Like the spores of ferns and other cryptogams, pollen grains are the product of meiosis, the cell division that produces haploid cells from diploid parents. Haploid fern spores are the first cells of gametophytes, the free-living haploid stage of the life cycle in which gametes are eventually produced by simple mitosis, fuse into a zygote, and finally develop into the diploid sporophyte which is the adult fern. Pollen grains are homologous to fern spores and equally haploid, but the gametophyte generation has been suppressed so that pollen grains are effectively no more than carriers of the male gamete. Pollen grains disperse to flowers, adhere to the stigma, and grow a pollen tube through which the gamete nucleus moves to enter the ovum. Thus the ecological functions of spores and pollen are subtly different. Both represent the reduction division necessary for a sexual life cycle, but spores also are dispersal agents. Structural homology between the two, however, remains clear and is the basis of their principal structural characteristics.