58 - The Cell 



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 nucleus divides 

 twice in rapid succession. In Spirogyra, these 

 are the meiotic divisions. Four haploid nu- 

 clei are thus produced, but three of these 

 degenerate prior to germination (Fig. 3-17. 

 stage 8). The remaining haploid nucleus gives 

 rise by mitosis to all the nuclei of the new 

 filament, and consequently all the cells of the 

 colony are haploid cells. 



Isogamy is also encountered in many other 

 algae, and in a few fungi and protozoans. 

 Most higher algae, however, have specialized 

 gametes, which are small flagellated cells, sim- 

 ilar to but smaller than the zoospores. Some- 



