persistent calyx but growing in size along with the fruit, e.g., Physalis, Shorea.

4. Modifications of Calyx:

Though sepals are generally green and leaf like structures, yet in some plants, they get modified in several forms, such as given below, for various purposes :-

(a) Pappus-Hairy or feathery sepals, e.g., Sonchus, Vernonia, sunflower etc.

(b) Spurred- When one or more sepals become beak-like outgrowth called spur, e.g., Impatiens, Delphinium.

(c) Leafy-Leaf-like sepals, e.g., in Mussaenda one of the sepals is modified into yellow leaf-like to attract insects for pollination.

(d) Spinous – Persistent sepals modified into spines, e.g., Trapa.

Such as in Aconitum, one of the sepals is modified into a hood like structure thus covering the whole flower.

In family Labiatae, the calyx is bilabiate, differentiated into an upper and a lower lip. Each lip is composed of one or more sepals, e.g., Ocimum (Tulsi – here there is one sepal in the upper lip and four in the lower lip), Salvia (three in upper lip and two in lower lip).

Corolla is the second floral whorl present inner to calyx and meant for attracting agents of pollination. It consists of individual units called petals. Each petal is differentiated into a narrow claw and an expanded limb.

(a) Petoloid – Coloured petals other than green.

(b) Sepal old – Petals green like sepals, e.g., Magnolia. Polyalthia.

(a) Polypetalous – Petals free, e.g. Brassica.

(b) Gamopetalous – Petals united, e.g., Datura, Petunia.

I. Polypetalous and Regular:

(a) Cruciform – Corolla with four petals arranged in form of a cross, e.g., Brassica, Iberis etc.

(b) Caryophyllaceous – Corolla with five petals arranged in such a manner that the limbs lie right angles to the claws, e.g., Silene, Dianthus etc.

(c) Rosaceous – Petals five or more without any claws i.e., sessile, e.g., Rose, tea, apple etc.

II. Polypetalous and Irregular:

Here corolla with five petals appears butterfly shaped. The posterior large petal is called standard or vexillum, two lateral petals are called wings or alae and two innermost fused petals are called keel or carina. It is the characteristic of family Papilionaceae.

III. Gamopetalous and Regular:

(a) Tubular – tube-like or cylindrical corolla, e.g., disc florets of sunflower.

(b) Campanulate-bell-shaped corolla, e.g., Campanula, Physalis.

(c) Infundibuliform-furmel-shapedcorolla, e.g.,Petunia, Datura

(d) Rotate – wheel-shaped corolla, e.g., Calotropis, brinjal.

(e) Hypocrateriform-Salver-shaped corolla, e.g., Vinca.

(f) Urceolate-Um-shapedcorolla, e.g.,Bryophyllum.

IV. Gamopetalous and Irregular:

(a) Ligulate-Strap-shaped corolla, e.g., ray florets.

(b) Bilabiate – two-lipped corolla where lips remain always open, e.g., Salvia, Ocimum etc.

(c) Personate – two-lipped corolla where lips remain closed by a projection called Palate e.g., Antirrhinum (snapdragon), Lindenbergia.

4. Aestivation of Corolla and Calyx:

Aestivation is the mode of arrangement of sepals or petals in relation to one another in a floral bud. It is useful in classification and identification of plants.

It is of following types:

The edges of sepals or petals touch or most not ‘ouch each other but do not overlap, e.g., mustard, coriander etc.

One edge of petal or sepal regularly overlaps the margin of the next one, e.g., petals of china rose.

The overlapping becomes irregular. Out of five members, one is outer, one is inner and the rest three remain in twisted condition.

It has two sub-types:

a. Ascending Imbricate:

Posterior petal is innermost i.e. being overlapped by the lateral petals, e.g., Cassia.

b. Descending Imbricate (= vaxillary):

The posterior petal is outermost and largest that overlaps the lateral petals (wings). They in turn enclose the two anterior smallest petals (keels). It is also called papilionaceous.

It is a modified imbricate type with 2 outer, 2 inner and one remain twisted, e.g., Ipomoea, guava etc.

When non-essential whorls (sepals and petals) are not distinct, they are collectively called periandi. The individual members of perianth are known as tepals, e.g.,Asphodelus, Onion. They may be sepaloid (greenish) orpetaloid (coloured other than green). The free and fused perianth is written as polyphyllous (= polytepalous) and gamophyllous (= gamotapelous) respectively.

Androecium, the male reproductive whorl of flower, is composed of stamens. A stamen (= microsprophyll) is made up of chiefly two parts: a large terminal portion, anther, and a stalk known as the filament. Each anther consists usually of two lobes connected together by a suture known as connective. Each anther lobe contains two cavities called pollen sacs, in which pollen-grains are produced (Fig. 6.10-A).

(c) Triandrous-three stamens

(d) Polyandrous- many stamens

2. (a) Insertedstamens remain inside the corolla tube, e.g., Petunia.

(b) Exserted – stamens are longer and exposed out the corolla tube, e.g., Hibiscus, Acacia.

(a) Isostemonous-when all stamens of a flower are of equal lengths, e.g., solarium.

(b) Heterostemnous – when length of stamens are unequal, e.g., Cassia

(c) Didynamous – stamens four, 2 short and 2 long, e.g., Ocimum

(d) Tetradynamous – stamens six, two outer short and inner four long, e.g., Brassica.

4. Arrangement of stamens:

(a) Diplostemonous – Stamens arranged in two whorls, outer whorl alternate with the petals (alternipetalous) and the inner whorl is opposite to petals (antipetalous), e.g., Cassia.

(b) Obdiplostemonous – When outer whorl of stamens is antipetalous and inner whorl is alternipetalous, e.g., Dianthus.

(c) Polystemonous – stamens arranged in more than 2 whorls.

5. (a) Fertile stamens-Stamens producing pollen.

(b) Staminode- stamens do not produce pollen i.e. non-functional, e.g., Salvia, Cassia.

6. (a) Monothecous -one-lobed anther, having 2 pollen chambers (bisporangiate), e.g., Malvaceae family.

(b) Dithecous-two-lobed anther, having4 pollen chambers (tetrasporangiate), e.g., Mustard.

7. Cohesion of Stamens:

(a) Adelpnous-When filaments are united but anthers remain free.

It is of three types –

i. Monoadelphous – Filaments of all stamens united in one bundle, e.g., Hibiscus.

ii. Diadelphous – Filaments of stamens are united to form two bundles, e.g., Pea.

iii. Polyadelphous – Filaments of stamens are united to form many bundles, e.g., Citrus, Castor, Cotton etc.

(b) Syngenesious – When anthers of stamens are fused and filaments remain free, e.g.,Helianthus, Tridax.

(c) Synandrous – When stamens are fused throughout their length, e.g., Cucurbita.

(d) Polyandrous – When stamens are free from one another, e.g., Ranunculus, Iberis etc.

8. Adhesion of stamens:

(a) Epipetalous – Fusion of stamens with petals, e.g., Datura, Ixora, tobacco, potato etc.

(b) Epitepalous (epiphyilous) – Stamens (used with tepals, e.g., Asparagus, Asphodelus etc.

(c) Gynandrous – Stamens fused with pistils, e.g., Calotropis.

The Flower [Flower—A Modified Shoot]

9. Fixation of anthers.

(a) Basifixed (Innate) – Filament attached to the base of the anther, e.g., Brassica, Datura.

(b) Dorsifixed- Filament attached to the dorsal (back) side of the anther, e.g. Passiflora, Sesbonia, Annona etc.

(c) Adnate-Filament attached along the entire length of anther, e.g. Magnolia, Nicotiana,Michelia, Nelumbium etc.

(d) Versatile – Filament atta­ched to a point on the back or base of anther so as to let it swing freely, e.g., Delo- nix, grasses etc.

(e) Divergent (divaricate) – When two anther lobes separate due to enlarged connective, e.g., Tilia.

(f) Distractile – When two anther lobes are far apart, e.g., Salvinia.

Gynoecium, the female reproductive whorl of flower, consists of carpels (= megasporophylls). A carpel is differentiated into 3 parts-stigama, style and ovary. When gynoecium is sterile or underdeveloped, it is called pistillode.

1. Gynoecium may be classified broadly into two types:

(a) Simple or Monocarpellary:

It is composed of only one carpel, e.g., pea, all legumes.

(b) Compound or Multi-capillary:

It comprises more than one carpel. Such a type of gynoecium occurs in majority of seed plants. Again, it may be of following two types: –

Each carpel is free from the other forming a separate gynoecium, e.g., Ranunculus, Clematis, etc.

All the carpels are fused with one another forming a compound gynoecium, e.g., Brassica (mustard), Hibiscus (China rose), Solanum species etc.

Depending upon the number of carpels, a syncarpous gynoecium may be of the following types:

(i) Bicarpellary: Comprises two carpels, e.g.,Sonchus, Coriandrum, Mussaenda.

(ii) Tricarpellary: With three carpels, e.g., Allium cepa (onion), etc.

(iii) Tetracarpellary: With four carpels, e.g., Duranta, Berberis, etc.

(iv) Pentacarpellary: With five carpels, e.g.. Hibiscus (China rose), Media (Neem), etc.

(v) Multicarpellary: With more than five carpels, e.g., Papaver. 3. Stigma:

It is the terminal part of pistil meant for receiving pollens at the time of pollination. On the basis of shape, stigma may be — capitate or round: Hibiscus, Citrus plumose or feathery: grasses Fid or Forked: Tridax Discoid: Melia Dumb-bell shaped: Thomoea Hood-Like: poppy Funnel-shaped: Crocus Striated Argemone

It is the tubular stalk that connect stigma with ovary.

It may be of following types-

When style lies in the same straight line with the ovary, e.g., Hibiscus, Dianthus, etc.

When style appears to be arising from the side of the ovary, such as in strawberry, mango.

Sometimes, such as in the family Labiatae, the ovary is lobed and the style arises from the depression in the centre of the ovary. Such a style is termed as gynobasic, e.g., Ocimum.

When the style becomes flattened and coloured like petals, e.g., Canna, Iris.

It is the lowermost (basal) part of the gynoecium, develops by the in rolling of the carpels (megasporophylls) along the median line.

1. Position of the ovary on thalamus:

In relation with other floral whorls, the ovary may occupy any of the following positions:

When Ovary occupies the highest position on thalamus, and the three other whorls (viz., sepals, petals and stamens) are successively insert d below it, the ovary is called superior, e.g., Citrus (lemon), Hibiscus, Brassica, etc.

Here the thalamus grows around the ovary to form a cup, and bears sepals, petals and stamens on the rim of the cup e.g., Rosa (Rose), Prunus, etc.

In this type, the thalamus completely covers the ovary and fuses with it. Sepals, petals and stamens emerge from the top of the ovary e.g., Coriandrum, Mussaenda, Cucurbita, etc.

2. Chambers (locules) of the Ovary:

Depending upon the numbers of locules, following types of ovaries can be recognized (Fig. 6.15). Mostly the number of locules corresponds to the number of carpels, but this is not the rule, because sometimes the number of locules may be more than the number of carpels due to the formation of false septa or less due to dissolution of septa.

Ovary with a single chamber, e.g., Pisum (pea).

Ovary with two chambers, e.g., Solarium, Murraya, etc.

Ovary with three chambers, e.g., Asphodelus, Euphorbia, Musa (Banana), etc.

The ovary of flower possesses one or more ovules which later on develop into seeds after fertilization. The ovule bearing region of the carpel is called placenta. The mode of arrangement of placentae and ovules within the ovary is called placentation.

It maybe of following types: (Fig. 6.16):

When the gynoecium is monocarpellary apocarpous, the placentae bearing ovules are borne on the ventral suture, where the margins of the ovary wall fuse, e.g., family Leguminosae.

Ovary multilocular and ovules borne on central placenta, e.g., Hibiscus, Citrus, Solarium, Allium, tomato, etc.

Ovary is unilocular but pistil is syncarpous. The ovules are borne on peripheral fused margins of carpels, e.g., Brassica, Papaya, Gourd etc.

Ovary is unilocular and ovules borne on a central column which is not connected to the ovary wall by any spetum, e.g., Dianthus, Silene, Primula etc.

Ovary is unilocular and a single ovule is borne at the base of the ovary.


