CONJUGA TJE 



269 



the half-cells have a smooth, sinuate, or beaked margin, with circular in- 

 flated protuberances ; in Cosmarium the half-cells are quite undivided, 

 and the whole outhne often nearly orbicular; in Xanthidium (Ehrb.) 

 and in most species of Staurastrum (Mey.) the surface is elevated into- 

 prominent tubercles or spines. 



The transparency of the cell-wall in desmids enables the rotation of 

 the protoplasm to be distinctly seen ; and at the colourless spaces at 

 the extremities of some species of Closterium and Docidium the dancing 

 'brownian' movement of particles suspended in the cell-sap is very 



Fig. 239. — A, Dcsmidiuin S-wnrtzii Ralfs ; B, Micrasterias rotattt Grev. ; c, Euastruin rostratuvz 

 Ralfs ; D, Cos-ma'/ in7n cmlatu-m Ralfs ; t, Xanthidium cristatum Br^b. ; F, Staurastrum A rachne 

 Ralfs ; G. Closterium DiatusYMrh. ; H, Docidium haculum Br^b. (All after Ralfs and variously 

 magnified.) 



evident. Klebs describes four kinds of movement in desmids, viz. : — 

 (i) A forward motion on the surface, one end of each cell touching the 

 bottom, while the other end is more 6r less elevated and oscillates- 

 backwards and forwards ; (2) an elevation in a vertical direction from 

 the substratum, the free end making wide circular movements ; (3) a 

 similar motion, followed by an alternate sinking of the free end and 

 elevation of the other end ; and (4) an oblique elevation, so that both 

 ends touch the bottom— lateral movements in this position ; then an 

 elevation and circular motion of one end, and a sinking again to an 

 oblique or horizontal position. These movements are, according to this 

 observer, all due to an exudation of mucilage, and the first two to the 



