236 



Chlorophycece 



furnished with a single pyrenoid. The connecting-threads are 

 often derived from the old mother-cell-walls, but in some cases it 

 is doubtful if they do arise in this manner. Multiplication takes 

 place normally by the formation of four daughter-cells within the 

 mother-cell. Reproduction by biciliated zoogonidia occurs, but 

 has been very rarely observed. 



D. Ehrenbergianum Nag. is a widely distributed British Alga, often 

 occurring in quantity in the surface waters of ponds and in the plankton of 

 lakes ; diam. of cells 4 10 p. D. pulchellum Wood (fig. 104), D. reniforme 

 Buln. and D. oviforme Lagerh. are more rarely found, but it is probable that 

 the five so-called species of this genus are merely forms of D. Ehrenbergianum. 



Genus Dictyocystis Lagerh., 1890 1 . The cells are ellipsoid 

 or oblong, and are arranged in radiating series to form a small 

 free-floating colony. The radiating series of cells frequently 

 branch and the cells are held in place by delicate mucous threads. 

 Each cell possesses a central chloroplast with one pyrenoid. 



D. Hitchcockii (Wolle) Lagerh. is a rare British Alga, occurring in the bogs 

 of N. Ireland and N. W. Scotland, and also in the Scottish plankton. Diam. 

 of cells 9 11 fj.; the American specimens are larger. 



Genus Tetracoccus West, 1892 2 . [Westella De Wild., 1897 

 (in part).] The cells are small, globose or subglobose, sometimes 



a little angular, and are closely 

 arranged in groups of four. These 

 groups are connected by the old 

 mother-cell-walls into free-floating 

 colonies of small size, consisting 

 of a maximum number of about 

 80 cells. The four cells of each 

 group are disposed in one plane, 

 and the old walls of the mother- 

 cells become transformed into deli- 

 cate connecting-threads. There 

 is one chloroplast which contains several large granules, but the 

 presence of pyrenoids has not yet been definitely demonstrated. 

 Multiplication takes place by the formation of four daughter-cells 

 within the mother-cell, which arise by the division of the cell- 

 contents in two directions in one plane. The colonies are almost 

 entirely free from enveloping mucus. 



1 Lagerheim in Nuovo Notarisia, 1890, p. 226. 



2 West in Journ. Boy. Micr. Soc. 1892, p. 735, t. x, f. 4348. 



Fig. 105. Tetracoccus botryoides 

 West, from Bowness, Westmoreland. 

 A and B, x450; B and C, two "te- 

 trads," x715. 



