Desmidiacece 



177 



girdle-like thickening which projects into the old semicells during 

 the earlier stages of division. The cells are united to form thread- 

 like colonies. 



Genus Desmidium Ag., 1824. [Didymoprium Kiitz., 1843; 

 Aptogonum Ralfs, 1848.] The cells are united to form twisted 

 filamentous colonies, often enveloped in a wide mucous coat. The 



Fig. 69. A, Desmidium Swartzii Ag., from near Preston, Lancashire (x365). 

 B, D. quadratum Nordst., showing cell-division (x475). C, zygospore of 

 D. cylindricum Grev. , from Donegal, Ireland (x350). D, zygospores of 

 D. aptogonum Breb. ( x 475). E and F, Gymnozyga moniliformis Ehrenb., 

 from Ehiconich, Sutherland ( x 475) ; F, showing cell-division. G, zygospores 

 of G. moniliformis var. gracilescens Nordst. ( x 475). 



median constriction is moderately deep and the semicells are much 

 depressed, so that the cells are generally much broader than their 

 length. The attachment of the cells is either by the close 

 apposition of their flat apices or by the apposition of corresponding 

 truncate apical projections. In the latter case there is a space of 

 variable width visible between the actual apices of two adjacent 

 cells. In vertical view the cells may be triangular, quadrangular, 

 or elliptical with mamillate poles. There is a single central 

 chloroplast in each semicell containing as many pyrenoids as there 

 are angles in the vertical view, and there are two longitudinal 

 plates diverging from each pyrenoid into the angle. The zygo- 

 w. A. 12 



