Mature pollen grains form on the anthers of the flower, where they may be carried by insects or wind to the stigma of a hospitable plant. When a pollen grain lands on a stigma, it hydrates and then germinates by sending forth a pollen tube from the grain. This tube grows down the style to the ovule, carrying the two sperm cells within it. When it reaches the ovary, the tip of the pollen tube ruptures, releasing the sperm cells, one of which fertilizes the egg and the other fertilizes the central cell of the female gametophyte. The pollen tube in many ways behaves as a single cell and during the period in which the tube elongates, the sperm cells may be regarded as passive passengers that are just along for the ride. For that reason, the study of pollen tube growth has become an important field of biological research that reaches beyond questions of fertilization. From the point of view of the cell biologist, there are many fascinating questions that can be approached by the study of pollen tube growth. The growth of the pollen tube requires the insertion of new membrane and cell wall material at the growing tip. This is an extreme example of the phenomenon of tip growth, which is also how fungal hyphae and neurons grow.