Describe the structure of a pollen grain and explain the development of the male gametophyte within it.

(N/A) Structure of Pollen Grain:
$1$. Pollen grains represent the male gametophytes. They are generally spherical,measuring about $25-50 \ \mu m$ in diameter.
$2$. They possess a prominent two-layered wall. The hard outer layer,called the exine,is made of sporopollenin,which is one of the most resistant organic materials known. It can withstand high temperatures,strong acids,and alkali. No enzyme that degrades sporopollenin is known.
$3$. The exine has prominent apertures called germ pores where sporopollenin is absent. This allows for the emergence of the pollen tube during germination.
$4$. The inner wall of the pollen grain is called the intine. It is a thin,continuous layer made up of cellulose and pectin.
Development of Male Gametophyte:
$1$. The microspore nucleus undergoes mitosis to produce two unequal cells: a larger vegetative cell and a smaller generative cell.
$2$. The vegetative cell is abundant in food reserve and has a large,irregularly shaped nucleus.
$3$. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus.
$4$. In over $60\%$ of angiosperms,pollen grains are shed at this $2$-celled stage. In others,the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed ($3$-celled stage).