Relation to Deleterious Chemical Agents 451 



posure of three days, and flowers closed after an exposure 

 of twelve hours to an atmosphere of only 1 part to 2,000,000. 

 276. Stimulation by means of weak toxic agents. - -Small 

 quantities of acids and other substances may serve as 

 stimulants in several types of enzyme action, - - they may 

 increase the velocities of the chemical reactions. The 

 transformation of starch by diastase and of certain pro- 

 teins by pepsin are both accelerated by traces of acid. 

 Richards and Ono have shown conclusively that the dry 

 weight of certain fungi in nutrient solutions may be in- 

 creased two or three times by the addition of a small 

 quantity of one of several metallic salts. 1 In general, 

 zinc has afforded the best results. Spore production is 

 diminished in the stimulated cultures. Furthermore, it 

 has been shown by subsequent work that stimulated plants 

 are able " to dispose more economically of the sugar 

 used, . . . thereby permitting a more rapid production of 

 dry substance in a given time." 



1 The data from some experiments (Richards) in which the fungus was 

 grown on a nutrient solution containing sugar are as follows : - 



STIMULATION OF GROWTH IN ASPERGILLUS NIGER BY 

 (CULTURES KEPT AT 30 C. UNLESS OTHERWISE NOTED.) 



