GREEN ALG& 



237 



in spirogyra takes place by a process known as conjugation. When 

 the conditions are favorable two threads lying near each other 

 conjugate by tubes developed from opposite cells. The tubes 

 meet and the walls at the point of contact 

 dissolve, making an open communication 

 between the two cells. This is the conjuga- 

 tion tube, and where two conjugating threads 

 lie side by side for some distance the conju- 

 gation of the opposite cells presents a ladder- 

 like figure. The protoplasm from one of 

 the cells flows over through the tube into 

 the cell opposite, carrying with it the chloro- 

 phyll band and all of the cell-contents, but 

 some of the water is lost. The merged con- 

 tents of the two cells now contract into a 

 rounded or elliptical body called the zygo- 

 spore, or zygote as shown in fig. 1 8 1. A 

 thick and firm wall is formed and much of 

 the protoplasm is changed into an oily sub- 

 stance, in which condition it is more re- 

 sistant to unfavorable conditions of dryness 

 or cold. The two nuclei fuse into one 

 nucleus. The fusion of the cell contents 

 and fusion of the nucleus of the two 

 gametes into one is known as fertilization. 

 The zygospore is a resting spore and serves 

 to carry the plant over unfavorable condi- 

 tions or periods of weather. 



368. The gametes and gametangia. 

 The cell in which the zygospore is formed is 

 the receiving cell, while the other one is the 

 supplying cell. Each conjugating cell is a 

 gametangium (i.e., a gamete case), and the 

 portion of its content which takes part in the formation of 

 the zygospore is a gamete. When one cell of a thread in 

 ladder-like conjugation is a receiving gamete, all the others of 



Fig. 182. 



Spirogyra, showing con- 

 jugation by a buckle joint 

 between two adjacent 

 cells of the same thread. 



