<P> Pollen tube formation is important for sexual reproduction to occur in seed plants . Plants have separate structures such as microsporocytes and megasporocytes and the pollen grains are transported to the female gametophyte through wind, water or pollinators . The last step before fertilization is the growth of the pollen tube to transfer non-motile sperm to the protected egg . Pollen tubes are unique to plants and their structure had evolved over plant history . The pollen tube formation is complex and the mechanism is not fully understood, but is of great interest to scientists . </P> <P> Pollen is produced by the stamen, the male reproductive organ of the flower . Each pollen grain contains a vegetative cell, and a generative cell that divides to form two sperm cells . The pollen is delivered by the opening of anthers for subsequent pollination (transfer of pollen grains to the pistil, the female reproductive organ). Transfer of pollen is usually carried out by wind, water, or animals . The ovaries hold the ovules that produce the female gamete: the egg cell, which waits in place for fertilization . </P> <P> Once a pollen grain settles on a compatible pistil, it may germinate in response to a sugary fluid secreted by the mature stigma . Lipids at the surface of the stigma may also stimulate pollen tube growth for compatible pollen . Plants that are self - sterile often inhibit the pollen grains from their own flowers from growing pollen tubes . The presence of multiple grains of pollen has been observed to stimulate quicker pollen tube growth in some plants . The vegetative cell then produces the pollen tube, a tubular protrusion from the pollen grain, which carries the sperm cells within its cytoplasm . The sperm cells are the male gametes that will join with the egg cell and the central cell in double fertilization . </P> <P> The germinated pollen tube must then drill its way through the nutrient - rich style and curl to the bottom of the ovary to reach the ovule . Once the pollen tube reaches an ovule, it bursts to deliver the two sperm cells . One of the sperm fertilizes the egg cell which develops into an embryo, which will become the future plant . The other one fuses with both polar nuclei of the central cell to form the endosperm, which serves as the embryo's food supply . Finally, the ovary will develop into a fruit and the ovules will develop into seeds . </P>

What is role of pollen tube in spermopsids