. Principles of modern biology. Biology. Fig. 3-15. Conjugation of Spirogyro. Note how a bridge forms between the conjugating cells and how one of the cells flows over and fuses with the other. Also note the appearance of contractile vacuoles, which eliminate water from the cells, allowing them to shrink as they fuse. filament cells may act as gametes. When Spirogyra engages in sexual reproduction— and this usually occurs in the Tail ol the year—the pairs of cells that happen to lie oppositely on two nearby filaments begin to show signs of mutual stimulation. Short tube- like processes grow ou
. Principles of modern biology. Biology. Fig. 3-15. Conjugation of Spirogyro. Note how a bridge forms between the conjugating cells and how one of the cells flows over and fuses with the other. Also note the appearance of contractile vacuoles, which eliminate water from the cells, allowing them to shrink as they fuse. filament cells may act as gametes. When Spirogyra engages in sexual reproduction— and this usually occurs in the Tail ol the year—the pairs of cells that happen to lie oppositely on two nearby filaments begin to show signs of mutual stimulation. Short tube- like processes grow out from each of the pro- spective gametes (Fig. 3-15). These out- growths join in pairs, and the cell walls dis- solve away at the points of contact. In this way an open tube is formed connecting the members of each pair of conjugating cells (Fig. 3-15). Soon all the protoplasm of one gamete passes through the conjugation tube and fuses with the protoplasm of the other. Finally—and this is the essence of fertiliza- tion—the two haploid gamete nuclei pise to form the diploid zygote nucleus. In the case of Spirogyra the gamete cells are identical in appearance. Accordingly these gametes are specified as isogametes, and the production of such gametes is called isogamy. Conversely, cases where the gametes (heterogametes) are obviously not alike are designated as heterogamy. Although isogamy is fairly widespread among primitive organ- isms, it is not encountered in higher forms. Heterogamy became dominant early in evo- lution, and fully differentiated eggs and sperm are common to practically all multi- cellular animals and plants. Shortly after fertilization the zygote of Spirogyra develops a very thick cell wall and becomes a resting cell, called the zygospore (Fig. 3-16). The zygospore is resistant to cold and drought and is able to survive through the winter. Then in the spring, the zygospore germinates (Fig. 3-17), but just before a new filament is formed, the zygote nucl
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