Protococcacese 



191 



Diinorphococcus (fig. 117) there is comparatively little mucus, notwith- 

 standing the free-floating nature of the colonies, 

 and the relics of the mother-cell-walls, which are 

 to a great extent very irregular, are therefore 

 of great importance as binding-threads. In all 

 except Dictyosphterium Hitchcockii the chloro- 

 plast is parietal, often very massive, and with 

 or without a pyrenoid. 



Multiplication takes place by the formation 

 of four daughter-cells (sometimes only two in 

 Dictyosphs&rium) within the mother-cell. The 

 division-planes are oblique ; in Westelta the 

 four daughter-cells are always arranged in one 

 plane, but in Radiococcus they are tetrahe- 

 clrally disposed. The colonies frequently disso- 

 ciate, each group of four cells being the 

 commencement of a new colony. Reproduction 

 by biciliated zoogonidia has been observed in 

 Dictyosphasrium by both Zopf and Massee, but 

 does not occur under normal circumstances, nor 

 has it been observed in any of the other genera. 



The family may be conveniently divided as follows : 



Sub-family DICTYOSPHJERIE^E : Colonies completely enveloped in mucus ; colls in groups 

 of two or four. Dictyosphxrium Nageli, 1849 [inclus. Dictyocystis Lagerheim, 1890] ; 

 Radiococcus Schmidle, 1902. 



Sub-family QUATERNAT^E : Colonies with very little surrounding mucus; cells in 

 definite groups of four more or less in one plane. Dimorphococcus A. Braun, 1849 [inclus. 

 Steinidla Bernard, 1908] ; Westella De Wildeman (in part) [Tetracoccus W. West, 1892 1 ]. 



Fig. 117. Dimorphococcus lunatm 

 A. Br. x520. In A one cell 

 shows division of contents of 

 mother-cell into four daughter- 

 cells. 



Family Protococcacese. 



This family as here constituted is the ' Pleurococcacese ' of most authors. 

 The cells are for the most part aggregated to form a definite stratum, but 

 they may be scattered as unicells among other Alga?. 



The most important member of the family is Protococcus viridis Ag. 

 (= Pleurococcus vulgaris of nearly all authors) 2 which is found as a green 



1 In the Euphorbiaceas there is a valid genus Tetracoccus Englem. ex Parry in Wcst-Amer. 

 Scientist, i, 1885, p. 13. 



- Wille ('13) has examined the original specimens of Protococcus viridis from Agardh's 

 herbarium and finds that they are identical with the common Alga usually known as ' Pleuro- 

 coccus vulgaris,' which was described by Chodat ('02) as Pleurococcus Nagelii. This note 

 recently published by Wille is of the greatest general and taxonomic importance, for it not only 

 finally decides that what is perhaps the commonest Green Alga in the world must in future be 

 known as Protococcus viridis Ag., but it also determines the family Protococcacea3, which cannot 



