DEVELOPMENT OF PLANTS 



177 



in the process, as in some species of Zygnema, where the fusion 

 is effected in the middle of the tube. This is only a different 

 form of the sexual process with which you are familiar, the dif- 

 ference 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 gametospore becomes 

 surrounded by a thick wall and in this resting condition 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- 



FiG. 108. Common forms of Desmids: A, Closterium. B, Micrastcrias. 

 C, Xanthidiiim. D, Staurastrum. At left top view, at right side view. E, 

 Desmidiiim forming a chain of plants. At right, end view of chain. 



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- 

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