FOOD REQUIREMENTS 479 



which are found as general constituents of protoplasm, may be listed as 

 probably essential to growth: C, H, O, N, P, S, Ca, Fe, K, Na, Mg, CI. 

 Additional possibilities include Cu, Sr, Al, Mn, Zn, Ni, B, Rb, Ba, Si, Ti, 

 V, As, Co, and Cr, since these have all been demonstrated in plant or 

 animal tissues and some appear to be essential to the growth of higher 

 organisms. 



By a process of successive eliminations, it should be possible to deter- 

 mine which elements are and which are not essential to growth. Such 

 investigations, however, are entirely dependent upon adequately purified 

 chemicals. In certain investigations (Hall, 1938b, 1939a; Hall and 

 Schoenborn, 1939a; Osterud, 1938) analyzed reagents have been used 

 in the preparation of culture media and, within such limits, the composi- 

 tion of each medium is known. One of these media (EF) contains the 

 following elements: C, H, O, N, P, K, Mg, S, Ca, and CI in appreciable 

 amounts, and traces ( 1 X 10"" to 1 X 10"" g^- per cc. ) of Cu, Ba, Fe, As, 

 Mn, Na, Zn, and Pb. This medium has supported growth of Euglena 

 gracilis, E. viridis, E. anabaena, and Lobomonas pirijormis. Another 

 medium (EC) has supported growth of E. gracilis, E. viridis, and Chlo- 

 rogonium euchlorum. So far as the component elements are concerned, 

 this medium differs from EF in the absence of Ba, in lower concentrations 

 of Ca, CI, Mg, and Mn, and in higher concentrations of P and K. Media 

 EA and EAB, which have supported growth of E. gracilis, E. viridis, and 

 C. euchlorum, contain a trace of Al, but no Ba; except for concentrations, 

 the list of elements is otherwise the same as in EC and EF. Just how 

 many of the "trace" elements are actually essential to growth has not 

 been determined. The omission of Ba from three media and of Al from 

 two media seems to be of little significance, and the status of these two 

 elements as essential substances is questionable. By comparable methods 

 of elimination, it may be possible to determine whether various other 

 elements are actually essential in photoautotrophic nutrition. 



In a few cases there is evidence that particular elements exert signifi- 

 cant effects on growth. Calcium requirements of Euglena stellata (DusI, 

 1933b) are much greater than those of other Euglenidae investigated, 

 and manganese (Hall, 1937c) has been found to accelerate growth of 

 E. anabaena. In addition, a few similar observations on heteroautotrophic 

 flagellates have been reported. For instance, A. Lwoff (1930) has re- 

 ported that Fe is essential to growth of Polytoma uvella. Similarly, Mast 



