DesmidiacesB 



361 



becomes attached by the other pole, the movement being repeated. This 

 reversal of position occupies from 6 to 35 minutes according to the tem- 

 perature, the change of position occurring more rapidly as the temperature is 

 increased. At 33 C. the reversal occupied from 6 to 8 minutes. In a number 

 of other genera, such as Gosmarium, Euastrum, Micrasterias, etc., the move- 

 ments are mostly quite irregular and spasmodic, although sometimes of a 

 gliding character. In greatly compressed Desmids the flattened sides may 

 be kept towards the incident light. The movements have been chiefly ascribed 



Fig. 225. A and B, Onychonema compaction W. & G. S. West. CF, 0. laeve Nordst. ; D and 

 -B, zygospores; F, aplanospore. G, O. Iseve Nordst. var. latum W. & G. S. West. H, O. 

 uncinatum G. C. Wallich. a, front view ; b, vertical or end view. E, x 360 ; all the 

 remainder x 468. 



to heliotropic and geotropic responses, but there is little doubt that in most 

 cases the spasmodic nature of the movement is owing to the secretion of an 

 irregular stalk of mucilage through the larger pores at the extremity of the 

 cell (fig. 228 A), and that, moreover, a substratum is necessary for the move- 

 ment to be carried out. If a sediment of organic detritus containing a 

 number of living Desmids be exposed to the light, in a few hours the Desmids 

 will have moved towards the incident light, having collected in clusters sur- 

 rounded by abundant mucilage. 



