DESMIDIACEjE 



585 



The subsequent history of the zygospore has been followed out 

 in the case of Cosmariuni botrytis. After remaining at rest for a 

 considerable time, it germinates by the bursting of the two outer 

 coats, the protoplasmic contents escaping while still enclosed in the 

 innermost coat. In this body the protoplasm and endochrome are 

 already divided into two halves, which contract somewhat, and the 

 whole becomes enveloped in a new 7 cell-wall. A constriction has, 

 in the meantime, made its appearance between the two halves, 

 which are of somewhat unequal size, and thus the new desmid is 

 formed. <* . 



The subdivision of this family into genera, according to the 

 method of Mr. Ralfs (' British Desmidiese '), as modified by Mr. 

 Archer (Pritchard's ' Infusoria '), is based in the first instance upon 

 the connection or disconnection of the individual cells, two groups 

 being thus formed, of which one includes all the genera whose cells, 

 when multiplied by binary division, remain united into an elongated 

 filament ; whilst the 

 other and much larger 

 one comprehends all those 

 in which the cells become 

 separated by the comple- 

 tion of the fission. The 

 further division of the 

 filamentous group, in 

 which the zygospores are 

 always globular and 

 smooth (Plate IX, fig. 8), 

 is based on the fact that 

 in one set of genera the 

 joints are many times 

 longer than they are 

 broad, and that they are 

 neither constricted nor 

 furnished with lateral 

 teeth or projections ; 

 whilst in the other set 

 (fig. 440 ; Plate IX, fig. 3) the length and breadth of each 

 joint are nearly equal, and the joints are more or less con- 

 stricted, or have lateral teeth or projecting angles, or some other 

 figure ; and it is for the most part upon the variations in these last 

 particulars that the generic characters are based. The solitary 

 group presents a similar basis for primary division in the marked 

 difference in the proportions of its cells, such elongated forms as 

 Closterium (figs. 436, 439 ; Plate IX, fig. 2), in which the length is 

 many times the breadth, being thus separated from those in which, 

 as in Micrasteria* (fi$. 437 ; Plate IX, fig. 1), Cosmarium (fig. 438 ; 

 Plate VIII, fig. 2), and Staurastrum (Plate VIII ; figs. 5, 6, 10), 

 the breadth more nearly equals the length. In the former the 

 zygospores are smooth, whilst in the latter they are very commonly 

 spinous (Plate VIII, figs. 1, 4) and are sometimes quadrate. In this 

 group the chief secondary characters are derived from the degree of 



FIG. 439. Conjugation of Closterium striolatiun : 

 A, ordinary cell ; B, empty cell ; C, two cells in 

 conjugation, with zygospore. 



