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26.3A: Angiosperm Flowers - Biology


Flowers are modified leaves containing the reproductive organs of angiospems; their pollination is usually accomplished by animals or wind.

Learning Objectives

  • Describe the main parts of a flower and their purposes

Key Points

  • Sepals, petals, carpels, and stamens are structures found in all flowers.
  • To attract pollinators, petals usually exhibit vibrant colors; however, plants that depend on wind pollination contain flowers that are small and light.
  • Carpels protect the female gametophytes and megaspores.
  • The stigma is the structure where pollen is deposited and is connected to the ovary through the style.
  • The anther, which comprises the stamen, is the site of microspore production and their development into pollen.

Key Terms

  • sepal: a part of an angiosperm, and one of the component parts of the calyx; collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
  • corolla: an outermost-but-one whorl of a flower, composed of petals, when it is not the same in appearance as the outermost whorl (the calyx); it usually comprises the petal, which may be fused
  • stamen: in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
  • carpel: one of the individual female reproductive organs in a flower composed of an ovary, a style, and a stigma; also known as the gynoecium

Flowers

Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant. A whorl of sepals (collectively called the calyx) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Sepals and petals together form the perianth. The sexual organs (carpels and stamens) are located at the center of the flower.

Styles, stigmas, and ovules constitute the female organ: the gynoecium or carpel. Flower structure is very diverse. Carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil. The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma, where pollen is deposited, to the ovary, enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens (collectively called the androecium), surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther, where the microspores are produced by meiosis and develop into pollen grains.


26.3A: Angiosperm Flowers - Biology

Figure 1. These flowers grow in a botanical garden border in Bellevue, WA. Flowering plants dominate terrestrial landscapes. The vivid colors of flowers are an adaptation to pollination by animals such as insects, birds, and bats. (credit: Myriam Feldman)

From their humble and still obscure beginning during the early Jurassic period, the angiosperms—or flowering plants—have evolved to dominate most terrestrial ecosystems (Figure 1). With more than 250,000 species, the angiosperm phylum (Anthophyta) is second only to insects in terms of diversification.

The success of angiosperms is due to two novel reproductive structures: flowers and fruits. The function of the flower is to ensure pollination, often by arthropods, as well as to protect a developing embryo. The colors and patterns on flowers offer specific signals to many pollinating insects or birds and bats that have coevolved with them. For example, some patterns are visible only in the ultraviolet range of light, which can be seen by arthropod pollinators. For some pollinators, flowers advertise themselves as a reliable source of nectar. Flower scent also helps to select its pollinators. Sweet scents tend to attract bees and butterflies and moths, but some flies and beetles might prefer scents that signal fermentation or putrefaction. Flowers also provide protection for the ovule and developing embryo inside a receptacle. The function of the fruit is seed protection and dispersal. Different fruit structures or tissues on fruit—such as sweet flesh, wings, parachutes, or spines that grab—reflect the dispersal strategies that help spread seeds.


ANGIOSPERMS (FLOWERING PLANTS)

Angiosperms are most dominant and most evolved plants on this earth, appeared about 130 million years ago in Jurassic (dicot plant) and cretaceous (monocot plants) period of the Mesozoic era. Tertiary period of Cenozoic era is called as Age of angiosperms.

Angiosperms are found in every habitat. They constitute more than 50% of the total plants on this earth.

Angiosperms are seed bearing, flowering vascular plants in which seeds are enclosed in fruits. The term angiosperm means ‘enclosed seeds’ as seeds (ovules) are found enclosed in the ovary wall.

Instead of a cone, angiosperms bear flowers. The flower is the most characteristic structure of the angiosperms.

The flower is defined as a modified shoot meant essentially for the reproduction of the plant.

The flower usually develops as a branch from a bud, growing in the axil of a small leaflike structure known as bract. A stalk called pedicel supports the flower in the axil of the bract. The upper swollen end of the pedicel called receptacle or thalamus bears all the floral parts.

A typical angiospermic flower consists of 4 whorls, viz., calyx, corolla, androecium and gynoecium. Both calyx and corolla are known as accessory or helping whorls. The androecium and gynoecium are called reproductive or essential whorls.

Androecium (stamen) is microsporophyll and gynoecium (carpel) is megasporophyll. Megasporophyll is differentiated into ovary, style and stigma.

Angiosperms lack both archegonia and antheridia. The archegonium is replaced by gynoecium (pistil) and antheridium by pollen tube.

