FOOD REQUIREMENTS 489 



of heteromesotrophic or heterometatrophic nutrition. Hence the case 

 for primitive chlorophyll-bearing forms as the ancestors of all the 

 Protozoa may not be so strong as is generally assumed. 



The plant-like flagellates as a group, however, furnish a logical start- 

 ing point for the evolution of other groups of Protozoa. Primitive meth- 

 ods of nutrition are not the only methods to be observed in Phytomasti- 

 gophora, and it is obvious that some of these flagellates approach in their 

 growth requirements the Zoomastigophora, Sarcodina, and Ciliata, repre- 

 sentatives of which have been grown in peptone media comparable to 

 those required by E. pisciformis and H. klebsii. Accordingly, it seems 

 that, so far as physiological modifications are concerned, the evolution of 

 animal-like flagellates and other groups of Protozoa from an ancestral 

 stock of plant-like flagellates could have presented few problems. 



Specific Growth Factors, or Vitamins 



A concise definition of the term, growth factor, is not yet available. 

 The term is now usually restricted to an essential substance which the 

 organism in question cannot synthesize, or perhaps cannot synthesize 

 rapidly enough to meet the normal requirements for growth. Such a 

 growth factor may exert its characteristic effects, even when present in 

 low concentration. By general agreement, the concept excludes the es- 

 sential food substances and elements necessary for synthesis of proto- 

 plasm. While a growth factor may in itself accelerate growth, it is to 

 be distinguished from nonessential growth stimulants, which also pro- 

 duce noticeable efl^ects when present in low concentrations. 



A survey of the rapidly growing literature reveals that growth-factor 

 requirements may difl^er among the species of a single protozoan genus, 

 and that some species can be grown in media apparently containing no 

 growth factors, while related types are much more exacting. At present, 

 there is no sound basis for generalization. Lack of information concern- 

 ing food requirements makes it impossible in some cases to decide 

 whether or not a specific growth factor is necessary, and occasionally a 

 postulated need for growth factors has disappeared after further in- 

 vestigation. Thus Dusi (1936) suggested that growth of E. viridis in 

 inorganic media might be impossible without a growth factor, but the 

 species has since been grown as a photoautotroph (Hall, 1939a). Simi- 

 larly, Hutner (1936) concluded that a vitamin-like substance is a neces- 



