Reproduction in Angiosperm

Reproduction in Angiosperm (Flowering Plants)

Angiosperm reproduce both by vegetative as well as by sexual methods. In this section we will study the sexual reproduction in angiosperms. As you know sexual reproduction occurs by fusion of male and female gametes present in the flower. Thus, flower represents the reproductive unit of a plant.

How frequently do plants flower?

There is variation shown by the plants in this respect. Angiosperms cam be classified as annuals, biennials and perennials depending upon

The time they take to complete the life cycle and flowering.

(a) Annuals: Plants live for one year. The plants which produce flowers and seeds within one season are called annuals eg. pea

(b) Biennials: Plants which complete their life cycle in two seasons are called biennials. In the first year, the plants remain in the vegetative state. In the second year they produce flowers, fruits and seeds and then die e.g. radish.

(c) Perennials: Plants which live for several years are termed perennials. Their vegetative stage may last from one to a few years after which they produce flowers, fruits and seeds every year. e.g. Mango, Peepal, Neem etc.

(d) Monocarpic: However, some perennial plants reproduce only once in their lifetime and then die. Such plants are called Monocarpic. e.g. Bamboo.

Initiation of Flowering

As the seed germinates a new plantlet emerges from it. The young plant grows vigorously and continue to grow till it attains a definite shape and size with its vegetative parts (roots, stem, leaves) well developed. This phase of the life cycle represents the young or the juvenile phase.

Then, at a certain point of time on completion of vegetative growth the plant switches over to its reproductive phase or adult phase and vegetative shoot apex transforms into a reproductive or floral apex and starts bearing flowers. As listed above this transition from vegetative, the flowering stage may take several years in trees but only a few weeks or days in annuals.

Factors Affecting Flowering

Flowering in a plant is affected by temperature (Vernalisation) and light (photoperiodism).

Vernalisation: Low temperature treatment which stimulates early flower formation is called Vernalisation.

Photoperiodism: It is the response in growth and flowering of a plant to the duration of light and dark period per day.

Sex in Flowers: Flowers maybe bisexual (having both stamens and carpels) or unisexual (staminate or pistillate).

In some dioecious species there may be a (i) Chromosomal basis of sex determination, for example xx and xy chromosomes. (ii) The male and female plants may also exhibit differences in the levels of their growth substances.

For example – Plants of Cucumis which bear male flowers have a high gibberellin content as compared to those which bear only female flowers. The application of gibberellin from outside can induce the formation of male flowers even in genetically female plants and treating male plants with auxin or ethylene may develop functional female flowers. The above response has also been seen in Cannabis. V

Parts of Flower

As you have already studied a typical flower consist of four whorls born on a thalamus or stalk. These whorls are from outside

(a) Calyx – consisting of sepals. V V

(b) Corolla – consisting of petals

(d) Gynoecium or pistil – consisting of carpels.

Try to recollect their role in reproduction. The two outermost whorls are known as non-essential or accessory whorls as they aid in reproduction but do not directly take part in the process. The other two whorls i.e. Androecium (male reproductive organ) and Gynoecium (female reproductive organ) are known as the essential whorls as their absence from flowers will lead to failure of reproduction.

Pollination: Transfer of pollen grains from the anther to the stigma of flower.

Pollination is of two types. (a) Self-pollination (b) Cross pollination

Transfer of Pollen grains to stigma of same or another to stigma of a flower of another flower of same plant. As in the plant of the same species and as in China rose palm.

Importance of Pollination:

It results in fertilization and ovule is converted into seed.
New varieties of plants are formed through new combination of genes in case of cross pollination.
During pollination pollen tube produce growth hormones which convert ovary into fruit. Cross pollination is brought about by various external agencies such as, wind, insects, water, birds and other animals. Now let us study the various agencies of cross pollination which carry pollen grains from one flower to stigma of another flower.

Characteristics in flowers which favour Cross Pollination

1. Pollination by wind (Anemophily): (Anemos : wind, Phile: to love)

(i) Flowers are small, without colour, nectar and scent.

(ii) Flowers produce a large number of pollen grains to allow for wastage when carried with wind to another flower.

(iii) The pollen grains are small, light and sometimes provided with ‘Wings’.

(iv) The stigmas are comparatively large, protruding and sometimes hairy, to trap pollen grains for example, grasses and some cacti.

2. Pollination by insects (Entomophily): (entomo : insect, phile : to love)

(i) Flowers are usually large, coloured and showy to attract insects.

(ii) Some of these flowers secrete nectar to attract insects. Saia flowers show special adaptations for pollination by bees

3. Pollination by Water (Hydrophily) (Hydros: water)

This takes place in aquatic plans.

(i) Pollen grains are produced in large numbers.

