178 



REPRODUCTION OF DESMIDS 



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. 



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

 unicellular plants known as Desmids (Fig. io8). 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 

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

 structure of the cells and the sexual method of reproduction 



rP"^ 



Fig. 109. Reproduction of Cosmarium: 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. 



is essentially the same as in the filamentous forms which have 

 doubtless given rise to these unicellular plants. The asexual 

 method of reproduction, however, is rather peculiar. The region 

 connecting the two halves (Fig. 109, A, i), called the isthmus, 

 becomes somewhat elongated and swollen. Soon a constriction 

 appears midway between these two halves which deepens until 

 the desmid is cut in half. Each desmid now consists of one of 



