1.Describe the structure of microsporangium?

Ans:

• It is surrounded by four wall layers– the epidermis, endothecium, middle layers and the tapetum.
• The outer three wall layers perform the function of protection and help in dehiscence of anther to release the pollen.
• The innermost wall layer is the tapetum. It nourishes the developing pollen grains Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
• In a young anther a group of compactly arranged homogenous cells called the sporogenous tissue occupies the centre of each microsporangium.

2.What is microsporogenesis?

Ans:The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.

3.Explain the process of microsporogenesis in angiosperms.

Ans:

• As the anther starts developing, the cells of the sporogenous tissue undergo meiotic divisions that leads to the formation of microspore tetrads.
• Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad.
• Each one is a potential pollen or microspore mother cell (PMC).
• The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
• The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad.
• As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
• Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.

4.Describe the sequence of events that lead to the development of pollen grain from microspore mother cell in angiosperms.

Ans:

• The pollen grains represent the male gametophytes.
• Pollen grains are generally spherical and are made up of a two-layered wall.The outer layer and inner layer.
• The outer layer is called exine and the inner layer is called intine.
• The exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali. Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin.
• The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.
• The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell.
• The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is a spindle shaped with dense cytoplasm and a nucleus.
• In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
• When once they are shed, pollen grains have to land on the stigma before they lose viability if they have to bring about fertilisation.

5.Name the organic materials the exine and intine of an angiosperm pollen grains are made up of.

Ans:
The exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali.

The intine is a thin and continuous layer made up of cellulose and pectin.

6.What is the function or role of exine?

Ans:
The exine is made up of sporopollenin which is one of the most resistant organic materials known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin. The exine exhibits a fascinating array of patterns and designs.

7.What is apomixis?

Ans: Apomixis is a form of asexual reproduction that produce seeds without fertilisation.

8.What are wind pollinated flowers called?

Ans: Abiotic agents

9.Explain Wind Pollination.

Ans:

• Wind pollination requires pollen grains to be light and non-sticky so that they can be transported in wind currents.
• They mostly have well well-exposed stamens and large often-feathery stigma to easily trap air-borne pollen grains.
• Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence an example is corn cob.
• Wind-pollination is quite common in grasses.

10. Explain the functions of the tapetum.

Ans:

• Tapetum is the innermost wall layer of microsporangium.
• Tapetum helps in nourishing the developing pollen grains.
• Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.

11.Explain how the female gametophyte is formed.

Ans:

• Most of the flowering plants have four megaspores, but from that three degenerate and one is functional.The functional megaspore is developed into female gametophyte also called the embryo sac.This method of embryo sac formation from a single megaspore is termed monosporic development.
• Next the nucleus of the functional megaspore divides mitotically to form two nuclei and it moves to the opposite poles that form 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.
• 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.

12.Explain the meaning of emasculation and why and how it is done?

Ans: Removal of anthers from the flower bud before the anther burst open is referred to as emasculation. It is done to improve the crops using artificial hybridisation technique. To carry out emasculation the flowers have to be covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen.

13.Explain bagging process in artificial hybridisation.

Ans: Artificial hybridisation which is done to improve crops growth is done using emasculation and bagging techniques. Emasculated flowers are covered with a bag of suitable size, generally made up of butter paper, to prevent contamination of its stigma with unwanted pollen. This process is called bagging. When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma, and the flowers are rebagged, and the fruits allowed to develop.