A Pollen grain is the male gametophyte of a flowering plant. (A gametophyte is a the stage in the life cycle of a plant which undergoes alternation of generations). Each pollen grain contains both vegetative (non-reproductive) cells and a reproductive cell. The vegetative cell produces the pollen tube and the generative cell divides to form the two sperm cells. Pollination is the transfer of pollen grains to the female reproductive structure, called the pistil. A pistil typically consists of a basal portion called the ovary and an elongated section called a style and an apical structure, the stigma, which receives pollen. Pollen reaches the female pistil either by floating randomly in the wind where it lands on the stigma of an appropriate flower, or more directly by being carried to the pistil by a insect or other animal. The intricate sculptural design of the outer wall of a pollen grain is a species-specific feature which has evolved to facilitate the particular challenges facing the plant during reproduction.