II DEVELOPMENT AND ENVIRONMENT 23 



of a toxic substance 

 by the fungus into 

 the surrounding 

 medium. Such ex- 

 cretion takes place 

 more rapidly at 

 high temperatures 

 than at low ones. 

 Lastly, retracing 

 our series of con- 

 ditions, the cessa- 

 tion of growth in 

 fresh cultures raised 

 rapidly to a tem- 

 perature of 37*5° C. 

 is due to poisoning 

 of the protoplasm 

 by the toxin, which 

 is produced most 

 rapidly at this tem- 

 perature. We shall 

 denote this hypo- 

 thetical substance, 

 or mixture of sub- 

 stances, by the sym- 

 bol "a?," for con- 

 venience. 



The chemical nature of " a? " is obscure. It is not an 

 enzyme, however. On prolonged boiling, or on exposure 

 to air in thin films of water, it is decomposed, and by 

 either of these methods we can rejuvenate a stale culture 

 medium. Since "x" is thus unstable, and its chemical 

 nature unknown, we are thrown back upon the growth- 

 curve as a test for its presence. Denoting completely 

 stale culture media as 100 per cent " x." we find that the 

 depression of our growth stopping-point from the normal 



30 



60 90 



Minutes 



150 



maximum rate = 0026 mm. per minute 



Fig. 32. —Growth and Temperature of a 

 Root. 



Cotton seedling. 



