Botany Chapter -02 Sexual Reproduction in Flowering Plants

Textual Based Activities Part-01

1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place.     

In angiosperms flowering plants, male gametes developed from anther and female gametes developed within ovary

2. Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.

Microsporogenisis

Megasporogenisis

As the anther develops, the cells of the sporogenous

tissue undergo meiotic divisions to form microspore tetrads.

Ovules generally

differentiate a single megaspore mother cell (MMC) in the micropylar region

As each cell of the sporogenous tissue is capable of giving rise to a

microspore tetrad.

one of the

megaspores is functional while the other three degenerate. Only the

functional megaspore develops into the female gametophyte (embryo

sac).

It occurs inside pollen sac.

Occurs inside ovules

3. Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.       

Sporogenous tissue ----pollen mother cell--- microspore tetrad ---  pollen grain ---- male gametes

4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule



The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle. Each ovule has one or two protective envelopes called integuments. Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the 

chalaza, representing the basal part of the ovule. Enclosed within the integuments is a mass of cells called the nucellus. Cells of the nucellus have abundant reserve food materials. Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore . 

5. What is meant by monosporic development of female gametophyte?         

In most of flowering plants the only one megaspore is functional other three are degenerated. Only the functional megaspore develops into female gametophyte. The development of embryosac from single megaspore is called monosporic development.

6. With a neat diagram explain the 7celled, 8-nucleate nature of the female gametophyte. 

• Embryo sac is formed by meiotic divisionThe nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac.  

• Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac.  

• It is of interest to note that these mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation.  

• After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.  

•  Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. 

• There is a characteristic distribution of the cells within the embryo sac. Three cells are grouped together at the micropylar end and constitute the egg apparatus.  

• The egg apparatus, in turn, consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in 

guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The large central cell as mentioned earlier, has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, though 8nucleate is 7-celled. 



7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.  

Some plants such as Viola (common pansy), Oxalis, and Commelinan produce two types of flowers  chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all . In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators. 

Botany Chapter -02 Textual Exercise Part-02 Click Here