Cultural significance

Flowers have been symbols of beauty in most civilizations of the world, and flower giving is still among the most popular of social amenities. As gifts, flowers serve as expressions of affection for spouses, other family members, and friends as decorations at weddings and other ceremonies as tokens of respect for the deceased as cheering gifts to the bedridden and as expressions of thanks or appreciation. Most flowers bought by the public are grown in commercial greenhouses or horticultural fields and then sold through wholesalers to retail florists. See also articles on individual flowers (e.g., carnation lotus petunia rose tulip).

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.


Contents

Flower is from the Middle English flour, which referred to both the ground grain and the reproductive structure in plants, before splitting off in the 17th century. It comes originally from the Latin name of the Italian goddess of flowers, Flora. The early word for flower in English was blossom, [2] though it now refers to flowers only of fruit trees. [3]

Floral parts Edit

The essential parts of a flower can be considered in two parts: the vegetative part, consisting of petals and associated structures in the perianth, and the reproductive or sexual parts. A stereotypical flower consists of four kinds of structures attached to the tip of a short stalk. Each of these kinds of parts is arranged in a whorl on the receptacle. The four main whorls (starting from the base of the flower or lowest node and working upwards) are as follows:

Perianth Edit

Collectively the calyx and corolla form the perianth (see diagram).

  • Calyx: the outermost whorl consisting of units called sepals these are typically green and enclose the rest of the flower in the bud stage, however, they can be absent or prominent and petal-like in some species.
  • Corolla: the next whorl toward the apex, composed of units called petals, which are typically thin, soft and colored to attract animals that help the process of pollination.
  • Perigone: in monocots the calyx and corolla are indistinguishable thus the whorls of the perianth or perigone are called tepals. [4]

Reproductive Edit

  • Androecium (from Greek andros oikia: man's house): the next whorl (sometimes multiplied into several whorls), consisting of units called stamens. Stamens consist of two parts: a stalk called a filament, topped by an anther where pollen is produced by meiosis and eventually dispersed.
  • Gynoecium (from Greek gynaikos oikia: woman's house): the innermost whorl of a flower, consisting of one or more units called carpels. The carpel or multiple fused carpels form a hollow structure called an ovary, which produces ovules internally. Ovules are megasporangia and they in turn produce megaspores by meiosis which develop into female gametophytes. These give rise to egg cells. The gynoecium of a flower is also described using an alternative terminology wherein the structure one sees in the innermost whorl (consisting of an ovary, style and stigma) is called a pistil. A pistil may consist of a single carpel or a number of carpels fused together. The sticky tip of the pistil, the stigma, is the receptor of pollen. The supportive stalk, the style, becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma. The relationship to the gynoecium on the receptacle is described as hypogynous (beneath a superior ovary), perigynous (surrounding a superior ovary), or epigynous (above inferior ovary).

Structure Edit

Although the arrangement described above is considered "typical", plant species show a wide variation in floral structure. [5] These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species.

The four main parts of a flower are generally defined by their positions on the receptacle and not by their function. Many flowers lack some parts or parts may be modified into other functions and/or look like what is typically another part. In some families, like Ranunculaceae, the petals are greatly reduced and in many species the sepals are colorful and petal-like. Other flowers have modified stamens that are petal-like the double flowers of Peonies and Roses are mostly petaloid stamens. [6] Flowers show great variation and plant scientists describe this variation in a systematic way to identify and distinguish species.

Specific terminology is used to describe flowers and their parts. Many flower parts are fused together fused parts originating from the same whorl are connate, while fused parts originating from different whorls are adnate parts that are not fused are free. When petals are fused into a tube or ring that falls away as a single unit, they are sympetalous (also called gamopetalous). Connate petals may have distinctive regions: the cylindrical base is the tube, the expanding region is the throat and the flaring outer region is the limb. A sympetalous flower, with bilateral symmetry with an upper and lower lip, is bilabiate. Flowers with connate petals or sepals may have various shaped corolla or calyx, including campanulate, funnelform, tubular, urceolate, salverform or rotate.

Referring to "fusion," as it is commonly done, appears questionable because at least some of the processes involved may be non-fusion processes. For example, the addition of intercalary growth at or below the base of the primordia of floral appendages such as sepals, petals, stamens and carpels may lead to a common base that is not the result of fusion. [7] [8] [9]

Many flowers have a symmetry. When the perianth is bisected through the central axis from any point and symmetrical halves are produced, the flower is said to be actinomorphic or regular, e.g. rose or trillium. This is an example of radial symmetry. When flowers are bisected and produce only one line that produces symmetrical halves, the flower is said to be irregular or zygomorphic, e.g. snapdragon or most orchids.

Flowers may be directly attached to the plant at their base (sessile—the supporting stalk or stem is highly reduced or absent). The stem or stalk subtending a flower is called a peduncle. If a peduncle supports more than one flower, the stems connecting each flower to the main axis are called pedicels. The apex of a flowering stem forms a terminal swelling which is called the torus or receptacle.

Inflorescence Edit

In those species that have more than one flower on an axis, the collective cluster of flowers is termed an inflorescence. Some inflorescences are composed of many small flowers arranged in a formation that resembles a single flower. The common example of this is most members of the very large composite (Asteraceae) group. A single daisy or sunflower, for example, is not a flower but a flower head—an inflorescence composed of numerous flowers (or florets). An inflorescence may include specialized stems and modified leaves known as bracts.

Floral diagrams and floral formulae Edit

A floral formula is a way to represent the structure of a flower using specific letters, numbers and symbols, presenting substantial information about the flower in a compact form. It can represent a taxon, usually giving ranges of the numbers of different organs, or particular species. Floral formulae have been developed in the early 19th century and their use has declined since. Prenner et al. (2010) devised an extension of the existing model to broaden the descriptive capability of the formula. [10] The format of floral formulae differs in different parts of the world, yet they convey the same information. [11] [12] [13] [14]

The structure of a flower can also be expressed by the means of floral diagrams. The use of schematic diagrams can replace long descriptions or complicated drawings as a tool for understanding both floral structure and evolution. Such diagrams may show important features of flowers, including the relative positions of the various organs, including the presence of fusion and symmetry, as well as structural details. [11]

A flower develops on a modified shoot or axis from a determinate apical meristem (determinate meaning the axis grows to a set size). It has compressed internodes, bearing structures that in classical plant morphology are interpreted as highly modified leaves. [15] Detailed developmental studies, however, have shown that stamens are often initiated more or less like modified stems (caulomes) that in some cases may even resemble branchlets. [9] [5] Taking into account the whole diversity in the development of the androecium of flowering plants, we find a continuum between modified leaves (phyllomes), modified stems (caulomes), and modified branchlets (shoots). [16] [17]

Flowering transition Edit

The transition to flowering is one of the major phase changes that a plant makes during its life cycle. The transition must take place at a time that is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success. To meet these needs a plant is able to interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes. [18] Many perennial and most biennial plants require vernalization to flower. The molecular interpretation of these signals is through the transmission of a complex signal known as florigen, which involves a variety of genes, including Constans, Flowering Locus C and Flowering Locus T. Florigen is produced in the leaves in reproductively favorable conditions and acts in buds and growing tips to induce a number of different physiological and morphological changes. [19]

The first step of the transition is the transformation of the vegetative stem primordia into floral primordia. This occurs as biochemical changes take place to change cellular differentiation of leaf, bud and stem tissues into tissue that will grow into the reproductive organs. Growth of the central part of the stem tip stops or flattens out and the sides develop protuberances in a whorled or spiral fashion around the outside of the stem end. These protuberances develop into the sepals, petals, stamens, and carpels. Once this process begins, in most plants, it cannot be reversed and the stems develop flowers, even if the initial start of the flower formation event was dependent of some environmental cue. [20]

Organ development Edit

The ABC model is a simple model that describes the genes responsible for the development of flowers. Three gene activities interact in a combinatorial manner to determine the developmental identities of the primordia organ within the floral apical meristem. These gene functions are called A, B, and C. A genes are expressed in only outer and lower most section of the apical meristem, which becomes a whorl of sepals. In the second whorl both A and B genes are expressed, leading to the formation of petals. In the third whorl, B and C genes interact to form stamens and in the center of the flower C genes alone give rise to carpels. The model is based upon studies of aberrant flowers and mutations in Arabidopsis thaliana and the snapdragon, Antirrhinum majus. For example, when there is a loss of B gene function, mutant flowers are produced with sepals in the first whorl as usual, but also in the second whorl instead of the normal petal formation. In the third whorl the lack of B function but presence of C function mimics the fourth whorl, leading to the formation of carpels also in the third whorl. [21]

The principal purpose of a flower is the reproduction of the individual and the species. All flowering plants are heterosporous, that is, every individual plant produces two types of spores. Microspores are produced by meiosis inside anthers and megaspores are produced inside ovules that are within an ovary. Anthers typically consist of four microsporangia and an ovule is an integumented megasporangium. Both types of spores develop into gametophytes inside sporangia. As with all heterosporous plants, the gametophytes also develop inside the spores, i. e., they are endosporic.

In the majority of plant species, individual flowers have both functional carpels and stamens. Botanists describe these flowers as "perfect" or "bisexual", and the species as "hermaphroditic". In a minority of plant species, their flowers lack one or the other reproductive organ and are described as "imperfect" or "unisexual". If the individual plants of a species each have unisexual flowers of both sexes then the species is "monoecious". Alternatively, if each individual plant has only unisexual flowers of the same sex then the species is "dioecious".

The primary purpose of the flower is reproduction. [22] Since the flowers are the reproductive organs of the plant, they mediate the joining of the sperm, contained within pollen, to the ovules — contained in the ovary. [23] Pollination is the movement of pollen from the anthers to the stigma. [24] Normally pollen is moved from one plant to another, known as cross-pollination, but many plants are able to self-pollinate. Cross-pollination is preferred because it allows for genetic variation, which contributes to the survival of the species. [25] Many flowers are dependent, then, upon external factors for pollination, such as: the wind, water, animals, and especially insects. Larger animals such as birds, bats, and even some pygmy possums, [26] however, can also be employed. [27] [28] To accomplish this, flowers have specific designs which encourage the transfer of pollen from one plant to another of the same species. The period of time during which this process can take place (when the flower is fully expanded and functional) is called anthesis, [29] hence the study of pollination biology is called anthecology. [30]

Flowering plants usually face evolutionary pressure to optimize the transfer of their pollen, and this is typically reflected in the morphology of the flowers and the behaviour of the plants. [31] Pollen may be transferred between plants via a number of 'vectors,' or methods. Around 80% of flowering plants make use of biotic, or living vectors. Others use abiotic, or non-living, vectors and some plants make use of multiple vectors, but most are highly specialised. [32]

Though some fit between or outside of these groups, [33] most flowers can be divided between the following two broad groups of pollination methods:

Biotic pollination Edit

Flowers that use biotic vectors attract and use insects, bats, birds or other animals to transfer pollen from one flower to the next. Often they are specialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate). [34] In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas—arranged with equally pointed precision—of all of the flowers it visits. [35] Many flowers rely on simple proximity between flower parts to ensure pollination, while others have elaborate designs to ensure pollination and prevent self-pollination. [25] Flowers use animals including: insects (entomophily), birds (ornithophily), bats (chiropterophily), lizards, [28] and even snails and slugs (malacophilae). [36]

Attraction methods Edit

Plants cannot move from one location to another, thus many flowers have evolved to attract animals to transfer pollen between individuals in dispersed populations. Most commonly, flowers are insect-pollinated, known as entomophilous literally "insect-loving" in Greek. [38] To attract these insects flowers commonly have glands called nectaries on various parts that attract animals looking for nutritious nectar. [39] Birds and bees have color vision, enabling them to seek out "colorful" flowers. [40] Some flowers have patterns, called nectar guides, that show pollinators where to look for nectar they may be visible only under ultraviolet light, which is visible to bees and some other insects. [41]

Flowers also attract pollinators by scent, thought not all flower scents are appealing to humans a number of flowers are pollinated by insects that are attracted to rotten flesh and have flowers that smell like dead animals. These are often called Carrion flowers, including plants in the genus Rafflesia, and the titan arum. [40] Flowers pollinated by night visitors, including bats and moths, are likely to concentrate on scent to attract pollinators and so most such flowers are white. [42]

Flowers are also specialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant. Other flowers use mimicry or pseudocopulation to attract pollinators. Many orchids for example, produce flowers resembling female bees or wasps in colour, shape, and scent. Males move from one flower to the next in search of a mate, pollinating the flowers. [34] [43]

