Chapter 1 Solutions — Sexual Reproduction in Flowering Plants

In angiosperms, the two reproductive structures where gametophytes develop are located within the flower.

Male gametophyte (pollen grain) develops inside the microsporangia (pollen sacs) of the anther. The anther is the terminal part of the stamen. Each anther is bilobed and dithecous, with four microsporangia in total. The sporogenous tissue within the microsporangia undergoes meiosis (microsporogenesis) to produce microspore tetrads, and individual microspores mature into pollen grains — the male gametophytes.

Female gametophyte (embryo sac) develops inside the megasporangium (ovule), which is present within the ovary of the pistil. The megaspore mother cell in the nucellus of the ovule undergoes meiosis (megasporogenesis) to produce four megaspores, of which one functional megaspore develops into the embryo sac — the female gametophyte.

The correct developmental sequence in microsporogenesis and pollen development is:

Sporogenous tissue → Pollen mother cell (PMC) → Microspore tetrad → Pollen grain → Male gametes

Explanation:

1. Sporogenous tissue: A compact group of homogenous cells in the centre of each microsporangium in the young anther.

2. Pollen mother cell (PMC): Each cell of the sporogenous tissue is a potential pollen mother cell (microspore mother cell).

3. Microspore tetrad: The PMC undergoes meiosis to produce a cluster of four haploid microspores (microspore tetrad).

4. Pollen grain: Individual microspores dissociate and mature into pollen grains (male gametophytes), each containing a vegetative cell and a generative cell.

5. Male gametes: The generative cell divides mitotically to produce two male gametes (either within the pollen grain before shedding, in 3-celled pollen, or during pollen tube growth).

Monosporic development refers to the development of the female gametophyte (embryo sac) from a single (mono = one) megaspore.

In a majority of flowering plants, the megaspore mother cell (MMC) in the nucellus undergoes meiosis to produce four haploid megaspores (a megaspore tetrad). Of these four megaspores, only one — called the functional megaspore — survives and develops into the female gametophyte (embryo sac). The other three megaspores degenerate.

The nucleus of the functional megaspore divides mitotically three times (free nuclear divisions) to produce 8 nuclei. After the 8-nucleate stage, cell walls are laid down, organising the 7-celled, 8-nucleate mature embryo sac.

Because the entire female gametophyte originates from a single functional megaspore, this method is called monosporic development. This is the most common pattern of embryo sac development in angiosperms.