DesmidiacesB 



355 



others increase their floating capacity in the plankton ; and occasionally, when 

 the conditions have been favourable for rapid multiplication, enormous num- 

 bers of individuals occur embedded in masses of jelly. Sometimes the 

 secreted mucus is very tough. In some species of the genus Spondylosium 

 (fig. 224, /, M and 0} the cells are united into filaments by mucous threads 

 passing between their apposed ends, and the filaments break much more 

 readily across the isthmus of a cell than at the points of apical attachment. 



Fig. 219. Various species of Cosmarium. A, C. Botrytis Menegh. var. depressum W. & G. S. 

 West, x 430 ; B, C. Regncsi Beinsch var. montanum Schmidle, x 1200; C, C. decoratum var. 

 dentiferum W. & G. S. West, x 520; D, C. pseudoconnatum Nordst. , x520; E, C. prsemorsum 

 Breb., x520; F, C. Prainii W. & G. S. West, x520; G, C. Pappekuilense G. S. West, 

 x 500. /, front view of cell; s, side view; v, vertical or end view. 



The colonial Desmids mostly occur as filaments (figs. 224 and 225), the 

 degree of firmness of attachment of the cells being very variable. It is in 

 Gonatozygon (fig. 238 C and D) that the attachment is most fragile, so much 

 so that individual cells of this genus are much more frequently met with 

 than filaments. The mere adhesion by tough mucus secreted between 

 apposed apices of cells is in a few cases supplemented by the development of 

 overlapping apical outgrowths of the cell-wall, as in Onychonema (fig. 225). 

 In Sph&rozosma there are also supplementary button-like outgrowths, which 



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