Flower-pollinator relationships Edit

Many flowers have close relationships with one or a few specific pollinating organisms. Many flowers, for example, attract only one specific species of insect, and therefore rely on that insect for successful reproduction. This close relationship an example of coevolution, as the flower and pollinator have developed together over a long period of time to match each other's needs. [34] This close relationship compounds the negative effects of extinction, however, since the extinction of either member in such a relationship would almost certainly mean the extinction of the other member as well. [44]

Abiotic pollination Edit

Flowers that use abiotic, or non-living, vectors use the wind or, much less commonly, water, to move pollen from one flower to the next. [32] In wind-dispersed (anemophilous) species, the tiny pollen grains are carried, sometimes many thousands of kilometres, [45] by the wind to other flowers. Common examples include the grasses, birch trees, along with many other species in the order fagales, [46] ragweeds, and many sedges. They have no need to attract pollinators and therefore tend not to grow large, showy, or colorful flowers, and do not have nectaries, nor a noticeable scent. Because of this, plants typically have many thousands of tiny flowers which have comparatively large, feathery stigmas to increase the chance of pollen being received. [47] Whereas the pollen of entomophilous flowers is usually large, sticky, and rich in protein (to act as a "reward" for pollinators), anemophilous flower pollen is typically small-grained, very light, smooth, and of little nutritional value to insects. [48] [49] In order for the wind to effectively pick up and transport the pollen, the flowers typically have anthers loosely attached to the end of long thin filaments, or pollen forms around a catkin which moves in the wind. Rarer forms of this involve individual flowers being moveable by the wind (Pendulous), or even less commonly the anthers exploding to release the pollen into the wind. [48]

Pollination through water (hydrophily) is a much rarer method, occurring in only around 2% of abiotically-pollinated flowers. [32] Common examples of this include Calitriche autumnalis, Vallisneria spiralis and some sea-grasses. One characteristic which most species in this group share is a lack of an exine, or protective layer, around the pollen grain. [50] Paul Knuth identified two types of hydrophilous pollination in 1906 and Ernst Schwarzenbach added a third in 1944. Knuth named his two groups Hyphydrogamy and the more common Ephydrogamy. [51] In Hyphydrogamy pollination occurs below the surface of the water and so the pollen grains are typically negatively buoyant. For marine plants that exhibit this method the stigmas are usually stiff, while freshwater species have small and feathery stigmas. [52] In Ephydrogamy pollination occurs on the surface of the water and so the pollen has a low density to enable floating, though many also use rafts, and are hydrophobic. Marine flowers have floating thread-like stigmas and may have adaptations for the tide, while freshwater species create indentations in the water. [52] The third category, set out by Schwarzenbach, is those flowers which transport pollen above the water through conveyance. This ranges from floating plants, (Lemnoideae), to staminate flowers (Vallisneria). Most species in this group have dry, spherical pollen which sometimes forms into larger masses, and female flowers which form depressions in the water the method of transport varies. [52]

Mechanisms Edit

Flowers can be pollinated by two mechanisms cross-pollination and self-pollination. No mechanism is indisputably better than the other as they each have their advantages and disadvantages. Plants use one or both of these mechanisms depending on their habitat and ecological niche. [53]

Cross-pollination Edit

Cross-pollination is the pollination of the carpel by pollen from a different plant of the same species. Because the genetic make-up of the sperm contained within the pollen from the other plant is different, their combination will result in a new, genetically distinct, plant, through the process of sexual reproduction. Since each new plant is genetically distinct, the different plants show variation in their physiological and structural adaptations and so the population as a whole is better prepared for an adverse occurrence in the environment. Cross-pollination, therefore, increases the survival of the species and is usually preferred by flowers for this reason. [25] [54]

Self-pollination Edit

Self-pollination is the pollination of the carpel of a flower by pollen from either the same flower or another flower on the same plant, [25] leading to the creation of a genetic clone through asexual reproduction. This increases the reliability of producing seeds, the rate at which they can be produced, and lowers the amount energy needed. [55] But, most importantly, it limits genetic variation. The extreme case of self-fertilization, when the ovule is fertilized by pollen from the same flower or plant, occurs in flowers that always self-fertilize, such as many dandelions. [56] Some flowers are self-pollinated and have flowers that never open or are self-pollinated before the flowers open these flowers are called cleistogamous many species in the genus Viola exhibit this, for example. [57] Conversely, many species of plants have ways of preventing self-pollination and hence, self-fertilization. Unisexual male and female flowers on the same plant may not appear or mature at the same time, or pollen from the same plant may be incapable of fertilizing its ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-incompatible. [58] [59] In Clianthus puniceus, (pictured), self-pollination is used strategically as an "insurance policy." When a pollinator, in this case a bird, visits C. puniceus it rubs off the stigmatic covering and allows for pollen from the bird to enter the stigma. If no pollinators visit, however, then the stigmatic covering falls off naturally to allow for the flower's own anthers to pollinate the flower through self-pollination. [55]

Allergies Edit

Pollen is a large contributor to asthma and other respiratory allergies which combined affect between 10 and 50% of people worldwide. This number appears to be growing, as the temperature increases due to climate change mean that plants are producing more pollen, which is also more allergenic. Pollen is difficult to avoid, however, because of its small size and prevalence in the natural environment. Most of the pollen which causes allergies is that produced by wind-dispersed pollinators such as the grasses, birch trees, oak trees, and ragweeds, and the allergens in pollen are proteins which are thought to be necessary in the process of pollination. [60] [61]

Fertilization, also called Synagmy, occurs following pollination, which is the movement of pollen from the stamen to the carpel. It encompasses both plasmogamy, the fusion of the protoplasts, and karyogamy, the fusion of the nuclei. When pollen lands on the stigma of the flower it begins creating a pollen tube which runs down through the style and into the ovary. After penetrating the centre-most part of the ovary it enters the egg apparatus and into one synergid. At this point the end of the pollen tube bursts and releases the two sperm cells, one of which makes its way to an egg, while also losing its cell membrane and much of its protoplasm. The sperm's nucleus then fuses with the egg's nucleus, resulting in the formation of a zygote, a diploid (two copies of each chromosome) cell. [62]

Whereas in fertilization only plasmogamy, or the fusion of the whole sex cells, results, in Angiosperms (flowering plants) a process known as double fertilization, which involves both karyogamy and plasmogamy, occurs. In double fertilization the second sperm cell subsequently also enters the synergid and fuses with the two polar nuclei of the central cell. Since all three nuclei are haploid, they result in a large endosperm nucleus which is triploid. [62]

While land plants have existed for about 425 million years, the first ones reproduced by a simple adaptation of their aquatic counterparts: spores. In the sea, plants—and some animals—can simply scatter out genetic clones of themselves to float away and grow elsewhere. This is how early plants reproduced. But plants soon evolved methods of protecting these copies to deal with drying out and other damage which is even more likely on land than in the sea. The protection became the seed, though it had not yet evolved the flower. Early seed-bearing plants include the ginkgo and conifers.

Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin.

Recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. An early fossil of a flowering plant, Archaefructus liaoningensis from China, is dated about 125 million years old. [63] [64] Even earlier from China is the 125–130 million years old Archaefructus sinensis. In 2015 a plant (130 million-year-old Montsechia vidalii, discovered in Spain) was claimed to be 130 million years old. [65] In 2018, scientists reported that the earliest flowers began about 180 million years ago. [66]

Recent DNA analysis (molecular systematics) [67] shows that Amborella trichopoda, found on the Pacific island of New Caledonia, is the only species in the sister group to the rest of the flowering plants, and morphological studies suggest that it has features which may have been characteristic of the earliest flowering plants. [68]

Besides the hard proof of flowers in or shortly before the Cretaceous, [69] [70] there is some circumstantial evidence of flowers as much as 250 million years ago. A chemical used by plants to defend their flowers, oleanane, has been detected in fossil plants that old, including gigantopterids, [71] which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail one of the earliest examples of petrification.

The similarity in leaf and stem structure can be very important, because flowers are genetically just an adaptation of normal leaf and stem components on plants, a combination of genes normally responsible for forming new shoots. [72] The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior".

The general assumption is that the function of flowers, from the start, was to involve animals in the reproduction process. Pollen can be scattered without bright colors and obvious shapes, which would therefore be a liability, using the plant's resources, unless they provide some other benefit. One proposed reason for the sudden, fully developed appearance of flowers is that they evolved in an isolated setting like an island, or chain of islands, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example), the way many island species develop today. This symbiotic relationship, with a hypothetical wasp bearing pollen from one plant to another much the way fig wasps do today, could have eventually resulted in both the plant(s) and their partners developing a high degree of specialization. Island genetics is believed to be a common source of speciation, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Note that the wasp example is not incidental bees, apparently evolved specifically for symbiotic plant relationships, are descended from wasps.

Likewise, most fruit used in plant reproduction comes from the enlargement of parts of the flower. This fruit is frequently a tool which depends upon animals wishing to eat it, and thus scattering the seeds it contains.

While many such symbiotic relationships remain too fragile to survive competition with mainland organisms, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life.

Flower evolution continues to the present day modern flowers have been so profoundly influenced by humans that many of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection. [73]

Many flowering plants reflect as much light as possible within the range of visible wavelengths of the pollinator the plant intends to attract. Flowers that reflect the full range of visible light are generally perceived as white by a human observer. An important feature of white flowers is that they reflect equally across the visible spectrum. While many flowering plants use white to attract pollinators, the use of color is also widespread (even within the same species). Color allows a flowering plant to be more specific about the pollinator it seeks to attract. The color model used by human color reproduction technology (CMYK) relies on the modulation of pigments that divide the spectrum into broad areas of absorption. Flowering plants by contrast are able to shift the transition point wavelength between absorption and reflection. If it is assumed that the visual systems of most pollinators view the visible spectrum as circular then it may be said that flowering plants produce color by absorbing the light in one region of the spectrum and reflecting the light in the other region. With CMYK, color is produced as a function of the amplitude of the broad regions of absorption. Flowering plants by contrast produce color by modifying the frequency (or rather wavelength) of the light reflected. Most flowers absorb light in the blue to yellow region of the spectrum and reflect light from the green to red region of the spectrum. For many species of flowering plant, it is the transition point that characterizes the color that they produce. Color may be modulated by shifting the transition point between absorption and reflection and in this way a flowering plant may specify which pollinator it seeks to attract. Some flowering plants also have a limited ability to modulate areas of absorption. This is typically not as precise as control over wavelength. Humans observers will perceive this as degrees of saturation (the amount of white in the color).

Many flowers have important symbolic meanings in Western culture. [74] The practice of assigning meanings to flowers is known as floriography. Some of the more common examples include:

  • Red roses are given as a symbol of love, beauty, and passion. [75] are a symbol of consolation in time of death. In the United Kingdom, New Zealand, Australia and Canada, red poppies are worn to commemorate soldiers who have died in times of war. /Lily are used in burials as a symbol referring to "resurrection/life". It is also associated with stars (sun) and its petals blooming/shining. are a symbol of innocence.

Because of their varied and colorful appearance, flowers have long been a favorite subject of visual artists as well. Some of the most celebrated paintings from well-known painters are of flowers, such as Van Gogh's sunflowers series or Monet's water lilies. Flowers are also dried, freeze dried and pressed in order to create permanent, three-dimensional pieces of floral art.

Flowers within art are also representative of the female genitalia, [76] as seen in the works of artists such as Georgia O'Keeffe, Imogen Cunningham, Veronica Ruiz de Velasco, and Judy Chicago, and in fact in Asian and western classical art. Many cultures around the world have a marked tendency to associate flowers with femininity.

The great variety of delicate and beautiful flowers has inspired the works of numerous poets, especially from the 18th–19th century Romantic era. Famous examples include William Wordsworth's I Wandered Lonely as a Cloud and William Blake's Ah! Sun-Flower.

Their symbolism in dreams has also been discussed, with possible interpretations including "blossoming potential". [77]

The Roman goddess of flowers, gardens, and the season of Spring is Flora. The Greek goddess of spring, flowers and nature is Chloris.

