140 



MORPHOLOG Y. 



tinues until the content of the supplying cell has passed over into that of the 

 receptive cell. The protoplasm of this one is now slipping away from the 

 cell wall, until finally the two masses round up into the one zygospore. 



292. The zygospore. This zygospore now acquires a thick wall which 

 eventually becomes brown in color. The chlorophyll color fades out, and a 

 large part of the protoplasm passes into an oily substance which makes it 

 more resistant to conditions which would be fatal to the vegetative threads. 

 The zygospores are capable therefore of enduring extremes of cold and dry- 

 ness which would destroy the threads. They pass through a "resting" 

 pefiod, in which the water in the pond may be frozen, or dried, and with the 

 oncoming of favorable conditions for growth in the spring or in the autumn 

 they germinate and produce the green thread again. 



293. Life cycle.- The growth of the spirogyra thread, the conjugation, of 

 the gametes and formation of the zygospore, and the growth of the thread 

 from the zygospore again, makes what is called a complete life cycle. 



294. Fertilization. While conjugation results in the fusion of the two 

 masses of protoplasm, fertilization is accomplished when the nuclei of the 

 two cells come together in the zygospore and fuse into a single nucleus. The 



Fig. 131. 



Fertilization in spirogyra ; shows different stages of fusion of the two nuclei, with mature 

 zygospore at right. (After Overton.) 



different stages in the fusion of the two nuclei of a recently formed zygospore 

 are shown in figure 131. 



In the conjugation of the two cells, the chlorophyll band of the supplying 

 cell is said to degenerate, so that in the new plant the number of chlorophyll 

 bands in a cell is not increased by the union of the two cells. 



295. Simplicity of the process. In spirogyra any cell of the thread 

 may form a gamete (excepting the holdfasts of some species). Since all of 

 the cells of a thread are practically alike, there is no structural difference 

 between a vegetative cell and a cell about to conjugate. The difference is a 

 physiological one. All the cells are capable of conjugation if the physiolog- 

 ical conditions are present. All the cells therefore are potential gametes. 

 (Strictly speaking the wall of the cell is the gametangium, while the content 

 forms the gamete.) 



While there is sometimes a slight difference in size between the conjugal- 