Flowers are adapted for various modes of pollination by birds, air, water, insects, etc.

The outermost axillary whorl of the flower is the calyx, represented by the sepals.

Corolla is the second whorl of flower present inner to calyx and is composed of petals. The bright color of the petals combined with the scent of essential oils present in some flowers makes the flower highly attractive to insects which act as agents for pollination.

The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from tiny, almost microscopic Wolffi too tall trees of Eucalyptus (over 100 metres). They provide food, fodder, fuel, medicines and several other commercially important products.

The male sex organs in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers after meiosis, produce pollen grains.

The female sex organs in a flower are the pistil or the carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed embryosacs. The embryo-sac formation is preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

Pollen grain shed at 2-3 celled stage after dispersal from the anthers is carried by wind or various other agencies to the stigma of a pistil. This is termed as pollination.

The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule.

The pollen tubes enter the embryo-sac where two male gametes are discharged.

One of the male gametes fuses with the egg cell to form a zygote (syngamy).

The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN) known as triple fusion.

Because of the involvement of two fusions, this event is termed as double fertilisation, and event unique to angiosperms.

The zygote develops into an embryo (with one or two cotyledons) and the PEN develops into endosperm which provides nourishment to the developing embryo.

The synergids and antipodals degenerate after fertilisation.

During these events, the ovules develop into seeds and the ovaries develop into a fruit.

A fruit is a ripened ovary. The ovary wall enlarges due to cell division and cell enlargement and is termed the pericarp. In fl eshy fruits, the pericarp is often differentiated into epicarp,
mesocarp and endocarp.

A true fruit is one which develops directly from an ovary. Sometimes other parts of the flower such as thalamus, sepals, etc. become a part of the fruit. Such fruits are termed false, spurious or pseudocarp, as in cashew-nut, apple, strawberry, etc.

They are divided into two classes:

the dicotyledons and the monocotyledons.

The dicotyledons are characterised by having two cotyledons in their seeds while the monocotyledons have only one.


Flowers were an evolutionary development that allowed the plant kingdom to boom into a huge diversity of species. They were developed as an alternative method of reproduction and proved to be very successful.

Fruit is found on all angiosperm plants and is produced following the pollination of flowers. They come in a huge variety of colors, shapes, and sizes.

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The stamen is the male part of a flower. It includes the anther and the filament. The filament is the elongated structure that supports the anther on its tip. The anther is where pollen is produced and displayed for animals to collect while they are feeding at flowers. A single flower can have many stamens each with an anther that will be covered in pollen.

Pollen is the substance that most plants use to reproduce. It contains the sperm of a plant and is built to withstand travelling long distances between flowers, often many hundreds of metres apart.

Pollen is generally lightweight and sticky. It easily rubs onto insects and other animals as they feed and again rubs off onto the stigma of another flower. Most flowers will produce millions of pollen grains but only a handful might need to reach another flower in order for reproduction to be successful.


Learn About Angiosperms, Flowers, Fruits, Monocots and Dicots

The main differences between monocots and dicots are: the number of cotyledons (seed leaves) in seeds, with one in monocots and two in dicots the pattern of leaf veins, which is parallel in monocots and reticulated in dicots the multiplicity of petal number, which comes in multiples of three in monocots and multiples of four or five in dicots and the position of vascular bundles in the stem, which are scattered in monocots and concentrically ringed in dicots.

Grasses, banana trees, sugar cane and orchids are examples of monocots. Sunflowers, oak and water lilies are examples of dicots.

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Flower Structure

4. What are the androecium and the gynoecium? What other structures compose flowers?

The androecium is the set of male reproductive structures of flowers. It consists of the stamens formed of the filament and anther. One flower has one androecium that may have one or several stamens. The gynoecium is the set of female reproductive structures of flowers. It is generally composed of a single pistil that includes the stigma, the style and the ovary. The androecium usually surrounds the central gynoecium.

In addition to the androecium and the gynoecium, typical flowers are also made of a peduncle, sepals and petals.

Pollination

5. What is pollination? What are the main forms of pollination?

The process by which pollen grains (the male gametophytes of phanerogamic plants) reach female gametophytes is called pollination.

The main forms of pollination are: anemophily, in which pollen is carried by wind hydrophily, in which pollination is helped by water entomophily, in which pollen is carried by insects ornitophily, which is pollination by birds and chiropterophily, which is the dissemination of pollen by bats.