In Hindu mythology, flowers have a significant status. Vishnu, one of the three major gods in the Hindu system, is often depicted standing straight on a lotus flower. [78] Apart from the association with Vishnu, the Hindu tradition also considers the lotus to have spiritual significance. [79] For example, it figures in the Hindu stories of creation. [80]

In modern times, people have sought ways to cultivate, buy, wear, or otherwise be around flowers and blooming plants, partly because of their agreeable appearance and smell. Around the world, people use flowers to mark important events in their lives:

  • For new births or christenings
  • As a corsage or boutonniere worn at social functions or for holidays
  • As tokens of love or esteem
  • For wedding flowers for the bridal party, and for decorations for the hall
  • As brightening decorations within the home
  • As a gift of remembrance for bon voyage parties, welcome-home parties, and "thinking of you" gifts
  • For funeral flowers and expressions of sympathy for the grieving
  • For worship. In Christianity, chancel flowers often adorn churches. [81] In Hindu culture, adherents commonly bring flowers as a gift to temples[82]

People therefore grow flowers around their homes, dedicate parts of their living space to flower gardens, pick wildflowers, or buy commercially-grown flowers from florists.

Flowers provide less food than other major plant parts (seeds, fruits, roots, stems and leaves), but still provide several important vegetables and spices. Flower vegetables include broccoli, cauliflower and artichoke. The most expensive spice, saffron, consists of dried stigmas of a crocus. Other flower spices are cloves and capers. Hops flowers are used to flavor beer. Marigold flowers are fed to chickens to give their egg yolks a golden yellow color, which consumers find more desirable dried and ground marigold flowers are also used as a spice and colouring agent in Georgian cuisine. Flowers of the dandelion and elder are often made into wine. Bee pollen, pollen collected from bees, is considered a health food by some people. Honey consists of bee-processed flower nectar and is often named for the type of flower, e.g. orange blossom honey, clover honey and tupelo honey.

Hundreds of fresh flowers are edible, but only few are widely marketed as food. They are often added to salads as garnishes. Squash blossoms are dipped in breadcrumbs and fried. Some edible flowers include nasturtium, chrysanthemum, carnation, cattail, Japanese honeysuckle, chicory, cornflower, canna, and sunflower. [83] Edible flowers such as daisy, rose, and violet are sometimes candied. [84]

Flowers such as chrysanthemum, rose, jasmine, Japanese honeysuckle, and chamomile, chosen for their fragrance and medicinal properties, are used as tisanes, either mixed with tea or on their own. [85]

Flowers have been used since prehistoric times in funeral rituals: traces of pollen have been found on a woman's tomb in the El Miron Cave in Spain. [86] Many cultures draw a connection between flowers and life and death, and because of their seasonal return flowers also suggest rebirth, which may explain why many people place flowers upon graves. The ancient Greeks, as recorded in Euripides's play The Phoenician Women, placed a crown of flowers on the head of the deceased [87] they also covered tombs with wreaths and flower petals. Flowers were widely used in ancient Egyptian burials, [88] and the Mexicans to this day use flowers prominently in their Day of the Dead celebrations [89] in the same way that their Aztec ancestors did.

Flower-giving Edit

The flower-giving tradition goes back to prehistoric times when flowers often had a medicinal and herbal attributes. Archaeologists found in several grave sites remnants of flower petals. Flowers were first used as sacrificial and burial objects. Ancient Egyptians and later Greeks and Romans used flowers. In Egypt, burial objects from the time around 1540 BC [ citation needed ] were found, which depicted red poppy, yellow Araun, cornflower and lilies. Records of flower giving appear in Chinese writings and Egyptian hieroglyphics, as well as in Greek and Roman mythology. The practice of giving a flower flourished in the Middle Ages when couples showed affection through flowers.

The tradition of flower-giving exists in many forms. It is an important part of Russian culture and folklore. It is common for students to give flowers to their teachers. To give yellow flowers in a romantic relationship means break-up in Russia. Nowadays, flowers are often given away in the form of a flower bouquet. [90] [91] [92]


Oriental Lilies

Oriental lilies have a much stronger fragrance than their Asiatic counterparts. Also desirable for their elaborately frilled flower petals, these lilies are hardy in USDA zones 7 through 9. Oriental lilies bloom in mid and late summer and grow best in full sunlight and moist, organic soil. Lilies in this group include Lilium auratum, which has white petals streaked with gold bands, Lilium japonicum, which thrives in cool soils and is a good companion to rhododendrons, Lilium rubellum, which has delicate pink flowers and grows at altitudes of 9,000 feet, and Lilium speciosum, which has a very strong, sweet scent.


Multiple Choice Question

1. Put a tick mark (✓) against the correct alternative in the following statements:

(a) In a germinating seed, the roots develop from:
(i) Radicle
(ii) Plumule
(iii) Tegmen
(iv) Hilum

(b) In a germinating seed, the shoot develops from:
(i) Radicle
(ii) Plumule
(iii) Tegmen
(iv) Hilum

(c) Which one of the following is a monocotyledonous seed ?
(i) Bean
(ii) Pea
(iii) Maize
(iv) Gram

(d) If the cotyledons are pushed above the soil, then such type of germination is called :
(i) Epigeal
(ii) Hypogeal
(iii) Perigeal
(iv) Progeal

(e) If the cotyledons remain under the soil, then such seeds type of germination is called:
(i) Epigeal
(ii) Hypogeal
(iii) Perigeal
(iv) Progeal

(f) Pollen is produced in the:
(i) Filament
(ii) Style
(iii) Pistil
(iv) Anther

(g) Reproductive whorls of a flower are:
(i) Stamens and carpels
(ii) Sepals and petals
(iii) Sepals and stamens
(iv) Petals and carpels

PQ. Vegetative propagation is not observed in:
(i) Potato
(ii) Tomato
(iii) Pea
(iv) Bean

(h) Which one of the following is a false fruit ?
(i) Tomato
(ii) Apple
(iii) Potato
(iv) Pea

(i) In a seed, food is generally stored in:
(i) Radicle
(ii) Plumule
(iii) Fruit
(iv) Catyledons or endosperms

1. Given below is a longitudinal section of a bean seed. Label the parts marked 1 to 5 and write their functions.

Answer :
1. Testa (seed coat)
2. Plumule
3. Radicle
4. Micropyle
5. Cotyledon

  1. Testa (seed coat) — It protects the seed from insects and bacteria as well as from mechanical injury.
  2. Plumule — Plumule develops into a shoot.
  3. Radicle — Radicle develops into a root
  4. Micropyle — The micropyle absorbs as much water as is required for germination.
  5. Cotyledon — Contain stored food material which is used by the seeding during germination.

2. Name the following The Flower ICSE Class-6th

(a) A seed which shows hypogeal germination.
Ans. pea seed, maize. seed
(b) Amonocotseed.
Ans. Maize seed, wheat seed
(c) A dicot seed.
Ans. Bean seed, gram seed, pea seed
(d) A seed which shows epigeal germination.
Ans. Bean seed castor seed, tamarind seed

Question 3.


Differentiate between the following pairs of terms:

Answer :

(a)

Radicle and plumule.
Radicle: In a seed the radicle lies downwards near the lower pointed end of the grain. It gives rise to the root.
Plumule: In a seed the plumule lies upwani near the cotyledon and gives rise to the shoot.

(b)

Ilium and micropyle.
Answer :
IlIum : On one side of seed câat, there is scar called hi hum, which marks the place where the seed was attached to the fiiüt wall.
Micropyle : Above the hilum is a small pore called micrope. The micropyle absorbs as much water as is required for germination

(c)

Testa and tegmen.
Answer :
Testa: The seed is protected by a thick outermost coat called the testa or seed coat.
Tegmen: Under the testa lies a very thin membrane called the tegmen.

Question 4.

Give two functions of a fruit.
Answer :
Functions of a fruit are:

  1. Fruit is a protective case for the seeds.
  2. Fruit is a temptation to animals and man to eat it and scatter the seeds

Question 5. Match the columns :

Column A, ColumnB

Question 6.


Radicle emerges out of the seed earlier than plumule.What one advantage is served by this ?
Answer :
Radicle comes out of the seed earlier than the plumule has advantages as it gets water and minerals from the soil and gives it to the growing plumule.

Question 7.


State whether the following statements are True or False.

(a) Some seeds have no cotyledons.
False
(b) Warmth is necessary for the germination of seeds.
True
(c) All seeds have two cotyledons.
False
(d) Oxygen is necessary for the germination of seeds.
True

Question 8.

State one function of the following:
(a) radicle
(b) cotyledons
(c) endosperm
(d) micropyle

Answer :

  1. Radicle — form the roots
  2. Cotyledons — On removing th& testa and the tegmen from a soaked bean seed, you will find that the seed is made up of two fleshy seed leaves called the cotyledolm. They contain stored food material which is used by the seedlling for growth.
  3. Endosperm—ovary forms the fruit.
  4. Micropyle —Above the hilum is a small pore called micropyle (micro = small, pyle = passage). The micropyle absorbs and allows as much water as is required for germination.

Question 9.


The three conditions necessary for germination of seeds are (tick the correct answer):
(a) Oxygen, suitable temperature and water.
(b) Good soil, water and air
(c) Good soil, suitable temperature and light.
(d) Light, oxygen, and temperature.
(e) Oxygen, carbon dioxide, and light.

Question 10.

Name the part of the seeds from which the following are given out:
(a) Roots : .
(b) Leaves :
Answer :
(a) Roots — Radicle give rise to roots.
(b) Leaves—Plumule gives rise to shoot bearing leaves

Question 11.


In the spaces provided below, draw labelled diagrams to show the three stages in the germination of any seed you have observed.
Answer :

Long Answer Questions (The Flower ICSE Class-6th)

(Write the answers in your note-book)

Question 1.


What is meant by pollination ? Name the two types of pollination.
Answer :
The transfer of the pollen grains from the anthers to the stigma of a flower is called pollination.
The two types of pollination found in flowering plants are.

  1. self-pollination – that occurs within the same plant.
  2. cross-pollination – that occurs between two flowers of two different plants but of the same kind.

Question 2.

Imagine that all the seeds produced by a plant happen to fall under the same plant and sprout into new plants. Mention any two problems that will be faced by the new plants.
Answer :
If all the seeds produced by a plant happen to fall under the same plant and sprout into new plants then in this situation plants will face the following problems:

  1. A large number of plants will grow in a small limited space. The water and the minerals available to them in the soil will be limited.
  2. The air surrounding them will not be enough and less sunshine will be available to them. As a result, most of these sprouted plants will die.

Question 3.


What is a flower ? Draw a typical flower and label its different parts.
Answer :

A flower is a reproductive part of a plant. It helps in sexual reproduction as it has male parts and female parts.
A fully opened flower has the following parts:
Stalk—A flower is attached to the shoot by means of stalk or pedicel stalk. The tip of the stalk is swollen or flattened. This is called toms or thalamus or receptacle.
The different parts of a flower are inserted on the thalamus. There are usually four whorls as Calyx

  • (Sepals)
  • Corolla (Petals)
  • Androecium (stamens)
  • Gynoecium (Carpels)
  • Present on the thalamus.
Sepals (Calyx):

These are the outermost part of the flower. These are leaf like and green in colour. This is the outer covering of the flower and form outer whorl in a flower. The Calyx (sepals) enclose the inner parts of the flower when it is a bud. It is protective in function.

Corolla (Petals):

Petals form the second whorl inner to the sepals. These are usually coloured, gaudy, or white in colour and scented and give sweet smell. The value of a flower is due to the attractive colour of the petals. These attract the insects for pollination.

Stamens (Androecium):

The third whorls inner to the petals are stamens. This third whorl is called Androecium. These are the male parts of the flower. Each stamen is formed of a long narrow, hair like structure called filament. On its tip it bears a rounded broad sac like structure called anther. Each anther has two anther lobes. Each anther lobe has two pollen sacs which have powdery mass called pollen grains.

Carpels (Gynoecium):

Carpels are the inner most or fourth whorl in a flower. It is lodged on the thalamus and forms the female part of a flower. This whorl of carpels is called gynoecium. Each carpel or pistil has three parts,

    1. The lower most, swollen part is ovary. It is attached to the thalamus
    2. The middle part is style which is narrow, thread like
    3. Stigma: The style ends in a knob like, rounded structure which is sticky in nature to receive the pollen grains.The ovaries contain ovules which later turn into seeds after fertilization and the ovary wall forms the fruit sometimes the thalamus also becomes a part of the fruit as in apple.

    Question 4.

    With the help of a suitable labelled diagram, describe the structure of a dicot seed.

    It is a dicotyledonous and non-endospermic seed. It is produced in a long cylindrical pod (fruit – phali) External characters. The seed is brown or whitish brown in colour. The seed is hard and smooth and kidney shaped i.e.. Convex on one side and concave on the other side. Concave side bears whitish scar called hilum. It is the place which is attached to the wall of the pod through a stalk called funicle. At one side of the hilum is a small pore called micropyle water enters through it.

