Angiosperms: General characteristics, Reproduction, Practice Problems and FAQs

You all like flowers. How beautiful they are. In all our rituals and functions we use them in one or another way. For example, it increases the beauty of the bride. Do you know, What is the name we use for these beautiful flower and seed bearing plants? Angiosperms are the seed-bearing plants. The word ‘Angiosperms’ has been derived from two words, ‘angion’ meaning vessel and ‘sperma’ meaning seeds.

Angiosperms are the plants in which the pollen grains and ovules are developed in specialised structures called flowers. The ovules remain enclosed by the ovary before fertilisation, same is the case with seeds after fertilisation in them. Now let’s understand more about these beautiful plants.

List of contents

Characteristics of angiosperms
Reproductive events of angiosperms
Practice Problems
FAQs

Characteristics of angiosperms

The major characteristics of the flowering plants will be discussed.

General features

Angiosperms are the most recent highly evolved group of plants. Prominent features of angiosperms are as follows:

Habitat

Angiosperms are a large group of plants occurring in a wide range of habitats. They range in size from tiny, almost microscopic Wolfia to tall trees of Eucalyptus (over 100 metres).

Habit

The plant body is divided into roots, stems and leaves in angiosperms. At maturity the plant bears flowers, fruits and seeds.

Vascular

They possess vascular tissues like xylem and phloem. It is used for the conduction of water and food respectively. They are arranged in the form of vascular bundles. Xylem possesses the vessels and phloem possess sieve tubes and companion cells.

Seed-bearing

It bears seed which contains the embryo.

Fruit-bearing

They bear fruits which are enclosed and protected by the fruit.

Flower-bearing

It bears flowers, which are the ornaments of the plant. It acts as the sexual organ too. Flowers attract insects which help in pollination.

Parts of a flower

The production of flowers is a unique feature of angiosperms. The sporophylls are aggregated to form the flowers. A typical flower has a short axis called thalamus and four whorls of floral leaves. Flowers have four major parts which are arranged in the form of four whorls as follows:

Calyx

It is the whorl of sepals. It is normally green in colour and protects the flower bud.

Corolla

It is the whorl of petals. It is coloured other than green and attracts pollinators like bees, butterflies etc.

Androecium

It is the whorl of stamens or microsporophylls. It is the male reproductive organ. Stamen is a modified leaf. It possesses anther and a filament. Anther is the bilobed portion present on the filament. It possesses pollen grains.

Gynoecium

It is the whorl of carpels or pistils or megasporophylls. It is the female reproductive organ. It possesses stigma, style and ovaries. Ovaries possess ovules. Ovule possesses nucellus, two integuments and a micropyle.

Details of floral parts

The details of floral parts are as follows:

Reproductive events of angiosperms

Reproductive events in angiosperms is divided into three types as follows:

Pre-fertilisation events
Fertilisation events
Post-fertilisation events

Pre-fertilisation events

Events occurring before the process of fertilisation are described as pre-fertilisation events. These include the production of gametes and their transfer.

Formation of male gametophyte

The microsporophylls produce the male gametophytes called pollen grains. The stamens are composed of anther and filament. Transverse section of anther shows the presence of two lobes with four pollen sacs. They carry microspore mother cells which are diploid. Each microspore mother cell undergoes meiosis to form four haploid microspores or pollen grains. The generative cells of the pollen grains undergo mitotic divisions which result in the formation of two male gametes. Thus pollen grain is the male gametophyte.

Formation of female gametophyte

The megasporophylls bear the female gametophyte. The carpels composed of style, stigma and ovary constitute the megasporophylls. Inside the ovary, ovules are present. The ovule is the megasporangium of the angiosperms. Megasporangium consists of the megaspore mother cell (diploid) that undergoes meiosis to form four haploid megaspores. Three of them usually degenerate, and one divides mitotically thrice to form the embryo sac. This is called monosporic development.

Embryo sac

Each embryo sac has a seven-celled, eight-nucleated condition. It possesses the three-celled egg apparatus composed of one egg cell and two synergids, three antipodal cells, and one central cell having two polar nuclei. Two polar nuclei fuse together to form a diploid secondary nucleus in the central cell.

Pollination

Pollination is the process of transfer of pollen grains from anther (male reproductive part of a plant) to the stigma of pistil (female reproductive part of a plant). Pollination is essential for the event of reproduction in dioecious plants. The staminate (male) or pistillate (female) flowers do not have both the male and female gametes in the same flower. Hence, pollen bearing male gametes have to be transferred to the stigma of the flower bearing female reproductive parts. Thus, pollination is a very crucial event which ensures survival of generations in plants.

