Gametogenesis is a biological process by which haploid male and female gametes are formed. This occurs in both plants and animals. In higher plants, there are two stages that are involved- sporogenesis and gametogenesis. Sporogenesis is the formation of spores whereas gametogenesis is the formation of gametes.
Gametogenesis: Formation of Male Gametes
Gametogenesis leading to the formation of male gametes in angiosperms occurs in two stages:
The androecium of the plant is made up of the stamen, filament, and anthers. The pollen grains that carry the male gametes are contained in the anthers. The anther is anatomically divided into lobes and the lobes are further divided into chambers called microsporangia.
Pollens present in these microsporangia. The microsporangia are also known as pollen sac. It is said that the pollen sac develops from a parent cell known as the archesporial cell. This archesporial cell divides into outer and inner layers. The inner layer forms the sporogenous tissue or cell that forms the pollen mother cell.
This sporogenous tissue is surrounded by the tapetum layer which provided nourishment to the pollen or microspore mother cell present in the sporogenous tissue. The microspore mother cell is diploid and divides meiotically to form haploid microspores or pollen. This describes the process of microsporogenesis.
The pollen grain is the haploid cell that is made up of two layers: the outer exine layer which is derived from the tapetum and the inner layer called intine. The exine is present all over the pollen grain except in one small part from where the pollen tube emerges post pollination. This small part is known as the germ pore.
The pollen grain divides into two halves- the small generative nucleus and the larger vegetative nucleus. The generative cell gives rise to two male nuclei whereas the vegetative cell gives rise to the pollen tube. This explains the process of microgametogenesis.
Gametogenesis: Formation of Female Gametes
Gametogenesis in angiosperms to form the female gametes, like the male gametes, occurs in two stages:
The word ‘mega-‘ is used instead of ‘micro-‘ in females because the female gametes are said to be larger in size as compared to the male gametes.
The ovules are present inside the ovary in multiple lobes. A cell in the ovule differentiates into a megaspore mother cell. The megaspore mother cell is diploid. This megaspore mother cell undergoes meiosis to form 4 haploid megaspores. 3 of the 4 megaspores degenerate and only one megaspore is left in each ovule. This process is known as megasporogenesis.
This megaspore nucleus now begins to divide mitotically to form 8 nuclei. 6 of the 8 nuclei migrate to opposing poles (3 each) while two nuclei remain at the center. The nuclei that remain at the center are known as polar nuclei. These polar nuclei fuse to form the secondary nucleus. The megaspore matures into an embryo sac. This entire process is called as megagametogenesis.
Solved Example for You
Q1: Part of the pollen grain not covered by exine is called?
- Pollen tube
- Germ pore
- Pollen nuclei
Sol. The correct answer is the option ”b”. The part of the pollen grain wall which is not covered by exine is called the germ pore. The germ pore gives rise to the pollen tube.