    Internal structure —

    The seed is covered by a hard, tough covering called testa. Inner to the tests is the embryo. Embryo consists of two cotyledons and embryo axis. Embryo axis has plumule and radicle.

    The plumule

    The plumule is present in between the two cotyledons and its top bears two folded tiny leaves. It forms future shoot and leaves of the growing seed.

    Radicle

    Radicle is rod shaped and is out of the two cotyledons. It forms the root of the growing seed. When the seed grows the two cotyledons come out of the soil and form cotyledonary leaves and turn green in colour. Cotyledons give food to the growing seedling as it has food. The germination in this seed is epigeal as cotyledons come outside the soil in the growing seed.

    Question 5.


    Define germination ? Name the two types of germination. Explain with examples.
    Answer :
    The growth and development of the embryo present in the seed into a seedling (or a young plant capable of independent existence) is called as seed germination.
    The embryo in a seed remains inactive or dormant. When the seed is put into the soil and given water and under suitable temperature, the embryo becomes active on absorbing the water and the embryo turns into a seedling.

    Types of germination — There are three types of germinations.
    (i) Epigeal
    (ii)Hypogeal
    (iii) Viviparous

    (i) Epigeal germination —

    Epi means above geo-ground (earth)
    When the cotyledons in growing seed come out of the soil it is epigeal type of germination as in case of castor seed, cucumber, tamarind, bean seed the cotyledons come out of the soil and turn green. These are called cotyledonary leaves. These cany on photosynthesis till new leaves arise. .

    (ii) Hypogeal germination—

    Hypo-below, gea soil.
    When in a growing seed the cotyledons remain under the ground as in case of gram, pea, groundnut and maize. The plumule firms the aerial shoot to which bears leaves and the radicle gives rise to roots. The growing seedling gets food from the cotyledons. As the seedling grows the cotyledons die in the soil.

    (iii) Viviparous germination—

    This is special type of germination.

    This occurs in plants growing along the sea coasts and in salt lakes. The seed start growing while it is still attached to the plant as in mangrove plants. The embryo comes out of the fruit with a long, dart like radicle. It falls directly into soft, slushy, wet mud. The radicle gives root and establishes as a seedling and the plumule give rise to shoot. This is in mangrove plants.

    Question 6.


    What are the three conditions necessary for the germination of seeds. How would you demonstrate this?
    Answer:
    For successful germination of any viable seed, three external conditions are necessary as:

    We can demonstrate this with “Three seed experiment This is a simple experiment to demonstrate the necessity of these factors for proper germination.

    Experiment:

    Three seed experiment to demonstrate germination Three mature dried bean seeds are taken and tied on a wooden strip at three different positions (above the figure). This strip is placed in a beaker containing water in such a way that the lower seed is completely submerged in water, the middle seed is partially submerged inside the water and the top seed is kept above water. This set-up is left in a warm place for few days.
    It is observed that the middle seed shows germination and gives out radicle and shoot leaves. The top seed shows no growth and the bottom one shows negligible growth.

    The middle seed gets fully germinated due to the fact that this seed has all the favourable conditions required for germination i.e., air (oxygen), moisture and warmth (favourable temperature), which are necessary for germination.

    Question 7.


    Give the main difference between hypogeal and epigeal and germination.

    Epigeal germination—

    Epi means above geo-means ground (soil). When the cotyledons in a growing seed come out of the soil it is epigeal type of germination as in case of castor seed, bean seeds, pulses, tamarind cucumber.

    The cotyledons come out of the soil and turn green these are cotyledonary leaves and carry on the function of photosynthesis till new leaves arise in the growing seedling.

    Hypogeal germination —

    Hypo-below, geo-soil it is that type of germination in which in the growing seed the cotyledons remain under the soil as in case of pea, gram, ground-nut, maize. The plumule forms the aerial shoot which later bears leaves and die radicle gives rise to root. The tiny seedling gets food from the cotyledons till it establishes itself in the soil by its roots and starts getting water and minerals and as well the new leaves arise on the ascending axis and they start making food by the process of food making.

    Differences—

    Epigeal germination

    1. In this type of germination the cotyledons come out of the soil.
    2. The cotyledons turn green as they come out of the soil and serve as cotyledonary cotyledons
      leaves and carry on photo¬synthesis to make food for the growing seedling.
    3. The cotyledons become pale and fall off when the ascending axis bear leaves
    Hypogeal germination
    1. In this type of germination the cotyledons remain under the soil.
    2. The cotyledons remain under the ground and the seedling gets food from the till it establishes in the soil.
    3. The cotyledons get rotten up in the soil when the seedling matures.

    Question 8.


    State the location of the following in a flower:
    Answer :

    Sepals:

    These are the outermost part of the flower. These are leaf like and green in colour. This is the outer covering of the flower and form outer whorl in a flower. The Calyx (sepals) enclose the inner parts of the flower when it is a bud. It is protective in function.

    Petals:

    Petals form the second whorl inner to the sepals. These are usually coloured, gaudy, or white in colour and scented and give sweet smell. The value of a flower is due to the attractive colour of the petals. These attract the insects for pollination.

    Anther—

    It is present at the end of a stamen. Anther has poller sacs in which pollen grains are formed. Pollen grains contain the male gametes.

    Stigma—

    It is the terminal knob-like part, it may be divided into two or more lobes and assume a feathery appearance. The stigma is covered with hair or with glandular papillae. It serves as the landing place grains for pollen during pollination.

    Question 9.


    Given below is the diagram of a typical flower. Label the parts marked by guidelines.

    Question 10.


    Give the difference in the function between the following parts:
    (a) Ovary and ovule
    (b) Petal and sepal
    (c) Filament and style
    (d) Pollen and ovule
    Answer :

    (a) Ovary and ovule —
    1. It is the female reproductive part of a flower and ovules are located inside the ovary.
    2. Ovary after fertilization turns into a fruit whereas ovules turn into seeds of fruit.
    (b) petal and sepal—

    Petals are colourful and attractive and helps to attract insects for pollination whereas the main function of sepals is to provide protection to the growing bud.

    (c) filament and style—

    The filament is a stalk like structure that attaches and support the flower and support the anther which is the structure that produces pollens whereas the style transfers the male gametes of the pollen grains into the ovary.

    (d) pollen and ovule —

    The function of pollen is to deliver male gametes (sperm) from stamen of a plant to an ovule whereas ovule, when fertilized, well developed into a seed. It is a female reproductive cell.


    Contents

    Early botany Edit

    There is evidence humans used plants as far back as 10,000 years ago in the Little Tennessee River Valley, generally as firewood or food. [6] Botany originated as herbalism, the study and use of plants for their medicinal properties. [7] The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BCE, [8] [9] Ancient Egypt, [10] in archaic Avestan writings, and in works from China purportedly from before 221 BCE. [8] [11]

    Modern botany traces its roots back to Ancient Greece specifically to Theophrastus (c. 371–287 BCE), a student of Aristotle who invented and described many of its principles and is widely regarded in the scientific community as the "Father of Botany". [12] His major works, Enquiry into Plants and On the Causes of Plants, constitute the most important contributions to botanical science until the Middle Ages, almost seventeen centuries later. [12] [13]

    Another work from Ancient Greece that made an early impact on botany is De Materia Medica, a five-volume encyclopedia about herbal medicine written in the middle of the first century by Greek physician and pharmacologist Pedanius Dioscorides. De Materia Medica was widely read for more than 1,500 years. [14] Important contributions from the medieval Muslim world include Ibn Wahshiyya's Nabatean Agriculture, Abū Ḥanīfa Dīnawarī's (828–896) the Book of Plants, and Ibn Bassal's The Classification of Soils. In the early 13th century, Abu al-Abbas al-Nabati, and Ibn al-Baitar (d. 1248) wrote on botany in a systematic and scientific manner. [15] [16] [17]

    In the mid-16th century, botanical gardens were founded in a number of Italian universities. The Padua botanical garden in 1545 is usually considered to be the first which is still in its original location. These gardens continued the practical value of earlier "physic gardens", often associated with monasteries, in which plants were cultivated for medical use. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens and their medical uses demonstrated. Botanical gardens came much later to northern Europe the first in England was the University of Oxford Botanic Garden in 1621. Throughout this period, botany remained firmly subordinate to medicine. [18]

    German physician Leonhart Fuchs (1501–1566) was one of "the three German fathers of botany", along with theologian Otto Brunfels (1489–1534) and physician Hieronymus Bock (1498–1554) (also called Hieronymus Tragus). [19] [20] Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification.

    Physician Valerius Cordus (1515–1544) authored a botanically and pharmacologically important herbal Historia Plantarum in 1544 and a pharmacopoeia of lasting importance, the Dispensatorium in 1546. [21] Naturalist Conrad von Gesner (1516–1565) and herbalist John Gerard (1545–c. 1611) published herbals covering the medicinal uses of plants. Naturalist Ulisse Aldrovandi (1522–1605) was considered the father of natural history, which included the study of plants. In 1665, using an early microscope, Polymath Robert Hooke discovered cells, a term he coined, in cork, and a short time later in living plant tissue. [22]

    Early modern botany Edit

    During the 18th century, systems of plant identification were developed comparable to dichotomous keys, where unidentified plants are placed into taxonomic groups (e.g. family, genus and species) by making a series of choices between pairs of characters. The choice and sequence of the characters may be artificial in keys designed purely for identification (diagnostic keys) or more closely related to the natural or phyletic order of the taxa in synoptic keys. [23] By the 18th century, new plants for study were arriving in Europe in increasing numbers from newly discovered countries and the European colonies worldwide. In 1753, Carl von Linné (Carl Linnaeus) published his Species Plantarum, a hierarchical classification of plant species that remains the reference point for modern botanical nomenclature. This established a standardised binomial or two-part naming scheme where the first name represented the genus and the second identified the species within the genus. [24] For the purposes of identification, Linnaeus's Systema Sexuale classified plants into 24 groups according to the number of their male sexual organs. The 24th group, Cryptogamia, included all plants with concealed reproductive parts, mosses, liverworts, ferns, algae and fungi. [25]

    Increasing knowledge of plant anatomy, morphology and life cycles led to the realisation that there were more natural affinities between plants than the artificial sexual system of Linnaeus. Adanson (1763), de Jussieu (1789), and Candolle (1819) all proposed various alternative natural systems of classification that grouped plants using a wider range of shared characters and were widely followed. The Candollean system reflected his ideas of the progression of morphological complexity and the later Bentham & Hooker system, which was influential until the mid-19th century, was influenced by Candolle's approach. Darwin's publication of the Origin of Species in 1859 and his concept of common descent required modifications to the Candollean system to reflect evolutionary relationships as distinct from mere morphological similarity. [26]

    Botany was greatly stimulated by the appearance of the first "modern" textbook, Matthias Schleiden's Grundzüge der Wissenschaftlichen Botanik, published in English in 1849 as Principles of Scientific Botany. [27] Schleiden was a microscopist and an early plant anatomist who co-founded the cell theory with Theodor Schwann and Rudolf Virchow and was among the first to grasp the significance of the cell nucleus that had been described by Robert Brown in 1831. [28] In 1855, Adolf Fick formulated Fick's laws that enabled the calculation of the rates of molecular diffusion in biological systems. [29]

    Late modern botany Edit

    Building upon the gene-chromosome theory of heredity that originated with Gregor Mendel (1822–1884), August Weismann (1834–1914) proved that inheritance only takes place through gametes. No other cells can pass on inherited characters. [30] The work of Katherine Esau (1898–1997) on plant anatomy is still a major foundation of modern botany. Her books Plant Anatomy and Anatomy of Seed Plants have been key plant structural biology texts for more than half a century. [31] [32]

    The discipline of plant ecology was pioneered in the late 19th century by botanists such as Eugenius Warming, who produced the hypothesis that plants form communities, and his mentor and successor Christen C. Raunkiær whose system for describing plant life forms is still in use today. The concept that the composition of plant communities such as temperate broadleaf forest changes by a process of ecological succession was developed by Henry Chandler Cowles, Arthur Tansley and Frederic Clements. Clements is credited with the idea of climax vegetation as the most complex vegetation that an environment can support and Tansley introduced the concept of ecosystems to biology. [33] [34] [35] Building on the extensive earlier work of Alphonse de Candolle, Nikolai Vavilov (1887–1943) produced accounts of the biogeography, centres of origin, and evolutionary history of economic plants. [36]

