SECT. ii. DESMIDIACEJE. 193 



Many of the Desmidiaceae, but more especially the 

 genus Closterium, are remarkable for having a double 

 circulation of the internal fluid in opposite directions, 

 maintained by a vital contractile energy. One current 

 flows between the cellulose horny coat, and the thin 

 film covering the chlorophyll, while the other spreads 

 in a broad stream in the contrary direction between 

 the thin film and the chlorophyll mass, carrying from 

 the latter some of its coloured particles to the ex- 

 tremities of the frond, where there seems to be a con- 

 nection between the two streams. 



The type of the Desmidiacese is continued by various 



Fig. 10. Economy of Closterinm Lunula : A, frond showing central separation ; 

 D, frond in a state of self -division. 



modes of bisection, depending upon the genus and 

 species of the plant. In the Closterium Lunula, which 

 has an elongated crescent shape, as in fig. 10 A, the 

 endochrome or internal matter divides into two equal 

 parts, which retreat from one another at the middle 

 line; and a constriction of the cellulose coat takes 

 place between them, which increases till it closes en- 

 tirely round the extremities, as in fig. 10 D; then one 

 of the halves remains at rest while the other moves 

 from side to side, and finally detaches itself from the 

 other with a jerk. In each of these halves a constric- 

 tion of the endochrome may be seen, dividing it into 

 an obtuse and an elongated part, and for some time 

 VOL. i. o 



