The component cells of the 'egg-apparatus' in an embryo sac include, two synergids, one egg cell and the filiform apparatus.

The pistil has the ability to recognise, the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). If it is of the right type, the pistil accepts the pollen and promotes post-pollination events that lead to fertilisation. If the pollen is of wrong type, the pistil rejects the pollen.

The ability of the pistil to recognise the pollen is followed by its acceptance or rejection. It is the result of a continuous dialogue between pollen grain and the pistil mediated by chemical components of the pollen interacting with those of the pistils.

The common functions that cotyledons and nucellus perform are as follows

(i) Storage of reserve food material.

(ii) Nourishment Cotyledons nourishes embryo and nucellus nourishes embryo sac.

$$ \begin{aligned} &\text { }\\ &\text { Pollen mother cell } \text { Pollen tetrad } \text { Pollen grain }\left\langle\begin{array}{l} \text {(i) Vegetative cell } \\ \text { (ii) Generative cell }\end{array}\right. \end{aligned} $$

The process of formation of microspores from a Pollen Mother Cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells i.e., the microspore tetrad.

As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains (male gametophyte). When the pollens mature, it contains two cells the vegetative cell (bigger) and generative cell (smaller).

Microsporogenesis : (a) A microspore tetrad

                                      (b) A microspore maturing into a pollen grain