Physiology 435 



not eliminate all potential organic contaminants, but it is possible that 

 such impurities could not account for the observed growth without cotton 

 plvigs. 



In contrast to Euglena anahaena, E. gracilis is said to require the py- 

 rimidine component (353), and E. pisciformis, both the pyrimidine and 

 thiazole components of thiamine (107) for growth as "photoautotrophs." 

 Whether or not any or all of the currently reported photoautotrophs will 

 eventually prove to be such, it appears at present that certain green flag- 

 ellates may not be photoautotrophs. For example, failures to demonstrate 

 "photoautotrophy" have been reported for Euglena deses (105, 188). Such 

 failures could have been caused by inadequate culture media. On the 

 other hand, E. deses may represent a more advanced stage in the type of 

 regressive evolution suggested for E. gracilis (353) and E. pisciformis 

 (107). 



Heteroautotrophic nvitrition was first noted by Pringsheim (452) in 

 Polytoma uvella. These observations have been repeated and comparable 

 findings have been reported for P. obtusum (348, 353, 355). The related 

 flagellates, Polytoma caudatum, P. ocellatum, and Polytomella caeca, have 

 been grown in inorganic salt and acetate media with supplementary 

 growth-factors (352, 353, 354). More recently. Astasia longa (503), Euglena 

 gracilis (504), and Lobomonas piriformis (422) have been maintained as 

 heteroautotrophs, the green species being incubated in darkness. Chilo- 

 monas Paramecium, also is a facultative heteroautotroph (73, 220, 390), 

 although there is one report that supplementary growth-factors are re- 

 quired under such conditions (354). At present, it appears that certain 

 flagellates are facultative heteroautotrophs for which thiamine and pos- 

 sibly other vitamins are stimulatory but not essential, while other species 

 require one or more supplementary growth-factors under such conditions. 

 This possibility needs further investigation with careful attention to 

 mineral requirements and with a variety of substrates. 



Mineral requirements 



In contrast to older beliefs that some ten or eleven elements are 

 essential to life, modern investigations have detected about fifty elements 

 in the tissues of different animals and plants. It remains to be determined 

 just how many are essential and how many are chance accumulations con- 

 ditioned by a particular chemical environment. So far as the Protozoa 

 are concerned, little is known about qualitative and quantitative mineral 

 requirements. For microorganisms in general, certain metal requirements 

 are related to particular enzyme systems. Such metals may be integral 

 parts of enzymes or may serve as "activators" whose exact functions are 

 not yet understood. Consequently, it is at least conceivable that certain 

 mineral requirements may vary quantitatively, and possibly even qualita- 

 tively, in the presence of different substrates. 



