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Arbitrarily begin with the haploid gametophyte generation, which is vastly reduced and dependent upon the diploid sporophyte generation. The pine tree with which we are familiar is the sporophyte. The gametophyte begins at meiosis, when microsporophylls (male cone scales) containing microsporocytes become microspores, and ovules on female cone scales develop megasporocytes that become megaspores. Meiosis produces a tetrad of microspores that develop into a pollen grain which, assisted by a pair of air-filled bladders, become wind-borne to the megagametophyte within an ovule. Meiosis also results in the production of four megaspores, three of which die, leaving a single functional megaspore. Within a pollen grain lies two cells, a generative sperm cell and a tube cell. Pollination occurs when the pollen grain penetrates the micropyle of the female scale. The tube cell elongates into a pollen tube that, over a period of a year or more, eventually reaches the egg cell. The sperm cell migrates through the pollen tube to the egg and fertilization results in the creation of a diploid zygote. The subsequent embryo develops within a naked seed and temporarily suspends development in order to overwinter. Upon receipt of the proper hormonal and environmental cues, germination occurs and a diploid sporophyte, or tree, develops. When the sporophyte is mature, diploid male and female cones emerge, and the alternation of generations completes a cycle. |
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