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Powdery grains that contain the male reproductive cells of most plants. In gymnosperms, pollen is produced by male cones or conelike structures. In angiosperms, pollen is produced by the anthers at the end of stamens in flowers. Each pollen grain contains a generative cell, which divides into two nuclei (one of which fertilizes the egg), and a tube cell, which grows into a pollen tube to conduct the generative cell or the nuclei into the ovule. The pollen grain is the male gametophyte generation of seed-bearing plants. In gymnosperms, each pollen grain also contains two sterile cells (called prothallial cells), thought to be remnants of the vegetative tissue of the male gametophyte.