. Principles of modern biology. Biology. Cell Division in Relation to Reproduction - 59. Fig. 3-16. Photograph of conjugating Spirogyra. The upper pair of filaments shows three pairs of cells at the outset of conjugation; and the lower filaments show a number of zygospores (dark oval bodies) which result from conjugation. (Photograph retouched. Copyright, General Biological Supply House, Inc.) times the same cells may act either as asexual zoospores, or as gametes, as in the case of Ulothrix, another filamentous green alga (Fig. 3-18). In this plant the number of re- productive cells produced
. Principles of modern biology. Biology. Cell Division in Relation to Reproduction - 59. Fig. 3-16. Photograph of conjugating Spirogyra. The upper pair of filaments shows three pairs of cells at the outset of conjugation; and the lower filaments show a number of zygospores (dark oval bodies) which result from conjugation. (Photograph retouched. Copyright, General Biological Supply House, Inc.) times the same cells may act either as asexual zoospores, or as gametes, as in the case of Ulothrix, another filamentous green alga (Fig. 3-18). In this plant the number of re- productive cells produced by each cell of the filament may vary from 4 to 64; the larger the number, the smaller the size of the cells. The larger spores soon settle down and de- velop asexually into a new filament by re- peated mitotic division (Fig. 3-18). The smaller "spores" may also germinate in the same way, but their growth is slower and the filaments produced by them are less vigorous. More frequently these smaller cells fuse in pairs, thus acting as gametes and forming a zygote (Fig. 3-18). After fertilization has oc- curred, the zygote develops into a thick-walled zygospore, as in Spirogyra. Likewise in Ulothrix, the diploid zygote undergoes two divisions, the meiotic divisions, just before germination. At first the four resulting hap- loid cells lie huddled together inside the old cell wall (Fig. 3-18) but finally, after this en- casement has disintegrated, each of the four haploid cells gives rise by mitosis to a sepa- rate new filament. The evolutionary transition between isog- amy, which is very common among simpler organisms, and heterogamy, which reaches a standardized condition in the eggs and sperm of higher plants and animals, can be found in a single group of free-swimming green algal plants. This group, the Volvocaceae, is made up of many species, but only three species will be mentioned in the present con- nection. All the Volvocaceae reproduce both sexu- ally and asexual
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