Q.(a) Explain the process of the development of a male gametophyte in an angiosperm.
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Start your 14-day free trial to unlock the full solution →The male gametophyte in angiosperms develops from a microspore through mitotic divisions, forming a pollen grain that contains a vegetative cell and two male gametes, which are essential for fertilisation.
In angiosperms, the life cycle involves an alternation of generations, where a dominant sporophytic plant produces spores, and these spores develop into highly reduced gametophytes. The male gametophyte, specifically, is a microscopic structure responsible for producing and delivering the male gametes for fertilisation.
(a) The Process of Development of a Male Gametophyte in an Angiosperm
The development of the male gametophyte begins within the anther, a part of the stamen. Here's a breakdown of the process:
- Microspore Mother Cell Formation: Inside the microsporangium (pollen sac) of the anther, specialised diploid cells called microspore mother cells (MMCs) or pollen mother cells are present.
- Meiosis and Microspore Tetrads: Each microspore mother cell undergoes meiosis, a reductional division, to form four haploid microspores. These microspores are initially arranged in a cluster called a microspore tetrad.
- Separation of Microspores: As the anther matures, the microspores in the tetrad separate from each other and develop into individual pollen grains. Each microspore is now a single-celled, haploid structure.
- First Mitotic Division: Each microspore then undergoes an unequal mitotic division. This division produces two cells:
- Vegetative Cell: This is a larger cell with abundant food reserve and a large, irregularly shaped nucleus. It is also known as the tube cell.
- Generative Cell: This is a smaller cell that floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
- Pollen Grain at Shedding: At this two-celled stage (vegetative cell + generative cell), the pollen grain is typically shed from the anther in about 60% of angiosperms.
- Second Mitotic Division and Mature Male Gametophyte: In the remaining species, the generative cell undergoes another mitotic division before the pollen grain is shed, forming two male gametes. More commonly, this second mitotic division of the generative cell occurs after the pollen grain lands on a suitable stigma and germinates, forming a pollen tube. The generative cell divides to produce two non-motile male gametes within the pollen tube.
The mature male gametophyte is the pollen grain containing the vegetative cell and the two male gametes. This three-celled stage is crucial for fertilisation.
(b) Why it is Called a Male Gametophyte …
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