(ii) Pollen grains float on surface of water till they land on the stigma of female flowers e.g. Hydrilla, Vallisneria.

4. Pollination by Animals (Zoophily) (Zoo: Animal)

Flowers of such plants attract animals by their bright colour, size, scent etc. for example sun bird, pollinates flowers of Canna, gladioli etc. Squirrels pollinate flowers of silk cotton tree. Humans carry out artificial pollination in a number of plants for producing hybrids. Some Adaptation to promote Cross Pollination.

Unisexuality: Flowers may be only male or female, on different plants e.g. papaya, palm.
Dichogamy: Male and female sex organs mature at different times. In sweet pea, and salvia. Anther matures earlier than the stigma and in custard apple (sharifa) carpel matures earlier than the anther.
Self Sterility: Pollen grains are incapable of affecting fertilization even after being placed on the stigma of the same flower e.g. petunia, apple.

Fertilization

Pollen grains reach the right stigma and begin to germinate.
Pollen grain from a small tube structure called pollen tube which emerges through the germ pore. The contents of the pollen grain move into the tube.
Pollen tube grows through the tissues of the stigma and style and finally the ovule through the micropyle.
Vegetative cell degenerates and the generative cell divides to form two sperms (or male gametes).
Tip of pollen tube bursts and the two sperms enter the embryo sac.
One sperm fuses with the egg (syngamy) and forms a diploid zygote.

The other sperm fuses with the secondary nucleus to form the primary endosperm nucleus which is triploid in nature. Since two types of fusion syngamy and triple fusion take palace in an embryo sac the process is termed as double fertilization.

After triple fusion, primary endosperm nucleus develops into an endosperm.
Endosperm provide food to the developing embryo.
Later the synergies and antipodal cells degenerate.

Significance of Fertilisation

(i) Gives stimulus for the growth of ovary, leading to fruit formation.

(ii) Helps in recombination of characters as genes from two different individuals combine and form the zygote.

Post Fertilisation change

Events that follow double fertilisation are development of endosperm and embryo and maturation of the ovule into seed and ovary into fruit.

(a) Endosperm: The endosperm development begins before embryo development. Can you tell why this is to establish the nutritive tissue for the growth of the zygote into an embryo. The primary endosperm cell divides repeatedly and forms an endosperm tissue. There are three way in which the endosperm may develop.

Nuclear Type: The nucleus undergoes repeated division to give rise to free nuclei which arrange themselves at the periphery leaving a large central space. Cell wall formation starts subsequently and endosperm becomes cellular at maturity. This is the most common type of endosperm development and is seen in maize, wheat, rice etc.

(b) In Cellular, each nuclear division is followed by cytoidnesis, making it cellular from the beginning.

Seed

The seed is defined as a ripened ovule.

Importance of Seed

It contains embryo which develops into a new plant.
The seen coat protects the embryo against dehydration and mechanical damage.
Seeds can be stored and transported from one place to another thus help in dispersal. A fruit is defined as a ripened ovary. Different parts are edible in different fruits.

Significance of Fruit:

1. It protects seeds. 2. On decay, fruits which contain chemical substances enrich the soil. 3. It helps in dispersal of seeds.

The unripe fruit has a different taste but no smell. But the same fruit when it ripens has a good taste and smell e.g. mango, banana. The following changes take place during the ripening of fruit:

(i) Starch is converted into sugar.

(ii) The production of various organic substances (esters) gives a different texture, taste and flavour.

(iii) The breakdown of chlorophyll leads to changes in colour of the skin of the fruit. Parthenocarpy. When fertilisation fails, seeds are not formed. But in certain plants the ovary develops into a fruit e.g. grapes, banana etc.

The Phenomenon of development of fruit from unfertilised ovary is called parthenocarpy and such fruits are called parthenocarpic fruits.

Commercial value of parthenocarpic fruits:

The fruits are seedless and are hence valued more, for eating purposes and contain seeds which cannot develop into a new plant.
The fruits contain sufficient growth hormones.

Seed

Seed is the final product of sexual reproduction and seed becomes relative. The metabolic activity of the embryo slows down and in majority of cases the embryo into phase of inactivity called dormancy or in some case if favourable cpnclitppspre available they germinate. Dormancy help the plants to survive under in favourable conditions and ensures its germination only under favourable conditions.

Germination: Embryo lies dormant in the seeds, but when the seed reaches the favourable signals and the inputs from the environment (moisture, temperature and oxygen) they germinate. Germination is the process the embryo grows and establishes itself as a seedling. Step of germination

Imbibition of water (through the micropyle) and by the seed coat
seed swells up as its gets hydrated
Enzyme activity converts the reserved seed food into soluble forms (glucose, amino acid, fatty acids)
The seed coat bursts and radicle emerges (grows into root) and then the plumule grows and develops into shoots.