164 



Chlorophycece 



middle and deeply cleft at each apex. The cells are usually 

 attenuated towards each pole and the apical cleft or incision is 

 tightly closed. There is a central chloroplast in each semicell 

 with a single axile row of pyrenoids. The zygospores are globose 

 and smooth, or subquadrate with rounded angles. 



There are only four British species, T. granulatus (Breb.) Ralfs (length 

 138238 p; breadth 3045 /*; fig. 59) and T. Icevis (Ktitz.) Ralfs (length 

 57 76 p, ; breadth 19 25 /x) being the most widely distributed. The former, 

 which possesses a sparsely scrobiculated cell-wall, is one of the most ubiqui- 

 tous of Desmids and the latter has a distinct preference for mountainous 

 districts. T. Brebissonii (Menegh.) Ralfs is a more or less frequent Desmid 

 in bogs and pools containing submerged Sphagnum. 



Genus Euastrum Ehrenb., 1832. In this genus the cells are 

 relatively shorter than in any of the preceding genera of the 



Fig. 60. A, Euastmm elegans (Breb.) Kiitz. (a form), from Capel Curig, N. Wales 

 ( x 474). B, zygospore of E. eleyans, from New Forest, Hants. ( x 474). C, E. 

 binale (Turp.) Ehrenb., from Thursley Common, Surrey ( x 474). D, zygo- 

 spore of E. oblong um (Grev.) Kalfs, from Pilmoor, N. Yorks. ( x 200). /, front 

 view; s, side or lateral view; v, vertical view. 



Cosmarieae and they are always distinctly flattened. Seen from 

 the front view they are elliptical, or narrowly elliptical, and they 

 possess a median constriction of considerable depth. The lateral 

 margins may be entire or lobed, and the apices are usually deeply 

 notched. In the lobed species there is always an odd number of 

 lobes to each semicell, the apical lobe (which bears the notch) 

 being termed the 'polar lobe.' In the vertical and side views 



