CHAPTER IV 



CHLOROPHYCEAE (cont.) 



CHAETOPHORALES, SIPHONOCLADIALES, SIPHONALES 



CHAETOPHORALES 



A family in which the fundamental structure is the possession of 

 both a basal and erect system, this type of thallus being known as 

 heterotrichous (cf. p. 263). In some of the genera, however, 

 reduction has taken place and only the basal or erect system is now 

 represented. 



*Pleurococcaceae : Pleiirococcus {pleuro^ box; coccus, berry). 

 Fig. 44. 



The systematic position of this alga has varied considerably. By 

 some authors it has been placed in the Chlorococcales whilst others 



Fig. 44. Pleurococcus Naegelii. A, single cell. B, single-celled colony. C, normal 

 colony. D-F, thread formation. (After Fritsch.) 



have placed it in a special group, the Pleurococcales, but as the alga 

 can occasionally develop branched threads there would seem to be 

 evidence for regarding it as a much reduced member of the 

 Chaetophorales. There are, it is true, almost equally sound argu- 

 ments for the other systematic treatments of the genus, and its place 

 at present must be largely a matter of opinion. Pleurococcus is 

 terrestrial and forms a green coat on trees, rocks and soil, growing in 

 situations where it may have to tolerate prolonged desiccation. The 

 cells, which are globose in shape and occasionally branched, are 

 single, or else as many as four may be united into a group. Under 

 certain cultural conditions branching may be copious. Each cell 



