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 ZNSO4 

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



