CONJUGA TyE 



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 outline 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 ven,' 



Fig. 239. — A, Desmidinvi Sivnrtzii Ralfs ; B, Jlicrasferias rotata Grev. ; c, Euastruin rostratmir 

 Ralfs ; D, Cosinarhcm coelatunt Ralfs ; t., Xatithidinni cristatum Br^b. ; f, Sta7irastr7ijn Arachnc 

 Ralfs ; G. Closterium DianceKhrh. ; h, Docidium baciduin 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 or 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 

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

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



