Xenogamy is a form of cross pollination in which pollen grains from anther of one flower are transferred to the stigma of another flower that belongs to the similar species in order to fertilise it. Example of xenogamy involves: Squash, onions, broccoli, and spinach.

Describe the structure of pollen grain in detail.

The pollen grains are the male gametophytes of angiosperms. These are single-celled, single nucleated, and haploid structures. The cytoplasm and the nucleus of the pollen grain are surrounded by two distinct wall layers. The outer layer is known as exine and the inner one is intine. The exine is thick and variously ornamented and is made up of sporopollenin. The inner intine is thin and made up of pectin and cellulose. At certain places in the wall layer of the pollen grain, the exine is thin or absent. These areas are known as germ pores. During the development of pollen grain, the nucleus shifts to peripheral position and divides by a mitotic division into a large vegetative nucleus and a small generative nucleus. Two male gametes are formed from the generative cell by mitosis.