Characteristics of the flowers of each plant species relate to the type of pollination used by the plant. Colored flowers are specialized in bird and insect attraction. Nocturnal flowers are generally white and perfumed, and many specialized in pollination by bats. Nectar is also a special adaptation to attract pollinator animals. Flowers that produce an exaggerated amount of pollen often use the wind as a pollinator. The position of more external or internal anthers next to the nectar is a way to facilitate the dissemination of pollen, via the wind or animals.

The Life Cycle of Angiosperms

6. How are male gametophytes and male gametes formed in angiosperms?

The anthers of each stamen contain pollen sacs. Within the pollen sacs are microspore mother cells, or microsporocytes. These cells undergo meiosis to form microspores. Each microspore undergoes mitosis to form a pollen grain containing one generative cell and one tube cell. The pollen grain is the male gametophyte.

When pollination occurs and the pollen grain makes contact with the stigma (the apex of the pistil), the tube cell elongates its cytoplasm, forming the pollen tube that grows towards the ovary. The generative cell divides to form two sperm nuclei (male gametes), which then migrate through the pollen tube.

7. How many cellular nuclei are contained by the pollen tube of angiosperms? What is the ploidy of each of these nuclei?

The pollen tube, which is the mature male gametophyte of angiosperms, has three cellular nuclei: two sperm nuclei and one tube cell nucleus.

All those nuclei are part of the male gametophyte of the plant and, therefore, each of them is haploid (n).

8. How is the female gametophyte formed in angiosperms?

The flower ovary contains megasporangia enclosed by a tegument with a small opening, the micropyle. Within the megasporangium is a megasporocyte, or megaspore mother cell, which undergoes meiosis to form four megaspores, three of which regress and only one of which is functional. The functional megaspore undergoes mitosis (three times) generating eight cells which, as a whole, form the embryonic sac.

9. What is the embryonic sac? Which cells form the embryonic sac? What are their ploidies?

The embryonic sac is the female gametophyte of angiosperms.

The embryonic sac is composed of: three cells that remain next to the micropyle, which are two lateral synergids and the central oosphere (egg) one binucleated cell, the polar nuclei, placed in the central region and three antipodal cells, which stay in the opposite side to the micropyle.

Since all these cells are produced by the mitosis of the functional megaspore, they are haploid (n).

10. After pollination, how does fertilization occur in angiosperms? In these plants, is fetilization dependent on water?

After pollination, one of the sperm nuclei from the pollen tube unites with the oosphere of the embryonic sac to form the diploid (2n) zygote. The other sperm nucleus fuses with the polar nuclei of the embryonic sac, producing a triploid (3n) cell that, by mitosis, will turn into the secondary endosperm of the seed. The synergids and the antipodal cells degenerate after the fertilization process.

Fertilization in these plants does not depend on water.

11. What is the difference between self-pollination and cross-pollination? Which of these two pollination methods contributes more to plant diversity?

Self-pollination occurs when pollen grains from a flowering plant fall into the pistils of the same plant and therefore gametes from the same individual unite to form a zygote. Cross-pollination occurs when pollinators carry pollen grains from a plant to other individual plants of the same species and, as a result, gametes of different individuals form the zygote.

Since it promotes the formation of zygotes containing genes from different individuals (new gene combinations), cross-pollination contributes more to biological diversity.

12. What is dichogamy?

Dichogamy is the phenomenon of the maturation of female reproductive structures of the plant during a different period than that of the maturation of the male reproductive structures. Dichogamy prevents self-pollination and makes cross-pollination almost obligatory. thus assisting in an evolutionary strategy to promote genetic recombination.

The Angiosperm Seed

13. What are the typical structures of a seed? What is the endosperm?

A typical seed is composed of the embryo, endosperm and shell. Within seeds of angiosperms, there are one or two cotyledons (seed leaves, one in monocots, two in dicots).

The endosperm is the tissue within the seed that has the function of nourishing the embryo.

14. How different is the endosperm of gymnosperms and the endosperm of angiosperms?

In gymnosperms, the endosperm is haploid (n) it is called a primary endosperm. In angiosperms, the endosperm is triploid (3n) it is called a secondary endosperm.

15. What are cotyledons?

Cotyledons, or seed leaves, are structures formed by the embryo of angiosperms to absorb nutrients from the endosperm and to store and transfer these nutrients to the embryo. (Cotyledons are auxiliary embryonic structures).