    Particularly since the mid-1960s there have been advances in understanding of the physics of plant physiological processes such as transpiration (the transport of water within plant tissues), the temperature dependence of rates of water evaporation from the leaf surface and the molecular diffusion of water vapour and carbon dioxide through stomatal apertures. These developments, coupled with new methods for measuring the size of stomatal apertures, and the rate of photosynthesis have enabled precise description of the rates of gas exchange between plants and the atmosphere. [37] [38] Innovations in statistical analysis by Ronald Fisher, [39] Frank Yates and others at Rothamsted Experimental Station facilitated rational experimental design and data analysis in botanical research. [40] The discovery and identification of the auxin plant hormones by Kenneth V. Thimann in 1948 enabled regulation of plant growth by externally applied chemicals. Frederick Campion Steward pioneered techniques of micropropagation and plant tissue culture controlled by plant hormones. [41] The synthetic auxin 2,4-Dichlorophenoxyacetic acid or 2,4-D was one of the first commercial synthetic herbicides. [42]

    20th century developments in plant biochemistry have been driven by modern techniques of organic chemical analysis, such as spectroscopy, chromatography and electrophoresis. With the rise of the related molecular-scale biological approaches of molecular biology, genomics, proteomics and metabolomics, the relationship between the plant genome and most aspects of the biochemistry, physiology, morphology and behaviour of plants can be subjected to detailed experimental analysis. [43] The concept originally stated by Gottlieb Haberlandt in 1902 [44] that all plant cells are totipotent and can be grown in vitro ultimately enabled the use of genetic engineering experimentally to knock out a gene or genes responsible for a specific trait, or to add genes such as GFP that report when a gene of interest is being expressed. These technologies enable the biotechnological use of whole plants or plant cell cultures grown in bioreactors to synthesise pesticides, antibiotics or other pharmaceuticals, as well as the practical application of genetically modified crops designed for traits such as improved yield. [45]

    Modern morphology recognises a continuum between the major morphological categories of root, stem (caulome), leaf (phyllome) and trichome. [46] Furthermore, it emphasises structural dynamics. [47] Modern systematics aims to reflect and discover phylogenetic relationships between plants. [48] [49] [50] [51] Modern Molecular phylogenetics largely ignores morphological characters, relying on DNA sequences as data. Molecular analysis of DNA sequences from most families of flowering plants enabled the Angiosperm Phylogeny Group to publish in 1998 a phylogeny of flowering plants, answering many of the questions about relationships among angiosperm families and species. [52] The theoretical possibility of a practical method for identification of plant species and commercial varieties by DNA barcoding is the subject of active current research. [53] [54]

    The study of plants is vital because they underpin almost all animal life on Earth by generating a large proportion of the oxygen and food that provide humans and other organisms with aerobic respiration with the chemical energy they need to exist. Plants, algae and cyanobacteria are the major groups of organisms that carry out photosynthesis, a process that uses the energy of sunlight to convert water and carbon dioxide [55] into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells. [56] As a by-product of photosynthesis, plants release oxygen into the atmosphere, a gas that is required by nearly all living things to carry out cellular respiration. In addition, they are influential in the global carbon and water cycles and plant roots bind and stabilise soils, preventing soil erosion. [57] Plants are crucial to the future of human society as they provide food, oxygen, medicine, and products for people, as well as creating and preserving soil. [58]

    Historically, all living things were classified as either animals or plants [59] and botany covered the study of all organisms not considered animals. [60] Botanists examine both the internal functions and processes within plant organelles, cells, tissues, whole plants, plant populations and plant communities. At each of these levels, a botanist may be concerned with the classification (taxonomy), phylogeny and evolution, structure (anatomy and morphology), or function (physiology) of plant life. [61]

    The strictest definition of "plant" includes only the "land plants" or embryophytes, which include seed plants (gymnosperms, including the pines, and flowering plants) and the free-sporing cryptogams including ferns, clubmosses, liverworts, hornworts and mosses. Embryophytes are multicellular eukaryotes descended from an ancestor that obtained its energy from sunlight by photosynthesis. They have life cycles with alternating haploid and diploid phases. The sexual haploid phase of embryophytes, known as the gametophyte, nurtures the developing diploid embryo sporophyte within its tissues for at least part of its life, [62] even in the seed plants, where the gametophyte itself is nurtured by its parent sporophyte. [63] Other groups of organisms that were previously studied by botanists include bacteria (now studied in bacteriology), fungi (mycology) – including lichen-forming fungi (lichenology), non-chlorophyte algae (phycology), and viruses (virology). However, attention is still given to these groups by botanists, and fungi (including lichens) and photosynthetic protists are usually covered in introductory botany courses. [64] [65]

    Palaeobotanists study ancient plants in the fossil record to provide information about the evolutionary history of plants. Cyanobacteria, the first oxygen-releasing photosynthetic organisms on Earth, are thought to have given rise to the ancestor of plants by entering into an endosymbiotic relationship with an early eukaryote, ultimately becoming the chloroplasts in plant cells. The new photosynthetic plants (along with their algal relatives) accelerated the rise in atmospheric oxygen started by the cyanobacteria, changing the ancient oxygen-free, reducing, atmosphere to one in which free oxygen has been abundant for more than 2 billion years. [66] [67]

    Among the important botanical questions of the 21st century are the role of plants as primary producers in the global cycling of life's basic ingredients: energy, carbon, oxygen, nitrogen and water, and ways that our plant stewardship can help address the global environmental issues of resource management, conservation, human food security, biologically invasive organisms, carbon sequestration, climate change, and sustainability. [68]

    Human nutrition Edit

    Virtually all staple foods come either directly from primary production by plants, or indirectly from animals that eat them. [69] Plants and other photosynthetic organisms are at the base of most food chains because they use the energy from the sun and nutrients from the soil and atmosphere, converting them into a form that can be used by animals. This is what ecologists call the first trophic level. [70] The modern forms of the major staple foods, such as hemp, teff, maize, rice, wheat and other cereal grasses, pulses, bananas and plantains, [71] as well as hemp, flax and cotton grown for their fibres, are the outcome of prehistoric selection over thousands of years from among wild ancestral plants with the most desirable characteristics. [72]

    Botanists study how plants produce food and how to increase yields, for example through plant breeding, making their work important to humanity's ability to feed the world and provide food security for future generations. [73] Botanists also study weeds, which are a considerable problem in agriculture, and the biology and control of plant pathogens in agriculture and natural ecosystems. [74] Ethnobotany is the study of the relationships between plants and people. When applied to the investigation of historical plant–people relationships ethnobotany may be referred to as archaeobotany or palaeoethnobotany. [75] Some of the earliest plant-people relationships arose between the indigenous people of Canada in identifying edible plants from inedible plants. [76] This relationship the indigenous people had with plants was recorded by ethnobotanists. [76]

    Plant biochemistry is the study of the chemical processes used by plants. Some of these processes are used in their primary metabolism like the photosynthetic Calvin cycle and crassulacean acid metabolism. [77] Others make specialised materials like the cellulose and lignin used to build their bodies, and secondary products like resins and aroma compounds.

    Plants and various other groups of photosynthetic eukaryotes collectively known as "algae" have unique organelles known as chloroplasts. Chloroplasts are thought to be descended from cyanobacteria that formed endosymbiotic relationships with ancient plant and algal ancestors. Chloroplasts and cyanobacteria contain the blue-green pigment chlorophyll a. [78] Chlorophyll a (as well as its plant and green algal-specific cousin chlorophyll b) [a] absorbs light in the blue-violet and orange/red parts of the spectrum while reflecting and transmitting the green light that we see as the characteristic colour of these organisms. The energy in the red and blue light that these pigments absorb is used by chloroplasts to make energy-rich carbon compounds from carbon dioxide and water by oxygenic photosynthesis, a process that generates molecular oxygen (O2) as a by-product.

    The light energy captured by chlorophyll a is initially in the form of electrons (and later a proton gradient) that's used to make molecules of ATP and NADPH which temporarily store and transport energy. Their energy is used in the light-independent reactions of the Calvin cycle by the enzyme rubisco to produce molecules of the 3-carbon sugar glyceraldehyde 3-phosphate (G3P). Glyceraldehyde 3-phosphate is the first product of photosynthesis and the raw material from which glucose and almost all other organic molecules of biological origin are synthesised. Some of the glucose is converted to starch which is stored in the chloroplast. [82] Starch is the characteristic energy store of most land plants and algae, while inulin, a polymer of fructose is used for the same purpose in the sunflower family Asteraceae. Some of the glucose is converted to sucrose (common table sugar) for export to the rest of the plant.

    Unlike in animals (which lack chloroplasts), plants and their eukaryote relatives have delegated many biochemical roles to their chloroplasts, including synthesising all their fatty acids, [83] [84] and most amino acids. [85] The fatty acids that chloroplasts make are used for many things, such as providing material to build cell membranes out of and making the polymer cutin which is found in the plant cuticle that protects land plants from drying out. [86]

    Plants synthesise a number of unique polymers like the polysaccharide molecules cellulose, pectin and xyloglucan [87] from which the land plant cell wall is constructed. [88] Vascular land plants make lignin, a polymer used to strengthen the secondary cell walls of xylem tracheids and vessels to keep them from collapsing when a plant sucks water through them under water stress. Lignin is also used in other cell types like sclerenchyma fibres that provide structural support for a plant and is a major constituent of wood. Sporopollenin is a chemically resistant polymer found in the outer cell walls of spores and pollen of land plants responsible for the survival of early land plant spores and the pollen of seed plants in the fossil record. It is widely regarded as a marker for the start of land plant evolution during the Ordovician period. [89] The concentration of carbon dioxide in the atmosphere today is much lower than it was when plants emerged onto land during the Ordovician and Silurian periods. Many monocots like maize and the pineapple and some dicots like the Asteraceae have since independently evolved [90] pathways like Crassulacean acid metabolism and the C4 carbon fixation pathway for photosynthesis which avoid the losses resulting from photorespiration in the more common C3 carbon fixation pathway. These biochemical strategies are unique to land plants.

    Medicine and materials Edit

    Phytochemistry is a branch of plant biochemistry primarily concerned with the chemical substances produced by plants during secondary metabolism. [91] Some of these compounds are toxins such as the alkaloid coniine from hemlock. Others, such as the essential oils peppermint oil and lemon oil are useful for their aroma, as flavourings and spices (e.g., capsaicin), and in medicine as pharmaceuticals as in opium from opium poppies. Many medicinal and recreational drugs, such as tetrahydrocannabinol (active ingredient in cannabis), caffeine, morphine and nicotine come directly from plants. Others are simple derivatives of botanical natural products. For example, the pain killer aspirin is the acetyl ester of salicylic acid, originally isolated from the bark of willow trees, [92] and a wide range of opiate painkillers like heroin are obtained by chemical modification of morphine obtained from the opium poppy. [93] Popular stimulants come from plants, such as caffeine from coffee, tea and chocolate, and nicotine from tobacco. Most alcoholic beverages come from fermentation of carbohydrate-rich plant products such as barley (beer), rice (sake) and grapes (wine). [94] Native Americans have used various plants as ways of treating illness or disease for thousands of years. [95] This knowledge Native Americans have on plants has been recorded by enthnobotanists and then in turn has been used by pharmaceutical companies as a way of drug discovery. [96]

    Plants can synthesise useful coloured dyes and pigments such as the anthocyanins responsible for the red colour of red wine, yellow weld and blue woad used together to produce Lincoln green, indoxyl, source of the blue dye indigo traditionally used to dye denim and the artist's pigments gamboge and rose madder. Sugar, starch, cotton, linen, hemp, some types of rope, wood and particle boards, papyrus and paper, vegetable oils, wax, and natural rubber are examples of commercially important materials made from plant tissues or their secondary products. Charcoal, a pure form of carbon made by pyrolysis of wood, has a long history as a metal-smelting fuel, as a filter material and adsorbent and as an artist's material and is one of the three ingredients of gunpowder. Cellulose, the world's most abundant organic polymer, [97] can be converted into energy, fuels, materials and chemical feedstock. Products made from cellulose include rayon and cellophane, wallpaper paste, biobutanol and gun cotton. Sugarcane, rapeseed and soy are some of the plants with a highly fermentable sugar or oil content that are used as sources of biofuels, important alternatives to fossil fuels, such as biodiesel. [98] Sweetgrass was used by Native Americans to ward off bugs like mosquitoes. [99] These bug repelling properties of sweetgrass were later found by the American Chemical Society in the molecules phytol and coumarin. [99]

    Plant ecology is the science of the functional relationships between plants and their habitats – the environments where they complete their life cycles. Plant ecologists study the composition of local and regional floras, their biodiversity, genetic diversity and fitness, the adaptation of plants to their environment, and their competitive or mutualistic interactions with other species. [100] Some ecologists even rely on empirical data from indigenous people that is gathered by ethnobotanists. [101] This information can relay a great deal of information on how the land once was thousands of years ago and how it has changed over that time. [101] The goals of plant ecology are to understand the causes of their distribution patterns, productivity, environmental impact, evolution, and responses to environmental change. [102]

    Plants depend on certain edaphic (soil) and climatic factors in their environment but can modify these factors too. For example, they can change their environment's albedo, increase runoff interception, stabilise mineral soils and develop their organic content, and affect local temperature. Plants compete with other organisms in their ecosystem for resources. [103] [104] They interact with their neighbours at a variety of spatial scales in groups, populations and communities that collectively constitute vegetation. Regions with characteristic vegetation types and dominant plants as well as similar abiotic and biotic factors, climate, and geography make up biomes like tundra or tropical rainforest. [105]

    Herbivores eat plants, but plants can defend themselves and some species are parasitic or even carnivorous. Other organisms form mutually beneficial relationships with plants. For example, mycorrhizal fungi and rhizobia provide plants with nutrients in exchange for food, ants are recruited by ant plants to provide protection, [107] honey bees, bats and other animals pollinate flowers [108] [109] and humans and other animals [110] act as dispersal vectors to spread spores and seeds.

