Protococcaceae 



191 



Dimorphococcus (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 Dictyosph&rium 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 

 Dictyosphf&rium) within the mother-cell. The 

 division-planes are oblique ; in Westella the 

 four daughter-cells are always arranged in one 

 plane, but in Radiococcus they are tetrahe- 

 drally disposed. The colonies frequently disso- 

 ciate, each group of four cells being the 

 commencement of a new colony. Reproduction 

 by biciliated zoogoniclia has been observed in 

 Dictyosphtvriuin by both Zopf and Massee, but 

 does not occur under normal circumstances, nor 

 has it been observed in any of the other genera. 



Fig. 117. Dimorphococcus lunatux 

 A. Br. x520. In A one cell 

 shows division of contents of 

 mother-cell into four daughter- 

 cells. 



The family may be conveniently divided as follows : 



Sub-family DICTYOSPH^ERIE/E : Colonies completely enveloped iii mucus ; cells in groups 

 of two or four. Dictyosphserium 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. 

 Steinielkt Bernard, 1908] ; Westella De Wildeman (in part) [Tctracoccus W. West, 1892 l \. 



Family Protocoecaceae. 



This family as here constituted is the ' Pleurococcaceae ' 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 Algae. 



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 Euphorbiace;e there is a valid genus Tctracoccus 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 Ncigelii. 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 Protococcacefe, which cannot 



