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The life cycle of plants is constituted by two generations: sporophytic and gametophytic. Gametophytic generation is the sexual generation. Differently from meiosis in animals, which gives rise to gametes, meiosis in plants originates spores. Heterospory occurs in some pterydophytes and in seed plants. This consists of the formation of two types of spores in separate sporangia (androsporangium and gynosporangium). In angiosperms, when meiosis occurs in anther sporangia, the spores are called androspores. When it occurs in seminal rudiment sporangia, they are called gynospores. The sporogenesis develops in a complete endosporic manner. In the case of gynospores formed, generally only one is viable in each sporangium. The viable spore germinates and, after three mitotic divisions, forms the female gametophyte, which develops in the sporangium tissue the nucellus. During the development of the male gametophyte, the first mitosis occurs inside the sporangium, the other ones may occur after male gametophyte release. The androgametophyte is called pollen grain and the gynogametophyte is called embryo sac. The two sperm cells formed in the second mitosis of the male gametophyte, are the male gametes. They are present in the tricellular pollen grain or after mitosis of the generative cells during the pollen tube germination. The female gametes are called egg cell and central cell. In this way, sexual

Embryology of Flowering Plants Applied to Cytogenetic Studies on Meiosis