100 Plant Tissue Culture 



complex source in the form of a mixture of a 

 number of amino-acids (White, 1935, 179, 1937, 

 181 ; Bonner and Addicott, 1937, 42) . In that case, 

 the synthetic acids should be used wherever pos- 

 sible, or where they are not available amino-acids 

 of the highest possible purity should be obtained. 

 This must be emphasized because amino-acids of 

 "natural" origin are likely to be themselves mix- 

 tures. Thus, even the best commercial samples of 

 leucine are known to contain considerable and 

 highly significant amounts of methionine (Mueller, 

 1935,447). 



Organic complexes. All tissue culture media 

 contain accessory organic (non-carbohydrate) ma- 

 terial of some sort. The early nutrients included 

 yeast extract (Robbins, 1922, 177; White, 1932, 

 63, 1934, 65, 1937, 180 ; Fiedler, 1936, 45), Liebig's 

 beef extract (Kotte, 1922, 51, 52; White, 1932, 89), 

 peptones (Robbins, 1922, 177), fibrin digests 

 ( White, 1932, 89 ) , etc. ( See also Haberlandt, 1902, 

 98 ; Loo and Loo, 1935, 216, 1936, 217) . The use of 

 thiamin, nicotinic acid, pyridoxine, the amino- 

 acids, etc., is the outcome of a long and pains- 

 taking study of these complexes and in most cases 

 permits us to dispense with the more complex 

 materials entirely. This has not always proved 

 true, however, and it is frequently desirable, when 

 a new tissue or a member taken from a previously 



