Question

3. The megaspore in most flowering plants grows into a megagametophyte that has seven cells and eight nuclei. Name the seven cells.

4. After pollen lands on a stigma, it is far away from the ovule with the megagametophyte which holds the egg (the mega-gamete). How are the two sperm cells transported from the stigma to the egg?

5. After pollination and fertilization, as the ovule develops into a seed, the ovary matures into a ____________. Many of these have three parts, the _____________ is the skin or peel, the ___________is the flesh, and the innermost layer, the _____________, may be tough like the pit of a cherry.

Answer 1

Answers with Explanation to the questions given :

Megaspores develops into the female gametophyte called as megagametophyte. It consists of 7 cells and 8 nucleus. Megagametophyte is also called as embryo sac in flowering plants. Such type of embryo sac develops after three cycles of continuous mitotic division. At the micropylar end, there is an egg cell and two synergid cells are by its side which helps the pollen to enter into the egg cell. At the opposite end means chalazal end, there are three antipodal cells which degenerates later. And then there is this large central cell of the embryo sac or female gametophyte which is megagametophyte. The central cell contains two polar nuclei.

So we can easily see that there are 7 cells out of which 1 egg cell, 2 synergid, 3 antipodal cells and one central cell each having one nucleus except for the the central cell which contains two polar nuclei which makes the megagametophyte a 7 celled, 8 nucleate gametophyte.

So it's rightly pointed out in the question that when the pollen lands on stigma, it's very far from the ovule which contains megagametophyte which in turn contains the mega gametes. The pollen grains reach upto ovule by germinating after landing on stigma and forming a pollen tube which extends through the style upto the ovule. And this pollen grains is the male gametophyte which contains two types of cells. They're vegetative cell and generative cells. Generative cells gives rise to two male gametes one which fertilize the egg while another male gamete fuse with the two polar nuclei.

After pollination and fertilisation, as the ovules develops into seed, ovary matures into a fruit. The epicarp is the skin or the peel, the mesocarp is the flesh, and the inner most layer, the endocarp maybe tough like the pit of a cherry.

Fleshy fruits have a pericarp which is made up of three layers. They're the outermost epicarp, the fleshy mesocarp and the hard endocarp which surrounds the seed.