THE AMP:RICAN BOTANIST 99 



velopnient uiitlcr iraining, psychologists find times when train- 

 ing seems to produce no results, and they designate such pauses 

 as "plateaux of learning." 



Dallinger subjected little animals called flagellata to 

 gradually increasing temperatures. Beginning with those 

 living in water at 60 degrees, Fahr., he found that he could 

 increase the temperature quite rapidly up to 73 degrees at 

 which point he iiad to hold it stationary for two months be- 

 fore he could increase it further without killing them. Past 

 this point he was able to increase the temperature rapidly 

 again up to 78 degrees, at which point he had to hold it for 

 ciglit months. He found several other sticking points on the 

 way of getting them to live at 158 degrees, which point w<!S 

 reached in about six years. 



Under continued training a horse will continue to gain 

 in trotting powder up to some point beyond 17 years of age, 

 l)ut not in every year of that time. Sometimes there will be 

 a lapse of one year and sometimes a lapse of two years in 

 which training appears to accomplish nothing, but if training 

 be continued, rapid improvement comes on again. In differ- 

 ent horses these sticking points come at different times in life. 

 The same thing is found in milk production by cows. When 

 regularly milked, production continues to increase up to 12 

 years of age, but not in every year of that time. As in horses, 

 tliese sticking points come at different times in different cows. 

 By a great many tests of different kinds we find that 

 powers in plants are developed the same as they are in ani- 

 mals, and that the development of disease-resisting powers 

 comes under the same laws as the development of other 

 powers. Applying these principles to the matter under con- 

 sideration, it will be seen that in a field of plants growing 

 under disease conditions, at any given moment some plants 



