<P> Pollen tubes were first discovered by Giovanni Battista Amici . </P> <P> Angiosperm reproduction is a complex process that includes several steps that may vary among species . Each step is a vast procedure in its own right . 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, that is, for the transfer of pollen grains to the pistil, the female reproductive organ . Pollination is usually carried out by wind, water or insects . 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 of certain plants . 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 successfully attains an ovule, it delivers the two sperm cells with a burst . One of them fertilizes the egg cell to form 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>

When you combine an egg with the material in pollen you get