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Figure 18.6 The alternation of generations in flowering plants. The pale blue area indicates the
sporophytic stages and the light tan area indicates the gametophyte stages. A. The germinating
seed gives rise to the young sporophyte generation. B. The mature vegetative sporophyte enters
the reproductive phase and produces flowers. C. The ovary of the pistil gives rise to the ovules,
or megasporangia. D. A specialized cell in the ovule undergoes meiosis to give rise to a single
functional megaspore, the first stage of the female gametophyte generation. E. The megaspore
divides mitotically to produce the embryo sac, or mature female gametophyte, containing an egg
cell and several other types of cells. F. At the same time, specialized microspore mother cells in
the anthers (microsporangia) give rise by meiosis to the microspores, the first stage of the male
gametophyte generation. G. Inside the anther, the microspore divides twice to produce the pollen
grain, or microgametophyte, consisting of a pair of sperm cells and a vegetative cell. H. The pollen
grain is shed from the anther and lands on the stigma, where it germinates and grows down to the
style. I. The tip of the pollen tube penetrates the embryo sac. J. The pollen tube tip bursts, releasing
the two sperm cells in the embryo sac. K. One sperm cell fuses with the egg to create the zygote,
the first cell of the sporophyte generation, and the other sperm fuses with the diploid central
cell nucleus to produce the triploid primary endosperm cell. L. Completing the cycle, the zygote
develops into an embryo (M), the seed matures and eventually germinates to produce a seedling
(A). The seedling then continues to grow, eventually forming a new mature sporophyte, and a new
cycle begins. See color insert.