THALLOPHYTA: ALGAE 



53 



(Fig. 40//). The zygote becomes diploid by the fusion of the two nuclei 

 derived from the conjugating protoplasts. When germination takes 

 place, the fusion nucleus undergoes two successive divisions that result in 

 a reduction of chromosomes (Fig. 40A-G). Of the four haploid nuclei 

 thus formed, three degenerate, leaving one to function. In this way the 

 zygote gives rise to a haploid filament. 



Zoospores are never produced in Spirogyra. If conjugation fails to 

 occur, a protoplast may round up and become a heavy-walled cell that, 

 after a period of rest, gives rise to a new filament. Such a cell is often 



Fig. 41. A species of Spirogyra with lateral conjugation, gametic union occurring between 

 adjacent cells of the same filament. A, B, C, development of conjugating tubes and forma- 

 tion of zygote, X300. 



called an aplanospore but would be more appropriately designated as a 

 gamete that develops without undergoing conjugation. 



Spirogyra, like the other Conjugales, is peculiar because an entire vege- 

 tative protoplast becomes a single large gamete that is not ciliated and 

 does not escape into the water. Although the gametes show no differ- 

 entiation in size, the active ones are regarded as male and the passive ones 

 as female. The ordinary type of conjugation is known as scalariform 

 (ladder-like) conjugation. In a few species lateral conjugation occurs 

 (Fig. 41). In this type conjugating tubes are developed between adjacent 

 cells of the same filament. At its completion a zygote is formed in one 

 of the conjugating cells. 



Zygnema. This is a genus closely related to Spirogyra and resembling 

 it in many ways. Both forms grow in the same sort of places and look 

 much alike to the naked eye. The filaments of Zygnema are unbranched 

 and consist of cyUndrical, more or less elongated cells (Fig. 42). Each 

 has two spherical chloroplasts between which, at the center of the cell, 

 lies the nucleus. Each chloroplast has a single pyrenoid surrounded by 



