DEVELOPMENT OF PLANTS 



169 



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 

 occurrence associated with coarse algae. The desmids are elabo- 

 rately 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 

 is essentially the sam.e as in the filamentous forms which have 

 probably been derived from these unicellular plants. The asex- 

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

 region connecting the two halves (Fig. 109, A, i), called the isth- 

 mus, becomes somewhat elongated and swollen. Soon a con- 

 striction appears midway between these two halves which deepens 



FIG. 109. Reproduction of Cosmarium: A, asexual 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. 



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

 of the original halves and one half of the isthmus. This latter 

 part gradually enlarges and forms the second half of the desmid. 

 Owing to the gelatinous character of the cell walls, the desmids 



