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pollen grain

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A microspore in flowering plants, which germinates to form the male gametophyte, a structure made up of the pollen grain plus a pollen tube. The grain contains 3 haploid nuclei (a tube nucleus and 2 sperm nuclei), which pass down the tube to the ovum. One of the sperm nuclei fertilizes the ovum, and the second fuses with the 2 polar nuclei forming the endosperm. The tube nucleus (which is considered to be vestigial, having been completely functional earlier in the evolution of flowering plants) degenerates after double fertilization (so called because of the two unions of nuclei).

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