Sexual Reproduction In Flowering Plants

The development of male gametophyte (pollen grain) takes place in pollen sac of anther.
The development of female gametophyte (embryo sac) takes place in ovule.

(a) Difference between microsporogenesis and megasporogenesis –

(b) During microsporogenesis and megasporogenesis, meiosis (reduction division) takes place.
During meiosis, a cell divides twice to form 4 cells containing half of the genetic information of the cell. This method is used to form haploid gametes.

(c)Structure formed at the end of microsporogenesis is haploid Microspores and the structure formed at the end of megasporogenesis is haploid Megaspores.

The correct sequence is –

Sporogenous tissues � Pollen mother cell � Microspore tetrad � Pollen grain � Male gametes

During microsporogenesis, the cells of the sporogenous tissue (which act as pollen mother cells), undergo meiotic divisions to form microspore tetrad containing 4 haploid microspores. As the anther matures, the microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.

1. Funicle – The ovule is a small structure attached to the placenta by means of a stalk called funicle.

2. Hilum – It is the junction between ovule and funicle. The body of the ovule fuses with funicle in the region called hilum.

3. Integument - These are the protective envelops of ovule. There are two integuments, one is outer and the other is inner.

4. Nucellus – Enclosed within integuments is a mass of cells called the nucellus. The cells of nucellus have abundant reserve food materials.

5. Embryo sac – An ovule has one embryo sac formed from a megaspore. It is located in the nucellus. It is the site of fertilization of egg and development of embryo.

6. Micropyle – Integuments encircle the nucellus except at the tip where a small opening is organized. This small opening is known as Micropyle.

7. Chalaza – Opposite the micropylar end, the swollen basal part of the ovule is represented by the chalaza.

In flowering plants, the megaspore mother cell undergoes reduction division to form four megaspores but only one functional megaspore out of four enlarges and develops into female gametophyte and the rest three generate. The development of female gametophyte or the embryo sac from a single megaspore is called monosporic development of female gametophyte.

i) The megaspore mother cell in flowering plants undergoes reduction division to form 4 haploid megaspores.

ii) Out of these 4 megaspores, 3 degenerate and the remaining one functional megaspore undergoes three successive mitotic divisions and form eight nucleate embryo sac.

iii) First mitotic division –

It occurs in the megaspores and results in the formation of 2 nuclei.

One nucleus moves towards the micopylar end.

Second nucleus moves towards the chalazal end.

This forms two-nucleate embryo-sac.

iv) Second Mitotic division –

The nucleus at the micopylar end divides mitotically.

The nucleus at the chalazal end divides mitotically.

This results in 2 nuclei at each end and four-nucleate embryo-sac forms.

v) Third mitotic division –

The 2 nuclei at each end re-divide mitotically

This results in 4 nuclei at each end and thus forms eight-nucleate embryo-sac.

vi) Out 4 nuclei at the micopylar end, only 3 differentiate. Two of them form synergids and one forms egg cell. Together they are known as egg apparatus.

vii) Out of 4 nuclei at the chalazal end, only 3 differentiate as antipodals.

viii) The remaining one nucleus from the micopylar end and one from the chalazal end move towards the centre and are known as polar nuclei, which are situated in a large central cell.

ix) This results in the formation of the 7-celled, 8-nucleate embryo-sac.

There are two types of flowers- chasmogamous and cleistogamous.

The flowers with their anther and stigma exposed for pollination are called chasmogamous flowers. E.g. peas, beans, etc.

The flowers which are closed and never open are called clesitogamous flowers. So cross-pollination can’t occur in cleistogamous flowers. The anther and stigma in these flowers lie close to each other so only self-pollination occurs. E.g. Oxalis, Viola, etc.

The two strategies evolved to prevent self-pollination in flowers are –

• Dichogamy – Maturation of anther and stigma of a flower at different times to prevent self-pollination.

When anther matures before stigma, it is called protandry.

When stigma matures before anther, it is called protogyny.

• Self-incompatibility – In some flowers, self-sterile genes are present due to which the pollen grains do not germinate on the stigma of that flower. E.g. potato, tobacco.

It is a genetic mechanism which prevents the pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen tube growth in the pistil

This prevents the fusion of the male and female gametes along with the formation of embryo. As a result, seed formation doesn’t take place.

In the self-incompatible species, when self-pollination takes place then the pollen tube doesn’t grow and fertilization doesn’t take place. So seed formation doesn’t take place.

i)In the flowers with both anther and stigma, the anther is removed before maturation carefully using forceps.

ii)The flower is then covered with a bag made up of butter paper.

iii)The covering of emasculated flowers with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollens is called bagging technique.

iv) When the stigma matures, the pollens from desired flower are dusted on the stigma with the help of pre-sterilized brush and the flower is re-bagged until the fruit is developed.

v) This technique is called artificial hybridization. Plant breeders often use this method to prevent the contamination of stigma of flowers from unwanted pollens. This technique is also helpful is developing the plant of desired variety.

Triple fusion – The process of fusion of the male gamete with the two polar nuclei inside the central cell located in the embryo-sac to form primary endospermic nucleus (PEN) is called triple fusion. The process takes place inside the embryo-sac.

This process takes place in the following steps –

• When the pollen grains fall on the stigma of a flower, they germinate and give rise to the pollen tube.

• The pollen tube passes through style and enters the embryo-sac from the micopylar end.

• One of the male gamete fuses with egg cell to form zygote (syngamy)

• The second male gamete fuses with the two polar nuclei present in the central cell and form triploid primary endosperm nucleus.

• This process involves the fusion of three haploid nuclei hence is known as triple fusion.

One male gamete nucleus and two polar nuclei are involved in this process.

The fusion of male gamete and nucleus of the egg cell leads to formation of zygote.

The zygote remains dormant for some time until certain amount of endosperm forms.

It happens because endosperm provides nutrition to the developing embryo.

After the formation of the endosperm, the further development of embryo from zygote happens.

A)

B)

C)

D)

In most of the species, the fruits develop from the ovary and the rest of the floral parts degenerate. Such fruits are called true fruits. But in some species like apple, the thalamus also contribute to the fruit formation. Such fruits are called false fruits.

The process of removal of the anthers before they mature from a bisexual flower carefully using the forceps without affecting the female reproductive part (pistil) is called emasculation.

It is one of the measures of artificial insemination.

To obtain a plant of desired variety, the plant breeders use this technique. They protect the stigma from being pollinated by undesired pollens from the same flower.

There are certain species like banana in which fruits develop without fertilization and such fruits are called parthenocarpic fruits. Such fruits are seedless.

Parthenocarpy can be induced through the application of growth hormones. So, the seedless fruits which are economically important like banana, grapes, pineapples, etc can be developed using this technique. The fruits with edible seeds like pomegranate can’t be developed using this technique.

The innermost wall layer of microsporangium is called tapetum. Cells of tapetum have dense cytoplasm and generally have more than one nucleus.

Functions of tapetum –

• It provides nourishment to the developing pollen grains.

• Helps in the formation of exine (the hard outer layer of pollen grain) which is composed of sporopollenin.

• It secretes hormones and enzymes

The process of production of seeds without fertilization is called apomixis. It is an asexual mode of reproduction present in certain species like grasses and Asteraceae. So it doesn’t involve the processes like syngamy and meiosis.

It is very important for producing hybrid seeds. The production of hybrid seeds by cultivation is very expensive and it is very difficult to maintain the hybrid characteristics by sowing the hybrid seeds due to segregation of characters. By introducing apomixis, the segregation of characters can be avoided.