Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 219

Sexual reproduction entails the transfer of gametes to specific female structures where they unite and are then transformed into an embryo, a miniature plant. Gametes are formed by the process of gametogenesis. They are produced from specialized diploid cells called microspore mother cells in anthers and megaspore mother cells in the ovary (Figure 5.4). Microspores derived from the mother cells are haploid cells each dividing by mitosis to produce an immature male gametophyte (pollen grain). Most pollen is shed in the two‐cell stage, even though sometimes, as in grasses, one of the cells later divides again to produce two sperm cells. In the ovule, four megaspores are similarly produced by meiosis. The nucleus of the functional megaspore divides three times by mitosis to produce eight nuclei, one of which eventually becomes the egg. The female gametophyte is the seven‐celled, eight‐nucleate structure. This structure is also called the embryo sac. Two free nuclei remain in the sac. These are called polar nuclei because they originate from opposite ends of the embryo sac.

Figure 5.4 Gametogenesis in plants results in the production of pollen and egg cells. Pollen is transported by agents to the stigma of the female flower, from which it travels to the egg cell to unite with it.