Male reproductive organ is called androecium and their unit is called stamen. Stamen is also known as microsporophyll. A typical stamen is differentiates into two parts-
- Long, thin structure is called filament which joins the stamen to the thalamus.
- Bilobed structure called anther.
- Each lobe of anther consists of two microsporangia or theca. So it is a dithecous condition. The microsporangia develops further and become pollen sacs. There are two lobes hence a typical anther has four pollen sac.
- Each microsporangium gives rise to thousands of microspores or pollen grains.
- Each pollen grain on germination gives rise to two male gametes.
DRGP Special: –
- Generally plants are dithecous and tetrasporangiate but in Malvaceae, the anther is monothecous and bisporangiate.
- In Arceuthobium there is only one microsporangium per anther. This condition is called Monosporangirate.
Structure of Microsporangium
Each anther is generally a bilobed structure. Each lobe of anther is dithecous. Each lobe has two microsporangia. Each microsporangium gives rise to thousands of microspores or pollen grains. Pollen grains are formed inside the Microsporangium through the meiotic division in pollen mother cells. After formation of pollen in microsporangium, it can also termed as pollen sac. A microsporangium appears near circular in outline. It is surrounded by four wall layers.
- Epidermis: It is the outermost single layered and protective in function.
- Endothecium: Inner to epidermis there is a single layer of radially elongated cells. Cells possesses bands of callose and help in dehiscence of anthers at maturity.
- Middle layers: Three to four layers of thin walled parenchymatous cells situated just below the endothecium are known as middle layers. Food is stored by parenchymatous cells. Middle layer is ephemeral (lasting for a very short time) in nature and absent in a mature anther.
- Tapetum: This is a single celled innermost layer surrounding the sporogenous tissue, having dense cytoplasm and are multinucleated. It has nutritional function. Nutritional functions like producing enzymes, IAA and to absorb food from the middle layer and provide it to microspore mother cells (MMC) and pollen grains. Tapetum also helps in formation of sporopollenin and pollenkitt.
Centre of microsporangium has compactly arranged homogenous cells called sporogenous cells or microsporocytes. A sporogenous cell has abundant cytoplasm and prominent nucleus. Each sporogenous cell undergoes mitotic division and form PMC. PMC undergoes meiotic division and form pollen grain.
Question: Tapetum cells are bi-nucleate, how?
Answer: The cells of Tapetum initially diploid but they become polyploid and multinucleate due to endomitosis, free nuclear division. Because nuclear division not follower by cell division Tapetum cells are bi-nucleate or sometimes multinucleate.
The process of formation of microspore or pollen or pollen grain from PMC (pollen mother cell) is known as Microsporogenesis. Each cell of the sporogenous tissue is capable of forming microspore. The cells of the sporogenous tissue undergo meiotic division to form microspore tetrad. Ploidy of cells of the tetrad is haploid. The microspores when formed are present in a cluster of four cell called microspore tetrad.
Structure of Pollen Grain
Microspore or pollen grain is haploid, uninucleate, minute spores produced in large numbers as a result of meiosis in microspore mother cells inside the microsporangia. The pollen grains represent the male gametophytes. Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has two layered wall exine and intine.
- Exine – It is outer layer and composed of sporopollenin which is one of the most resistant organic material. It can withstand high temperatures and strong acids and alkali. It enables the pollen grains to survive in unfavorable conditions. An enzyme that can degrade sporopollenin is not known so far. The pollen grains are well-preserved as fossils because of the presence of sporopollenin. At one or more places, the exine is very thin or absent. These regions are called germ pores. These are the places where sporopollenin is absent. A pollen tube emerges from these germ pores at the time of pollen germination.
- Intine – It is inner layer and composed of pectin and cellulose. It is thin and smooth layer. Being thin, it gives rise to pollen tube at the time of pollen germination.
The cytoplasm of pollen grain is surrounded by a plasma membrane. Mature pollen grain has 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 spindle shaped with dense cytoplasm and a nucleus. Now pollen grains becomes bicelled and binucleated stage. In angiosperm pollination of pollen grains take place in bicelled and binucleated stage in 60% Angiosperms and in 40 % angiosperms pollination occurs at 3-celled stage. In this case generative cell divides and form two male gametes.
Pollen grains and allergies
- Pollen grains of a number of species are known to cause severe allergies and bronchial afflictions in people. These may lead to chronic respiratory disorders like asthma and bronchitis.
- The carrot grass or Parthenium that came into India as a contaminant with the imported wheat has become widespread and cause pollen allergy.
Pollen grains as rich sources of nutrients
Pollen grains are very rich in nutrients. For this reason, they are being used as food supplements, in the form of pollen tablets. In Western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and race horses.
Pollen grains and their viability
After dehiscence of anther, pollen grains have to land on the stigma before they lose viability. The period for which the pollen grains retain the ability to germinate is known as pollen viability. It is highly variable in different plant species. Pollen viability is also depends on the temperature and humidity. In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes and in some members of Rosaceae, Leguminoseae and Solanaceae, they remain viable for months.
Question: Write the method to store pollen grains and what is the use of it?
Answer: Pollen grain can be stored in liquid nitrogen at -196oC. These stored pollens can be used as pollen banks. These pollens are used in crop breeding programs.
DRGP Special: –
- The number of germpore, structure and ornamentation of exine is a significant feature of taxonomy.
- The detail study of pollen grain is called Palynology.
- The plants in which pollination takes place by insects, their pollen grains having oily layer around the pollen grain. It is called pollen-kit. It is composed of lipids and carotenoids. Its sticky surface helps to attach with the insects. Its yellow colour attracts the insects. Pollen kit is present on the pollen of capsella.