The seeds of monocots have a single cotyledon. The seeds of dicots have two cotyledons.

Fruit Explained

16. What is the main function of fruit?

The main function of fruit is the protection and spreading of seeds.

17. Which floral structure produces fruit?

Fruits are modified flower ovaries.

18. How are fruits formed?

The fertilization in angiosperms triggers the release of hormones that act on the ovaries. The ovary wall then develops into a fruit that contains seeds.

19. Are fruits always the fleshy part of “fruit”? Is the edible part of an onion a fruit?

In some so-called fruits, the actual fruit is not the fleshy part. For example, the fleshy part of the strawberry is not the fruit. The fruits are the small hard dots on the surface of the strawberry. Another example: the fleshy part of the cashew is not the fruit. The fruit is the nut.

The edible part of the onion is the stem of the plant and not the fruit.

20. Why do some plants have fruits with one seed while other plants have fruits with more than one seed?

Plants that produce single-seeded fruits, for example, mangos and avocados, often have ovaries with only one egg inside. Fruits with more than one seed are produced from plants whose ovaries contain more than one egg.

21. What are infructescences, pseudofruits and parthenocarpic fruits?

Infructescences are aggregated fruits formed from inflorescences, which are aggregated flowers. Grape clusters are examples of infructescences. Pseudofruits are “fruits” not made in the ovaries and, in general, their true fruits lack development and are found within the flesh, such as in apples and pears. Parthenocarpic fruits are those made without fertilization, by means of hormonal stimuli, such as bananas.

22. What is the evolutionary importance of fruits for angiosperms?

Fruits contain seeds and can detach from the plant by falling on the ground. They can also serve as food for animals. Therefore, with the emergence of fruits, the seeds of angiosperms could be transported across long distances, thus contributing to the propagation of the species.

Evolutionary Trends in Plants

23. What are the trends of the gametophyte in the evolution of plants?

During the evolution of plantas, the tendency has been for gametophytes form gametes that are independent from water. In bryophytes and pteridophytes, fertilization is completely dependent on water. In phanerogamic plants, such a dependency does not exist.

Another tendency is the reduction in the size and duration of the gametophyte. In bryophytes, the gametophyte is the lasting stage. In pteridophytes, gymnosperms and angiosperms, it became the temporary stage and its relative size was successively reduced.

A third evolutionary trend relates to the interdependency between gametophytes and sporophytes. In bryophytes, the sporophyte is entirely dependent on the gametophyte to survive. In the remainder of plants, the sporophyte is the independent stage and the once autotrophic gametophyte in bryophytes and pteridophytes became dependent on the sporophyte in phanerogamic plants.


Ovaries

Most of the structure of the ovary is part of the sporophyte it's diploid. In cross section you can see that the ovary is three-sided, and it contains numerous ovules. Within each diploid ovule, meiosis occurs, producing a haploid female spore (called a megaspore because it's large the smaller male spore in the anther is called a microspore). The ovule is part of the sporophyte, but it will eventually contain the haploid gametophyte. This is one of the main things you should know about angiosperm life cycles: the gametophytes are microscopic and are formed completely inside the sporophyte. The megaspore eventually goes through three rounds of cell divison to form an 8-nucleus female gametophyte, also called the embryo sac. The gametophyte completes mitotic nuclear divisions but does not divide into 8 separate cells later, it divides the large cell into seven cells containing the 8 nuclei. One of these cells becomes the egg, and the others play supporting roles in development.

As you look at this slide, you should recognize that not every cross sectional slice will show a female gametophyte inside each ovule it's a matter of luck to slice it so that the female gametophyte is visible.


Biology Sexual Reproduction in Flowering Plants Chapter 2 – Exercise

Biology Sexual Reproduction in Flowering Plants Chapter 2 – Exercise : NCERT solutions of Biology chapter 2 exercises. We mentioned all important questions below check them, Or you can just directly download question paper. But you will get solutions in this page only. So you can book mark this page and you will find it easily.

Ans. Microsporogenesis occurs inside the pollen sacs of the anthers. During this process meiosis occurs in microspore mother cells resulting in the formation of microspores.

Megasporogenesis occurs inside the ovule where one of the sporogenous cell of nucleus acts as megaspore mother cell. During this process, meiosis occurs resulting in the formation of megaspore.

