Stamen,Microsporangium,Pollen Grain

Describe the Stamen

A typical stamen consists of

i) a long and slender stalk called filament

ii) a terminal and generally bilobed structure called anther

A typical angiosperm anther is bilobed, with 1 theca present in each lobe, therefore the anther is dithecous.

The anther is 4 sided (Tetragonal) and has 4 microsporangia, one located at each corner. 2 Microsporangia are located in one lobe, making the anther Tetrasporangiate.

These microsporangia further develop and become pollen sacs.

Describe the structure of the microsporangium

The typical microsporangium appears to be circular in outline. It is surrounded by 4 wall layers. The epidermis, The endothecium, The middle layers, and Tapetum. The outer 3 wall layers perform the function of protection and help in the dehiscence of the anther. The innermost layer is the tapetum that nourishes the developing pollen grains. Cells of tapetum possess dense cytoplasm and can have more than one nucleus.

When the anther is young, a group of compactly arranged homogenous cells called the sporogenous tissue occupy the center of each microsporangium

Define and explain the process of microsporogenesis

Definition: The process of formation of microspores (pollen grains) from a pollen mother cell (PMC) through meiosis is called as microsporogenesis

• Explanation: As the anther develops, the cells of the sporogenous tissue can differentiate into the pollen mother cell through mitosis, and then undergo meiotic divisions to form microspores.

• As a result of meiosis, 4 microspores are formed, which are arranged in a cluster of 4 cells called the microspore tetrad.

• When the anthers mature and dehydrate, the microspores dissociate from each other and then develop into pollen grains

• These pollen grains are then released with the dehiscence of anthers.

Define the pollen grains and describe its structure.

• Pollen grains are the male gametophytes and they are spherical in shape.

• Pollen grains have a 2 layered wall.

• i) The hard outer layer called exine which is made up of sporopollenin which is one of the most resistant organic materials that can withstand high temperatures and strong acids and alkali . No enzyme can degrade sporopollenin. Due to sporopollenin the the pollen grains are preserved as fossils.

• ii) The inner layer called intine which is thin and continuous and it is made up of cellulose and pectin.

• Pollen grains have apertures called germ pores where the sporopollenin is absent. The germ pores help to initiate the formation of the pollen tube.

Why are pollen grains called the 3-celled stage?

When the pollen grain is mature after mitotic divisions, it contains a vegetative cell and a generative cell.

The generative cell mitotically divides into 2 male gametes before the pollen is shed, increasing the number of cells within the pollen grain from 2 to 3. Therefore the pollen grain is called the 3-celled stage.

Vegetative cell is bigger, has abundant food reserves, and large irregular shaped nucleus

Generative cell is smaller, floats in the cytoplasm of vegetative cell, it is spindle shaped and has dense cytoplasm and nucleus.

Write the advantages and disadvantages of pollen consumption

Advantages

1. Pollen grains are rich in nutrients

2. Pollen Tablets are used as food supplements

3. Pollen consumption can increase the performance of athletes and race horses

Disadvantages

1. Pollen grains of many species can cause severe allergies and bronchial afflictions leading to respiratory disorders like asthma, bronchitis etc

2. Example: Carrot grass (Parthenium) causes pollen allergy.

What is Viability of pollen grains ? Explain.

Viability: It is the ability of the pollen grains to initiate the process of fertilization

When pollen grains are shed, they must land on the stigma before they lose their viability, for fertilization to take place.

The period of viability is variable for pollen grain species and depends on temperature and humidity.

Some cereals; Rice and wheat, The viability is ~ 30 mins

Members of Solanaceae, Rosaceae, Leguminaceae, can maintain viability for months.

Pollen grains of many species can be stored in liquid nitrogen (-196 C) to maintain viability for longer periods and be used as pollen banks.