Botany and Plant Science

In angiosperms, pollen grains are typically shed at either the 2-celled stage (vegetative and generative cell) or the 3-celled stage (vegetative cell and two male gametes).

The exine of a pollen grain is composed of sporopollenin, which is one of the most resistant organic materials known, protecting the pollen from degradation.

In angiosperms, the female gamete is the egg cell, which is located within the embryo sac inside the ovule.

An orthotropous ovule is one where the micropyle, chalaza, and funiculus lie in a straight vertical line.

Microspore mother cells undergo meiosis to produce microspores, which then develop into pollen grains.

The most common type of ovule in angiosperms is the anatropous ovule, not the orthotropous one.

After arriving at the wall of the ovary, the pollen tube enters the ovule either through the micropyle or by some other route. The entrance of the pollen tube through the micropyle is the normal condition and is known as porogamy. In some cases the pollen tube enters the ovule through the chalaza(chalazogamy e.g., Casuarina), or through integuments (mesogamy e.g. Cucurbita).

In the given figure A-central cell, B-egg nucleus, C-plasma membrane, D-synergid, E-filiform apparatus, F-male gametes, G-vegetative nucleus.

The body of the ovule consists of a mass of parenchymatous cells named nucellus. It is equivalent to megasporangium $(2n)$. Megaspore mother cell or megasporocyte is a diploid$(2n)$ cell which undergoes meiosis to form a linear tetrad of four haploid megaspores (n). Normally, the chalazal megaspore is the functional megaspore while the other three degenerate.

In many aquatic plants with emergent flowers, pollination occurs by wind or insects, e.g., lotus, water lily, water hyacinth.

One Microspore mother cell after meiosis produce 4 pollen grains.

The microspore or pollen grain is the first cell of male gametophyte. The microspore starts dividing and it is known as precocious. 

In tenuinucellate ovules, the nucellus is thin and often disappears. The endothelium (a specialized layer of the inner integument) then provides nourishment to the developing embryo sac.