Q.3 Arrange the following terms in a correct developmental sequence:

Pollen grains, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

Ans. The various parts of an ovule are –

  1. Funiculus – It is a stalk-like structure which represents the point of attachment of the ovule to the placenta of the ovary.
  2. Hilum – It is the point where the body of the ovule is attached to the funiculus.
  3. Integuments –They are the outer layers surrounding the ovule that provide protection to the developing embryo.
  4. Micropyle – It is a narrow pore formed by the projection of integuments. It marks the point where the pollen tube enters the ovule at the time of fertilization.
  5. Nucellus – It is a mass of the parenchymatous tissue surrounded by the integuments from the outside. The nucellus provides nutrition to the developing embryo. The embryo sac is located inside the nucellus.
  6. Chalazal – It is the based swollen part of the nucellus from where the integuments originate.


The female gametophyte of plants is formed from a single functional megaspore with the help of four unequal mitotic divisions. These mitotic divisions give rise to 8 nuclei. Later, these nuclei assemble into 7 cells. To both chalazal and micropylar end three cells each move. The three cells at chalaza are called antipodal cells while those at micropylar end constitute egg apparatus. The egg apparatus includes two larger cells called synergid while the egg is the smallest cell that acts as a female gamete. Two nuclei migrate to the centre and together they form a single cell called the central cell. This central cell consists of two nuclei called polar nuclei. Thus, the mature embryo sac possesses 7 cells ( 3 antipodal cells, 2 synergid cells, 1 egg cell and 1 central cell) and 8 nuclei ( 3 of antipodal cell, 2 of synergid cells, 1 of egg cell and 2 of central cell).

Ans. Cross-pollinating flowers develop the following strategies to prevent self-pollination:

  1. Protogyny (when gynoecium matures earlier than androecium) or Protandry (when androecium matures and shed pollen before maturation of gynoecium).
  2. Self incompatibility

Ans. During microsporogenesis, the cells of tapetum provide various enzymes, hormones, amino acids and other nutritive materials to the dividing microsporocytes. The main functions of tapetum are –

(a) Transportation of nutrients into anther locule at the time of meiosis in spore mother cells

(b) Secretion of enzymes and hormones

(c) Production of Ubisch bodies which are coated with sporopollenin to cause thickening of exine

(d) Secretion of an oily material (pollenkitt) over outerside of mature pollen

(e) Secretion of special proteins for pollen to recognise compatibility

Ans. The process of development of seeds from diploid cells of nucellus tissue without the process of fertilisation is called apomixis. This process is a form of asexual reproduction that mimics sexual reproduction. Apomixis is very commonly seen in grasses.


Section Summary

Angiosperms are the dominant form of plant life in most terrestrial ecosystems, comprising about 90 percent of all plant species. Most crop and ornamental plants are angiosperms. Their success results, in part, from two innovative structures: the flower and the fruit. Flowers are derived evolutionarily from modified leaves. The main parts of a flower are the sepals and petals, which protect the reproductive parts: the stamens and the carpels. The stamens produce the male gametes, which are pollen grains. The carpels contain the female gametes, which are the eggs inside ovaries. The walls of the ovary thicken after fertilization, ripening into fruit that can facilitate seed dispersal.

Angiosperms’ life cycles are dominated by the sporophyte stage. Double fertilization is an event unique to angiosperms. The flowering plants are divided into two main groups—the monocots and eudicots—according to the number of cotyledons in the seedlings. Basal angiosperms belong to a lineage older than monocots and eudicots.


Angiosperms

From their humble and still obscure beginning during the early Jurassic period, the angiosperms—or flowering plants—have evolved to dominate most terrestrial ecosystems (Figure). With more than 250,000 species, the angiosperm phylum (Anthophyta) is second only to insects in terms of diversification.

These flowers grow in a botanical garden border in Bellevue, WA. Flowering plants dominate terrestrial landscapes. The vivid colors of flowers are an adaptation to pollination by animals such as insects and birds. (credit: Myriam Feldman)

The success of angiosperms is due to two novel reproductive structures: flowers and fruit. The function of the flower is to ensure pollination. Flowers also provide protection for the ovule and developing embryo inside a receptacle. The function of the fruit is seed dispersal. They also protect the developing seed. Different fruit structures or tissues on fruit—such as sweet flesh, wings, parachutes, or spines that grab—reflect the dispersal strategies that help spread seeds.