    Plants, climate and environmental change Edit

    Plant responses to climate and other environmental changes can inform our understanding of how these changes affect ecosystem function and productivity. For example, plant phenology can be a useful proxy for temperature in historical climatology, and the biological impact of climate change and global warming. Palynology, the analysis of fossil pollen deposits in sediments from thousands or millions of years ago allows the reconstruction of past climates. [111] Estimates of atmospheric CO
    2 concentrations since the Palaeozoic have been obtained from stomatal densities and the leaf shapes and sizes of ancient land plants. [112] Ozone depletion can expose plants to higher levels of ultraviolet radiation-B (UV-B), resulting in lower growth rates. [113] Moreover, information from studies of community ecology, plant systematics, and taxonomy is essential to understanding vegetation change, habitat destruction and species extinction. [114]

    Inheritance in plants follows the same fundamental principles of genetics as in other multicellular organisms. Gregor Mendel discovered the genetic laws of inheritance by studying inherited traits such as shape in Pisum sativum (peas). What Mendel learned from studying plants has had far-reaching benefits outside of botany. Similarly, "jumping genes" were discovered by Barbara McClintock while she was studying maize. [115] Nevertheless, there are some distinctive genetic differences between plants and other organisms.

    Species boundaries in plants may be weaker than in animals, and cross species hybrids are often possible. A familiar example is peppermint, Mentha × piperita, a sterile hybrid between Mentha aquatica and spearmint, Mentha spicata. [116] The many cultivated varieties of wheat are the result of multiple inter- and intra-specific crosses between wild species and their hybrids. [117] Angiosperms with monoecious flowers often have self-incompatibility mechanisms that operate between the pollen and stigma so that the pollen either fails to reach the stigma or fails to germinate and produce male gametes. [118] This is one of several methods used by plants to promote outcrossing. [119] In many land plants the male and female gametes are produced by separate individuals. These species are said to be dioecious when referring to vascular plant sporophytes and dioicous when referring to bryophyte gametophytes. [120]

    Unlike in higher animals, where parthenogenesis is rare, asexual reproduction may occur in plants by several different mechanisms. The formation of stem tubers in potato is one example. Particularly in arctic or alpine habitats, where opportunities for fertilisation of flowers by animals are rare, plantlets or bulbs, may develop instead of flowers, replacing sexual reproduction with asexual reproduction and giving rise to clonal populations genetically identical to the parent. This is one of several types of apomixis that occur in plants. Apomixis can also happen in a seed, producing a seed that contains an embryo genetically identical to the parent. [121]

    Most sexually reproducing organisms are diploid, with paired chromosomes, but doubling of their chromosome number may occur due to errors in cytokinesis. This can occur early in development to produce an autopolyploid or partly autopolyploid organism, or during normal processes of cellular differentiation to produce some cell types that are polyploid (endopolyploidy), or during gamete formation. An allopolyploid plant may result from a hybridisation event between two different species. Both autopolyploid and allopolyploid plants can often reproduce normally, but may be unable to cross-breed successfully with the parent population because there is a mismatch in chromosome numbers. These plants that are reproductively isolated from the parent species but live within the same geographical area, may be sufficiently successful to form a new species. [122] Some otherwise sterile plant polyploids can still reproduce vegetatively or by seed apomixis, forming clonal populations of identical individuals. [122] Durum wheat is a fertile tetraploid allopolyploid, while bread wheat is a fertile hexaploid. The commercial banana is an example of a sterile, seedless triploid hybrid. Common dandelion is a triploid that produces viable seeds by apomictic seed.

    As in other eukaryotes, the inheritance of endosymbiotic organelles like mitochondria and chloroplasts in plants is non-Mendelian. Chloroplasts are inherited through the male parent in gymnosperms but often through the female parent in flowering plants. [123]

    Molecular genetics Edit

    A considerable amount of new knowledge about plant function comes from studies of the molecular genetics of model plants such as the Thale cress, Arabidopsis thaliana, a weedy species in the mustard family (Brassicaceae). [91] The genome or hereditary information contained in the genes of this species is encoded by about 135 million base pairs of DNA, forming one of the smallest genomes among flowering plants. Arabidopsis was the first plant to have its genome sequenced, in 2000. [124] The sequencing of some other relatively small genomes, of rice (Oryza sativa) [125] and Brachypodium distachyon, [126] has made them important model species for understanding the genetics, cellular and molecular biology of cereals, grasses and monocots generally.

    Model plants such as Arabidopsis thaliana are used for studying the molecular biology of plant cells and the chloroplast. Ideally, these organisms have small genomes that are well known or completely sequenced, small stature and short generation times. Corn has been used to study mechanisms of photosynthesis and phloem loading of sugar in C4 plants. [127] The single celled green alga Chlamydomonas reinhardtii, while not an embryophyte itself, contains a green-pigmented chloroplast related to that of land plants, making it useful for study. [128] A red alga Cyanidioschyzon merolae has also been used to study some basic chloroplast functions. [129] Spinach, [130] peas, [131] soybeans and a moss Physcomitrella patens are commonly used to study plant cell biology. [132]

    Agrobacterium tumefaciens, a soil rhizosphere bacterium, can attach to plant cells and infect them with a callus-inducing Ti plasmid by horizontal gene transfer, causing a callus infection called crown gall disease. Schell and Van Montagu (1977) hypothesised that the Ti plasmid could be a natural vector for introducing the Nif gene responsible for nitrogen fixation in the root nodules of legumes and other plant species. [133] Today, genetic modification of the Ti plasmid is one of the main techniques for introduction of transgenes to plants and the creation of genetically modified crops.

    Epigenetics Edit

    Epigenetics is the study of heritable changes in gene function that cannot be explained by changes in the underlying DNA sequence [134] but cause the organism's genes to behave (or "express themselves") differently. [135] One example of epigenetic change is the marking of the genes by DNA methylation which determines whether they will be expressed or not. Gene expression can also be controlled by repressor proteins that attach to silencer regions of the DNA and prevent that region of the DNA code from being expressed. Epigenetic marks may be added or removed from the DNA during programmed stages of development of the plant, and are responsible, for example, for the differences between anthers, petals and normal leaves, despite the fact that they all have the same underlying genetic code. Epigenetic changes may be temporary or may remain through successive cell divisions for the remainder of the cell's life. Some epigenetic changes have been shown to be heritable, [136] while others are reset in the germ cells.

    Epigenetic changes in eukaryotic biology serve to regulate the process of cellular differentiation. During morphogenesis, totipotent stem cells become the various pluripotent cell lines of the embryo, which in turn become fully differentiated cells. A single fertilised egg cell, the zygote, gives rise to the many different plant cell types including parenchyma, xylem vessel elements, phloem sieve tubes, guard cells of the epidermis, etc. as it continues to divide. The process results from the epigenetic activation of some genes and inhibition of others. [137]

    Unlike animals, many plant cells, particularly those of the parenchyma, do not terminally differentiate, remaining totipotent with the ability to give rise to a new individual plant. Exceptions include highly lignified cells, the sclerenchyma and xylem which are dead at maturity, and the phloem sieve tubes which lack nuclei. While plants use many of the same epigenetic mechanisms as animals, such as chromatin remodelling, an alternative hypothesis is that plants set their gene expression patterns using positional information from the environment and surrounding cells to determine their developmental fate. [138]

    Epigenetic changes can lead to paramutations, which do not follow the Mendelian heritage rules. These epigenetic marks are carried from one generation to the next, with one allele inducing a change on the other. [139]

    The chloroplasts of plants have a number of biochemical, structural and genetic similarities to cyanobacteria, (commonly but incorrectly known as "blue-green algae") and are thought to be derived from an ancient endosymbiotic relationship between an ancestral eukaryotic cell and a cyanobacterial resident. [140] [141] [142] [143]

    The algae are a polyphyletic group and are placed in various divisions, some more closely related to plants than others. There are many differences between them in features such as cell wall composition, biochemistry, pigmentation, chloroplast structure and nutrient reserves. The algal division Charophyta, sister to the green algal division Chlorophyta, is considered to contain the ancestor of true plants. [144] The Charophyte class Charophyceae and the land plant sub-kingdom Embryophyta together form the monophyletic group or clade Streptophytina. [145]

    Nonvascular land plants are embryophytes that lack the vascular tissues xylem and phloem. They include mosses, liverworts and hornworts. Pteridophytic vascular plants with true xylem and phloem that reproduced by spores germinating into free-living gametophytes evolved during the Silurian period and diversified into several lineages during the late Silurian and early Devonian. Representatives of the lycopods have survived to the present day. By the end of the Devonian period, several groups, including the lycopods, sphenophylls and progymnosperms, had independently evolved "megaspory" – their spores were of two distinct sizes, larger megaspores and smaller microspores. Their reduced gametophytes developed from megaspores retained within the spore-producing organs (megasporangia) of the sporophyte, a condition known as endospory. Seeds consist of an endosporic megasporangium surrounded by one or two sheathing layers (integuments). The young sporophyte develops within the seed, which on germination splits to release it. The earliest known seed plants date from the latest Devonian Famennian stage. [146] [147] Following the evolution of the seed habit, seed plants diversified, giving rise to a number of now-extinct groups, including seed ferns, as well as the modern gymnosperms and angiosperms. [148] Gymnosperms produce "naked seeds" not fully enclosed in an ovary modern representatives include conifers, cycads, Ginkgo, and Gnetales. Angiosperms produce seeds enclosed in a structure such as a carpel or an ovary. [149] [150] Ongoing research on the molecular phylogenetics of living plants appears to show that the angiosperms are a sister clade to the gymnosperms. [151]

    Plant physiology encompasses all the internal chemical and physical activities of plants associated with life. [152] Chemicals obtained from the air, soil and water form the basis of all plant metabolism. The energy of sunlight, captured by oxygenic photosynthesis and released by cellular respiration, is the basis of almost all life. Photoautotrophs, including all green plants, algae and cyanobacteria gather energy directly from sunlight by photosynthesis. Heterotrophs including all animals, all fungi, all completely parasitic plants, and non-photosynthetic bacteria take in organic molecules produced by photoautotrophs and respire them or use them in the construction of cells and tissues. [153] Respiration is the oxidation of carbon compounds by breaking them down into simpler structures to release the energy they contain, essentially the opposite of photosynthesis. [154]

    Molecules are moved within plants by transport processes that operate at a variety of spatial scales. Subcellular transport of ions, electrons and molecules such as water and enzymes occurs across cell membranes. Minerals and water are transported from roots to other parts of the plant in the transpiration stream. Diffusion, osmosis, and active transport and mass flow are all different ways transport can occur. [155] Examples of elements that plants need to transport are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. In vascular plants, these elements are extracted from the soil as soluble ions by the roots and transported throughout the plant in the xylem. Most of the elements required for plant nutrition come from the chemical breakdown of soil minerals. [156] Sucrose produced by photosynthesis is transported from the leaves to other parts of the plant in the phloem and plant hormones are transported by a variety of processes.

