DEVELOPMENT OF PLANTS 



189 



examples, where it will become still more obvious. 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 motility. 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. 



The Desmids. — A second group of the Zygnematales are strictly 

 unicellular plants known as Desmids (Fig. 108). They are the 

 most attractive of unicellular plants and are of common occur- 

 rence associated with coarse algae. The desmids are elaborately 

 and variously fashioned but can readily be recognized by the 



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Fig. 109. Reproduction of Cosmariuni: A, a sexual reproduction, show- 

 ing the elongation of the isthmus, i, and its gradual enlargement and divi- 

 sion to form the new lobes of the desmids. B, sexual reproduction. ' At 

 the left the cell contents of two desmids fusing to form a gametospore. On 

 the right the mature gametospore covered with a spiny coat. 



fact that they consist of two similar halves (Fig. 109, A). The 

 structure of the cells and the sexual method of reproduction 

 is essentially the same as in the filamentous forms which have 

 doubtless given rise to these unicellular plants. The asexual 



