i68 



REPRODUCTION OF DESMIDS 



is only a different form of the sexual process with which you are 

 familiar, the difference being, that but one gamete is formed in 

 each cell and these bodies are motionless or at least not provided 

 with cilia and do not escape from the mother plant. When the 

 fusion of the gametes has been effected the resulting gameto- 

 spore becomes surrounded by a thick wall and in this resting con- 

 dition it tides the plant over seasons, such as the winter or drought, 

 unfavorable for the growth of the plant. When conditions re- 

 turn that permit the growth of the plant, the gametospore germi- 

 nates by rupturing the outer wall and protruding the inner wall 

 of the spore as a delicate tube (Fig. 107, B). As this tube con- 

 tinues to elongate the characteristic plastids and cell partitions 

 arise, thus forming a new plant. The loss of motility in the 

 reproductive process in this order may possibly be associated 

 with their exposure to terrestrial conditions. They are often 

 exposed to the soil, owing to the drying up of the water and it 

 has been suggested that they have consequently lost their mo- 

 tility. Their mucilaginous coatings would be of great service 

 in enabling them to meet such conditions by retaining moisture, 

 and indeed, they are often able to flourish in many damp places 

 without the aid of surface water. 



Fig. 108. Common forms of Desmids : ./, Closterium. B, Microsterias. 

 C, Xanthidium. D, Straustrastrum. At left top view, at right side view. 

 E, Desmidium forming a chain of plants. At righl end view of chain. 



