GREEN ALG& 



217 



tube, and where two conjugating threads lie side by side for 



some distance the conjugation 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 chlorophyll band and all of 



the cell contents, but some of the water is lost 



The merged contents of the two cells now 



contract into a rounded or elliptical body called 



the zygospore, or zygote, as shown in fig. 172. 



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 resistant 



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 unfa- 



vorable conditions or periods of weather. 



355. The gametes and gametangia. The 

 cell in which the zygospore is formed is the 

 receiving cell, while the other one is the supply- 

 ing 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 the same thread are also receiv- 

 ing gametes. All the cells of a single thread 

 are likewise supplying gametes. Usually there 

 is no difference in the size of the supplying and 

 receiving cells, and it is impossible to say what 

 the nature of the gamete is until movement lt 

 of the protoplasm from the supplying gamete is taking place. 

 While the sex of the threads appears in these cases to be 

 distinct, there is no differentiation in the size and form of the egg 



Fig. 173- 



samc 



