Describe the development of the male gametophyte from the microspore mother cell to the production of sperm.
Hint: Pollen grains are formed from diploid microspore mother cells within the pollen sacs of the anther. The formation of the male gametophyte takes place in both the stamen and the female reproductive organ. The microspore mother cell eventually transforms into a sperm cell, which is the mature male gametophyte.
Complete step by step answer:
– The anther contains two lobes, each of which has two pollen sacs, also known as microsporangia. Inside each microsporangium are numerous microspore mother cells. These pollen sacs produce pollen grains. Each pollen grain consists of two cells: a larger generative cell and a smaller tube cell. It is enclosed by a two-layered sporoderm, with an outer layer called the exine and an inner layer called the intine. The outer exine is highly resistant to environmental factors, providing protection for the pollen grain during fertilization.
– This pollen grain is transported to the stigma of a flower. Secretions from the stigma apply pressure on the pollen grain, causing the intine to protrude and initiate the growth of the pollen tube. The tube cell contains stored nutrients and energy.
– The tube cell, generative cell, and cytoplasm all migrate into the pollen tube, which is guided by chemical signals from the ovary.
– This pollen tube, carrying two male gametes and a degenerating tube nucleus, serves as the male gametophyte.
Note:
The formation of the pollen grain, or male gametophyte, goes beyond just the creation of the pollen grain itself. The development of the male gametophyte also depends on the involvement of female reproductive structures, such as the stigma and style.