An outgrowth of the intine of the pollen grain that, on germination, emerges through an aperture in the exine and grows towards the egg, carrying the male gametes with it. It represents the reduced male gametophyte in seed plants. In angiosperms pollen tube growth in compatible stigmatic tissue is usually rapid and may reach 1-3 mm/h. In gymnosperms growth is arrested at the nucellus and may not recommence until the next growing season. In Cycas and Gingko the gametes are motile. In other gymnosperms and in angiosperms the gametes are nonmotile and the mechanism of their movements down the pollen tube is not fully understood. In angiosperms the pollen tube may reach the ovule from the stigma either by growing down the stylar canal or by enzymatically digesting its way between or through the. individual cells of the style. It then grows through the ovular cavity and reaches the egg apparatus, usually via the micropyle but sometimes through the chalaza (see chalazogamy ). There, the contents of the pollen tube are discharged. The * vegetative nucleus disintegrates and one of the gametes fuses with the egg cell to form the zygote and the other with the polar nuclei or definitive nucleus to form the endosperm. |
