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Angiosperm life-cycle.
As in the pine, the sporophyte is the dominant generation. Eggs form within the megagametophyte, or embryo sac, inside the ovules, which in turn are enclosed in the carpels-members of the inner whorl of the flower. The carpel is differentiated in most angiosperms into a slender portion, or style, ending in a stigma, the surface on which the pollen grains germinate. The pollen grains, meanwhile, are formed within the sporangia of the anthers and complete their differentiation to their mature, three-celled stage either before or after grains are shed. Fertilization is distinctive in angiosperms, being a double process. A sperm and an egg come together, producing a zygote, at the same time, another aperm fused with the two polar nuclei, same time, producing the primary endosperm nucleus, which is triploid. Both the zygote and the primary endosperm nucleus divide mitotically, giving rise, respectively, to the embryo and the endosperm. The endosperm is the tissue, unique to angiosperms, that nourishes the embryo and young plant.
แหล่งข้อมูล: Understanding Biology. Page 643.
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