FOOD REQUIREMENTS 493 



L. tropica (M. Lwoff, 1938a, 1939), Leptomonas ctenocephall, and 

 Strigomonas jasckidata (M. Lwoff, 1933a). Protohemin and protopor- 

 phyrin have been substituted for hematin in the case of T. cruzi. The 

 significance of such growth factors has been discussed by A. Lwoff 

 (1934, 1936), who suggested that these substances may enter into the 

 composition of respiratory catalysts (cytochrome). 



Cholesterol. — The investigations of Cailleau (1936a, 1936b, 1937a, 

 1937b, 1938a, 1938b) indicate that cholesterol and certain other sterols 

 serve as grown factors for the parasitic Polymastigida, Trichomonas 

 columhae, T. foetus, and Eutrichomastix colubrorum. The physiological 

 significance of these substances has not yet been determined for Protozoa. 



Extract of soil. — An aqueous extract of soil has been used extensively 

 by Pringsheim, who found it to accelerate the growth of a number of the 

 plant-like flagellates and also to facilitate the growth of certain species 

 in simple media. Accordingly, Pringsheim has considered this extract 

 a source of unknown growth factors. The accelerative action has been 

 verified by A. Lwoff and Lederer (1935), whose results suggest that soil 

 extracts contain organic nitrogen in concentrations sufficient for growth 

 of Polytomella agilis. Hence the status of soil extracts as a source of 

 growth factors is yet to be evaluated. 



Growth Stimulants 



Growth stimulants differ from growth factors in that they are not 

 essential to life. Like growth factors, however, they may be effective 

 in low concentrations. So far as their relation to Protozoa is concerned, 

 pantothenic acid and the plant "hormones" (auxins) may be placed in 

 this category. Elliott (1935c) has shown that pantothenic acid accelerates 

 the growth of Colpidium campylum, the maximal effect being noted 

 at pH 6.0. Above 7.0 there was either no acceleration, or else a slight 

 decrease in growth rate. Similar experiments with Haematococcus pluvi- 

 alis, within the pH range 4.5 to 8.5, showed no acceleration. Addition of 

 pantothenic acid to gelatin, gliadin, and zein media, which in themselves 

 did not support the growth of Colpidium (Hall and Elliott, 1935), was 

 without effect. These results indicate that pantothenic acid is not a 

 substitute for thiamine. 



The effects of several plant hormones, or auxins, on the growth of 

 Euglena gracilis, Khawkinea halli, and C. striatum have also been in- 



