Desmidiacece 153 



structureless, the minute prominences or small spines characteristic 

 of most of the tribe forming a part of the outer layer. 



Genus Gonatozygon De Bary, 1856. The cells are cylindrical 

 or narjowly fusiform, 10 20 (rarely 40) times longer than the 

 diametbr, and without any constriction. The apices are occasion- 

 ally subcapitate and always more or less truncate. The apical 

 attachment of the cells is very slight, the least disturbance causing 

 a general dissociation of the filament. The cell-wall is rarely 

 smooth, being usually covered with minute, sharp prominences or 

 short spines. There are generally two axile chloroplasts in each 

 cell (rarely only one), each of which is rather narrow, undulated, 

 sometimes twisted, and contains 4 16 equidistant pyrenoids. 

 Occasionally an apical vacuole containing moving granules is 

 present at each end of the cell, beyond the limit of the chloro- 

 plasts. On the formation of the zygospore the conjugating cells 

 frequently become geniculate. The zygospore has a smooth outer 

 coat. 



There are five British species of the genus, of which G. monotcenium De Bary 

 [G. Ralfsii De Bary] and Q. Brebissonii De Bary [? Docidium asperum Breb.] 

 (fig. 53 A and B) are the most abundant. Smooth varieties of both these 

 species are known and G. Kinahani (Arch.) Rabenh., which is the largest 

 species of the genus (length of cell 162 376 p; breadth 11 14 /i), is always 

 smooth. The smallest form is G. Brebissonii var. minutum W. & G. S. 

 West (fig. 53 F and G), which has a length of 47 '5 67 '5 p and a breadth of 

 4-27 p. 



Genus Genicularia De Bary, 1858. The cells are similar in 

 form to those of Gonatozygon, being cylindrical, elongate, uncon- 

 stricted and with truncate apices. The filaments are extremely 

 fragile and the cells always become free previous to conjugation. 

 The zygospore is globose and smooth, and the conjugating cells 

 become geniculate. The cell-wall is rough on the exterior almost 

 exactly like the cells of Gonatozygon monotcenium. There may be 

 two or three parietal chloroplasts in each cell, disposed as spiral 

 bands or rarely somewhat irregular. Each chloroplast contains a 

 number of pyrenoids. Genicularia is one of the rarest known genera 

 of Desmids. 



G. Spirotcenia De Bary has only been found in the British Islands from 

 Cornwall and the Shetland Is. ; length of cells 200400 /*, breadth 2025 M , 

 diam. zygosp. 48 57 p. (fig. 53 I and J). G. elegans W. & G. S. West is a 

 more slender species in which the chloroplasts form laxer spirals ; length 

 303 427 /i, breadth 14 16'3 ^. It is only known from the plankton of Loch 

 Fadaghoda, Lewis, and of Loch nan Eun, N. Uist, Outer Hebrides. 



