Heterococcales 403 



would be to a great extent artificial. The so-called ' Volvocales ' and 

 ' Tetrasporales ' are merely sections of the Protococcales. The ' Volvocales ' 

 have never emerged from the Protococcales ; nor have the ' Tetrasporales,' 

 although the tetrasporine tendency has resulted in the evolution of Alga3 of 

 greatly increased complexity and definiteness of form (vide p. 157). 



For the classification of the Heterokontse to be consistent with that of 

 the Isokontae adopted in this work it is necessary to include Pascher's 

 Heterochloriadales and Heterocapsales in the Heterococcales, which thus 

 becomes in the Heterokontae the precise equivalent of the Protococcales 

 in the Isokontae. The Heterotrichales and the Hetero- 

 siphonales (including Botrydium only) are legitimate 

 and well-founded groups. It is highly probable, as 

 both Luther and Bohlin contend, that the Heterokontse 

 were evolved along an independent line from the rest 

 of the Green Algaa, having arisen from Flagellate forms 



& ' & & Fig. 254. Chlorama-ba 



such as Chloramoeba (fig. 254) and passed through low iietemmorpha Bohlin. 

 intermediate types like Leuvenia Gardner ('10) and 



Chlorosaccus Luther ('99), in which the dominant phase has become non- 

 motile. 



A good general account of the group and a synopsis of many of the species has been 

 given by Heering ('06), but the more recent suggestions concerning the classification of the 

 group do not, of course, occur in his work. 



Order 1. HETEROCOCCALES. 



The Algae included in this order of the Heterokontae are unicellular or 

 colonial. The cells vary much in outward form and in the way in which 

 they are associated to form colonies. 



In the more primitive types (Cklorosaccus, Stipitococcus) the cell-wall 

 is very thin, but in the more advanced types (Chlorobotrys, Botrydiopsis, 

 Ophiocytium) the wall is firm and strong. In some genera, such as Chlorobotrys 

 and Pelagocystis, there is a great development of mucilage. Mischococcus 

 has a branched colony which is almost unique amongst the Green Algae, 

 and the shapeless colonies of Botryococcus are even yet somewhat of a puzzle. 

 In Ophiocytium the cells are elongate and curved or coiled, and in the 

 Sciadium-section of the genus the daughter-cells, which are developed from 

 zoogonidia or aplanospores, remain attached in an umbellate manner to the 

 open apex of the old mother-cell, the colony being technically of the nature 

 of a coenobium. The cells contain from one to many chromatophores, the 

 greatest number occurring in Botrydiopsis. 



262 