    Plant hormones Edit

    Plants are not passive, but respond to external signals such as light, touch, and injury by moving or growing towards or away from the stimulus, as appropriate. Tangible evidence of touch sensitivity is the almost instantaneous collapse of leaflets of Mimosa pudica, the insect traps of Venus flytrap and bladderworts, and the pollinia of orchids. [158]

    The hypothesis that plant growth and development is coordinated by plant hormones or plant growth regulators first emerged in the late 19th century. Darwin experimented on the movements of plant shoots and roots towards light [159] and gravity, and concluded "It is hardly an exaggeration to say that the tip of the radicle . . acts like the brain of one of the lower animals . . directing the several movements". [160] About the same time, the role of auxins (from the Greek auxein, to grow) in control of plant growth was first outlined by the Dutch scientist Frits Went. [161] The first known auxin, indole-3-acetic acid (IAA), which promotes cell growth, was only isolated from plants about 50 years later. [162] This compound mediates the tropic responses of shoots and roots towards light and gravity. [163] The finding in 1939 that plant callus could be maintained in culture containing IAA, followed by the observation in 1947 that it could be induced to form roots and shoots by controlling the concentration of growth hormones were key steps in the development of plant biotechnology and genetic modification. [164]

    Cytokinins are a class of plant hormones named for their control of cell division (especially cytokinesis). The natural cytokinin zeatin was discovered in corn, Zea mays, and is a derivative of the purine adenine. Zeatin is produced in roots and transported to shoots in the xylem where it promotes cell division, bud development, and the greening of chloroplasts. [165] [166] The gibberelins, such as Gibberelic acid are diterpenes synthesised from acetyl CoA via the mevalonate pathway. They are involved in the promotion of germination and dormancy-breaking in seeds, in regulation of plant height by controlling stem elongation and the control of flowering. [167] Abscisic acid (ABA) occurs in all land plants except liverworts, and is synthesised from carotenoids in the chloroplasts and other plastids. It inhibits cell division, promotes seed maturation, and dormancy, and promotes stomatal closure. It was so named because it was originally thought to control abscission. [168] Ethylene is a gaseous hormone that is produced in all higher plant tissues from methionine. It is now known to be the hormone that stimulates or regulates fruit ripening and abscission, [169] [170] and it, or the synthetic growth regulator ethephon which is rapidly metabolised to produce ethylene, are used on industrial scale to promote ripening of cotton, pineapples and other climacteric crops.

    Another class of phytohormones is the jasmonates, first isolated from the oil of Jasminum grandiflorum [171] which regulates wound responses in plants by unblocking the expression of genes required in the systemic acquired resistance response to pathogen attack. [172]

    In addition to being the primary energy source for plants, light functions as a signalling device, providing information to the plant, such as how much sunlight the plant receives each day. This can result in adaptive changes in a process known as photomorphogenesis. Phytochromes are the photoreceptors in a plant that are sensitive to light. [173]

    Plant anatomy is the study of the structure of plant cells and tissues, whereas plant morphology is the study of their external form. [174] All plants are multicellular eukaryotes, their DNA stored in nuclei. [175] [176] The characteristic features of plant cells that distinguish them from those of animals and fungi include a primary cell wall composed of the polysaccharides cellulose, hemicellulose and pectin, [177] larger vacuoles than in animal cells and the presence of plastids with unique photosynthetic and biosynthetic functions as in the chloroplasts. Other plastids contain storage products such as starch (amyloplasts) or lipids (elaioplasts). Uniquely, streptophyte cells and those of the green algal order Trentepohliales [178] divide by construction of a phragmoplast as a template for building a cell plate late in cell division. [82]

    The bodies of vascular plants including clubmosses, ferns and seed plants (gymnosperms and angiosperms) generally have aerial and subterranean subsystems. The shoots consist of stems bearing green photosynthesising leaves and reproductive structures. The underground vascularised roots bear root hairs at their tips and generally lack chlorophyll. [180] Non-vascular plants, the liverworts, hornworts and mosses do not produce ground-penetrating vascular roots and most of the plant participates in photosynthesis. [181] The sporophyte generation is nonphotosynthetic in liverworts but may be able to contribute part of its energy needs by photosynthesis in mosses and hornworts. [182]

    The root system and the shoot system are interdependent – the usually nonphotosynthetic root system depends on the shoot system for food, and the usually photosynthetic shoot system depends on water and minerals from the root system. [180] Cells in each system are capable of creating cells of the other and producing adventitious shoots or roots. [183] Stolons and tubers are examples of shoots that can grow roots. [184] Roots that spread out close to the surface, such as those of willows, can produce shoots and ultimately new plants. [185] In the event that one of the systems is lost, the other can often regrow it. In fact it is possible to grow an entire plant from a single leaf, as is the case with plants in Streptocarpus sect. Saintpaulia, [186] or even a single cell – which can dedifferentiate into a callus (a mass of unspecialised cells) that can grow into a new plant. [183] In vascular plants, the xylem and phloem are the conductive tissues that transport resources between shoots and roots. Roots are often adapted to store food such as sugars or starch, [180] as in sugar beets and carrots. [185]

    Stems mainly provide support to the leaves and reproductive structures, but can store water in succulent plants such as cacti, food as in potato tubers, or reproduce vegetatively as in the stolons of strawberry plants or in the process of layering. [187] Leaves gather sunlight and carry out photosynthesis. [188] Large, flat, flexible, green leaves are called foliage leaves. [189] Gymnosperms, such as conifers, cycads, Ginkgo, and gnetophytes are seed-producing plants with open seeds. [190] Angiosperms are seed-producing plants that produce flowers and have enclosed seeds. [149] Woody plants, such as azaleas and oaks, undergo a secondary growth phase resulting in two additional types of tissues: wood (secondary xylem) and bark (secondary phloem and cork). All gymnosperms and many angiosperms are woody plants. [191] Some plants reproduce sexually, some asexually, and some via both means. [192]

    Although reference to major morphological categories such as root, stem, leaf, and trichome are useful, one has to keep in mind that these categories are linked through intermediate forms so that a continuum between the categories results. [193] Furthermore, structures can be seen as processes, that is, process combinations. [47]

    Systematic botany is part of systematic biology, which is concerned with the range and diversity of organisms and their relationships, particularly as determined by their evolutionary history. [194] It involves, or is related to, biological classification, scientific taxonomy and phylogenetics. Biological classification is the method by which botanists group organisms into categories such as genera or species. Biological classification is a form of scientific taxonomy. Modern taxonomy is rooted in the work of Carl Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to align better with the Darwinian principle of common descent – grouping organisms by ancestry rather than superficial characteristics. While scientists do not always agree on how to classify organisms, molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions along evolutionary lines and is likely to continue to do so. The dominant classification system is called Linnaean taxonomy. It includes ranks and binomial nomenclature. The nomenclature of botanical organisms is codified in the International Code of Nomenclature for algae, fungi, and plants (ICN) and administered by the International Botanical Congress. [195] [196]

    Kingdom Plantae belongs to Domain Eukarya and is broken down recursively until each species is separately classified. The order is: Kingdom Phylum (or Division) Class Order Family Genus (plural genera) Species. The scientific name of a plant represents its genus and its species within the genus, resulting in a single worldwide name for each organism. [196] For example, the tiger lily is Lilium columbianum. Lilium is the genus, and columbianum the specific epithet. The combination is the name of the species. When writing the scientific name of an organism, it is proper to capitalise the first letter in the genus and put all of the specific epithet in lowercase. Additionally, the entire term is ordinarily italicised (or underlined when italics are not available). [197] [198] [199]

    The evolutionary relationships and heredity of a group of organisms is called its phylogeny. Phylogenetic studies attempt to discover phylogenies. The basic approach is to use similarities based on shared inheritance to determine relationships. [200] As an example, species of Pereskia are trees or bushes with prominent leaves. They do not obviously resemble a typical leafless cactus such as an Echinocactus. However, both Pereskia and Echinocactus have spines produced from areoles (highly specialised pad-like structures) suggesting that the two genera are indeed related. [201] [202]

    Judging relationships based on shared characters requires care, since plants may resemble one another through convergent evolution in which characters have arisen independently. Some euphorbias have leafless, rounded bodies adapted to water conservation similar to those of globular cacti, but characters such as the structure of their flowers make it clear that the two groups are not closely related. The cladistic method takes a systematic approach to characters, distinguishing between those that carry no information about shared evolutionary history – such as those evolved separately in different groups (homoplasies) or those left over from ancestors (plesiomorphies) – and derived characters, which have been passed down from innovations in a shared ancestor (apomorphies). Only derived characters, such as the spine-producing areoles of cacti, provide evidence for descent from a common ancestor. The results of cladistic analyses are expressed as cladograms: tree-like diagrams showing the pattern of evolutionary branching and descent. [203]

    From the 1990s onwards, the predominant approach to constructing phylogenies for living plants has been molecular phylogenetics, which uses molecular characters, particularly DNA sequences, rather than morphological characters like the presence or absence of spines and areoles. The difference is that the genetic code itself is used to decide evolutionary relationships, instead of being used indirectly via the characters it gives rise to. Clive Stace describes this as having "direct access to the genetic basis of evolution." [204] As a simple example, prior to the use of genetic evidence, fungi were thought either to be plants or to be more closely related to plants than animals. Genetic evidence suggests that the true evolutionary relationship of multicelled organisms is as shown in the cladogram below – fungi are more closely related to animals than to plants. [205]

    In 1998, the Angiosperm Phylogeny Group published a phylogeny for flowering plants based on an analysis of DNA sequences from most families of flowering plants. As a result of this work, many questions, such as which families represent the earliest branches of angiosperms, have now been answered. [52] Investigating how plant species are related to each other allows botanists to better understand the process of evolution in plants. [206] Despite the study of model plants and increasing use of DNA evidence, there is ongoing work and discussion among taxonomists about how best to classify plants into various taxa. [207] Technological developments such as computers and electron microscopes have greatly increased the level of detail studied and speed at which data can be analysed. [208]


    Contents

    The microspore has three different types of wall layers. The outer layer is called the perispore, the next is the exospore, and the inner layer is the endospore. The perispore is the thickest of the three layers while the exospore and endospore are relatively equal in width. [4]

    In heterosporous seedless vascular plants, modified leaves called microsporophylls bear microsporangia containing many microsporocytes that undergo meiosis, each producing four microspores. Each microspore may develop into a male gametophyte consisting of a somewhat spherical antheridium within the microspore wall. Either 128 or 256 sperm cells with flagella are produced in each antheridium. [3] The only heterosporous ferns are aquatic or semi-aquatic, including the genera Marsilea , Regnellidium , Pilularia , Salvinia , and Azolla. Heterospory is also known in the lycopod genus Selaginella and in the quillwort genus Isoëtes.

    Types of seedless vascular plants:

    In seed plants the microspores develop into pollen grains each containing a reduced, multicellular male gametophyte. [5] The megaspores, in turn, develop into reduced female gametophytes that produce egg cells that, once fertilized, develop into seeds. Pollen cones or microstrobili usually develop toward the tips of the lower branches in clusters up to 50 or more. The microsporangia of gymnosperms develop in pairs toward the bases of the scales, which are therefore called microsporophylls. Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. These develop into pollen grains, each consisting of four cells and a pair of external air sacs. The air sacs give the pollen grains added buoyancy that helps with wind dispersal. [3]

    As the anther of a flowering plant develops, four patches of tissue differentiate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis producing a quartet of microspores. Four chambers (pollen sacs) lined with nutritive tapetal cells are visible by the time the microspores are produced. After meiosis, the haploid microspores undergo several changes:

    1. The microspore divides by mitosis producing two cells. The first of the cells (the generative cell) is small and is formed inside the second larger cell (the tube cell).
    2. The members of each part of the microspores separate from each other.
    3. A double-layered wall then develops around each microspore.

    These steps occur in sequence and when complete, the microspores have become pollen grains. [3]

    Although it is not the usual route of a microspore, this process is the most effective way of yielding haploid and double haploid plants through the use of male sex hormones. [6] Under certain stressors such as heat or starvation, plants select for microspore embryogenesis. It was found that over 250 different species of angiosperms responded this way. [6] In the anther, after a microspore undergoes microsporogenesis, it can deviate towards embryogenesis and become star-like microspores. The microspore can then go one of four ways: Become an embryogenic microspore, undergo callogenesis to organogenesis (haploid/double haploid plant), become a pollen-like structure or die. [6]


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