88 THE METABOLISM OF ALGAE 



which may act as growth factors and different factors may be 

 concerned in different cases.^^^ There is some evidence that 

 Gloeotrichia echinulata requires a thermolabile growth factor 

 present in garden soil.^^* It is commonly observed that algae 

 which have grown well in impure culture cease to thrive 

 after isolation in bacteria-free culture. Gloeotrichia nutans, 

 for example, is of this type.^^ This is an indication that 

 such species require organic growth factors. 