Pollen grain

It is the powdery substance produced by the anthers of a flower. These either get carried away by wind, water, insects etc., to the stigma of flowers.

Fertilisation

Fertilisation is considered as the process of fusion of the male and female gametes. For this process to occur first pollen tubes need to be formed.

Pollen tube formation

Once the pollen grains land on the stigma, they start forming pollen tubes.

Each pollen grain, after one round of mitosis, contains two cells, one being vegetative cell and the other being a generative cell. Vegetative cell produces a pollen tube. Generative cell divides normally by mitosis to form two male gametes.

Fusion of gametes

Angiosperms have a unique feature of two fertilisation events taking place. Of the two male gametes entering the pollen tube, one male gamete fuses with the egg cell to form a zygote. This gradually develops into an embryo. The other male gamete fuses with the central cell having two polar nuclei to produce a triploid primary endosperm cell (PEC) containing the triploid primary endosperm nucleus (PEN). This subsequently results in endosperm formation. Since the fertilisation process involves the fusion of three nuclei, it is referred to as triple fusion. Thus, the entire fertilisation event is referred to as double fertilisation.

Post-fertilisation events

After double fertilisation, the zygote develops into an embryo and the triploid primary endosperm cell (PEC) develops into the endosperm. The ovary develops into a fruit and the ovule becomes the seed. Ovary wall develops into the fruit wall.

Practice problems

Q1. Which of the following structures degenerate in the embryo sac of angiosperms normally after double fertilisation?

a. Synergids only
b. Synergids and antipodals
c. Antipodals only
d. Polar nuclei

Solution: Synergids and antipodals are the cells which are located in the embryo sac present at time of fertilisation. Once the fertilisation occurs and the zygote is formed, the synergids and antipodal cells degenerate and the ovary converts into a fruit. Polar nuclei combined with male gametes to form the PEN. Hence option b is correct.

Q2. The innermost layer wall of anther aids in ______________

a. Providing protection
b. Formation of pollen grains
c. Dispersal of pollens
d. Providing nourishment to pollen grain which is developing

Solution: The innermost layer of anther wall (tapetum), comprises only one layer of nutritive cells. It gives nourishment to the developing pollen grains.

Q3. Which of the following given combinations are diploid?

a. Antipodals, PEN, Integuments
b. PEN, Funicle, Nucellus
c. Nucellus, Antipodals, Integuments
d. Integuments, Funicle, Nucellus

Solution: Nucellus is present in the megasporangium where the embryo sac develops. It is diploid. Funicle is the stalk that attaches an ovule to the ovary wall. Nucellus is the central part of an ovule, which has the embryo sac. Funicles and nucellus are part of the ovule and are diploid.

Q4. Fill in the blank.

If stamen is considered as microsporophyll, then ______ is considered as the megasporophyll.

Answer: Stamens are equivalent to microsporophylls. It is the male reproductive part of the angiosperms. Carpels or pistils are equivalent to megasporophylls. It is the female reproductive part of the angiosperms.

Q5. During double fertilisation, one of the two male gamete fuses with egg and the other remaining male gamete fuses with ________________.

Answer: Double fertilisation includes syngamy and triple fusion. The male gamete fuses with the female gamete forming a zygote in syngamy that develops into an embryo. In triple fusion, the male gamete fuses with the diploid secondary nucleus to form a triploid primary endosperm nucleus (PEN) which divides and forms the endosperm (the nutritive tissue).

FAQs

Question 1. Where does the development of pollen grains occur?
Answer: Pollen grains are the male gametophytes of angiosperms that develop inside the microsporangium (pollen sac) of the anther. Filament is the stalk of the stamen or microsporophyll. Female gametophytes or embryo sac develop inside the megasporangium or ovule. It is enclosed inside the carpel or pistil.

Question 2. What are the male gametophyte and female gametophyte of the flowering plants?
Answer: The gametophytic generation is highly reduced in angiosperms. Pollen grain normally represents the male gametophyte generation. Embryo sac normally represents the female gametophyte generation in angiosperms. Pollen grains possess two cells, namely vegetative and generative cells. Generative cell divides and forms two male gametes. Embryo sac is 7 celled and eight nucleated.

Question 3. Why have the angiosperms dominated the land flora?
Answer: Angiosperms are widespread in all plant groups. Their dominance is due to the power of adaptability in diverse habitats. They are mostly adapted to terrestrial life. They occur in diverse habitats like Tundra, hot tropical regions and deserts.

Question 4. Which structures make angiosperms most successful?
Answer: Flowers and fruits make angiosperms most successful. The flower is the reproductive part of the angiosperm which ensures pollination. They normally provide protection for the ovule. It also protects the developing embryos inside the receptacle of the flower. The main function of the fruit is normally seed dispersal.

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