Male Reproductive Unit
Stamen, Microsporangium and Pollen Grain is the part of male reproductive units of flowers.
Stamen or Male gametophyte is the male reproductive unit and consists of the following two parts:
(i) A long and slender stalk called filament.
(ii) A bilobed terminal structure called anther.
Stamen
It is the male reproductive unit of angiosperms. It consists of two parts.
- The long and slender stalk called the filament.
- The terminal, generally bilobed structure called the anther.
- The proximal end of the filament is attached to the thalamus or the petal (i.e. epipetalous) of the flower. The number and length of the stamens are variable in flowers of different species.
- A typical angiosperm anther is a bilobed structure with each lobe having two theca (duteous) and are separated by a longitudinal groove running lengthwise.
- In a cross section, the anther is a tetragonal structure consisting of four microsporangia located at the corners, two in each lobe. Later, the microsporangia develop and become pollen sacs, which are packed with pollen grains.
Structure of Microsporangium
- A typical microsporangium is surrounded by four wall layers, i.e., the epidermis, endothecium, middle layers and the tapetum. The outer three wall layers are protective in function and help in dehiscence of anther to release pollen grains.
- Tapetum (innermost layer) nourishes the developing microspores or pollen grains and the cells of tapetum possess dense cytoplasm and generally have more than one nucleus.
(a) Transverse section of a young anther (b) Enlarged view of a microsporangium showing four wall layers
Microsporogenesis
- Each cell of the pyrogenous tissue is a potential Pollen Mother Cell (PMC) or microspore mother cell and can give rise to microspore tetrad. This process of formation of microspore from a pollen mother cell through the process of meiosis is called microsporogenesis.
- As the anthers mature and dehydrate, the microspores dissociate from one another and form tetrad 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.
- In general, dehiscence of anther occurs through the rupture of the anther lobe walls, which causes the release of the pollen grains.
Male Gametophyte (Pollen Grain)
- A pollen grain is a partly germinated microspore representing the male gametophyte. It is a haploid, uninucleate and minute spore produced in large numbers by meiosis in the microspore mother cell. They vary in their size, shape, color, design, etc., from species to species.
- Pollen grains are generally spherical measuring about 25-50 micromere in diameter. It has a two layered wall (also called sporidium), outer hard layer exine is made up of sporopollenin. It is the most resistant organic material as it can withstand high temperature as well as strong acids and alkali. No known enzyme can degrade it. Thus, pollens are well-preserved as fossils.
- Pollen grains have prominent distal aperture for germination called germ pore, where sporopollenin is absent. The inner layer intine is thin and chiefly composed of cellulose and pectin.
- Each mature pollen grain in angiosperms has two cells, the generative cell and vegetative cell.
- The vegetative cell is larger, has abundant reserve food material and a large nucleus, which is irregular in shape. Whereas the generative cell is small, spindle-shaped and floats in the cytoplasm of the vegetative cell.
- Pollen grains are generally shed at this 2-celled stage in over 60% of angiosperms. In the remaining species, the generative cell divides mitotically and gives rise to two male gametes before pollen grains are shed (3-celled stage).
- Pollen grains of several plant species may cause severe allergies and bronchial infections leading to asthma, bronchitis, etc., e.g., Parthenium or carrot grass came to India as a contaminant with imported wheat, became ubiquitous in occurrence and caused pollen allergy.
- Pollen grains are rich in nutrients. In Western countries, large number of pollen products in the form of tablets and syrups are used to increase the performance of athletes and racehorses.
- Viability of pollen depends on temperature and humidity. It varies from species to species, e.g., 30 minutes in case of wheat, rice and several months as in some members of Solanaceae, Rosaceae, Leguminosae, etc.
- It is possible to store pollen grains for years in liquid nitrogen (–196°C) in pollen banks for later use in plant breeding programmers.
FAQ
What is the male reproductive unit of a flower?
The male reproductive unit of a flower is called the stamen.
What are the parts of the stamen?
The stamen typically consists of two main parts: the anther and the filament.
What is the function of the anther?
The anther is the part of the stamen that produces pollen, which contains the male gametes (sperm cells) necessary for fertilization.
What is the function of the filament?
The filament is the stalk-like structure that supports the anther and positions it for optimal pollen dispersal.
How does pollen reach the female reproductive organs of a flower?
Pollen is typically dispersed from the anther and carried by various agents such as wind, insects, birds, or other animals to the stigma of the flower, where it may then germinate and fertilize the ovules.
