Nutrients 101 



unstudied species of plant is under investigation, 

 to employ a complex source of organic adjuncts 

 and only gradually to approach the simpler media. 

 While peptone or fibrin proteose preparations 

 (Fischer and Demuth, 1928, 194; Baker, 1933, 183) 

 may sometimes be used with success, the complex 

 of most uniform usefulness is yeast extract. 

 Brewer's yeast is apparently superior to baker's 

 yeast and the well known "Brewer's Yeast — Har- 

 ris" has proved very satisfactory for this pur- 

 pose. The standard procedure has been to weigh 

 out 10 grams of dry "Brewer's Yeast — Harris" 

 into a liter of distilled water, boil for one-half 

 hour, then centrifuge, decant the supernatant, 

 make up to one liter, divide into 50 ml. aliquots in 

 Pyrex test tubes, and freeze at - 15° C. Ten ml. 

 of this concentrate is sufficient to make one liter 

 of complete nutrient (White, 1937, 180) (see 

 later). In the frozen state this will keep indefi- 

 nitely without deterioration. Fibrin digest may 

 be prepared in a similarly concentrated form by 

 use of pepsin or by bacterial digestion (Fischer 

 and Demuth, 1928, 194). 



Coagulants. While most tissues grow satisfac- 

 torily in liquid media, there are some circum- 

 stances under which it is useful to provide a semi- 

 solid substratum (see Bobbins, 1922, 57; Kotte, 

 1922, 51, 52; trlehla, 1928, 115; White, 1933, 64, 



