36o RESEARCHES ON IRRITABILITY OF PLANTS 



may be gauged by the waxing or waning of its response. 

 It has been shown also how numerous and varied are the 

 factors that go to make up the complexity of the responses 

 in the plant. It has been shown that stimulus may be 

 modified in its effect, according as it is direct or indirect, 

 according as it is feeble or strong. The modifying influence 

 of the tonic condition of the tissue has also been shown, 

 depending on whether it was normal, sub-tonic, or fatigued. 

 In the numberless permutations and combinations of these 

 varied factors lies the infinite complexity of the responsive 

 phenomena of life. 



In surveying the response of living tissues we find that 

 there is hardly any phenomenon of irritability observed in 

 the animal which is not also found in the plant. The 

 various manifestations of irritability in the plant have been 

 shown to be identical with those in the animal. From the 

 standpoint of the theory of evolution this will be found 

 highly significant. It may be confidently predicted that 

 the recognition of this unity of response in animal and plant 

 will in no small degree further the progress of plant 

 physiology. Many difficult problems in animal physiology, 

 moreover, will find their solution in the experimental study 

 of corresponding problems under simpler conditions of 

 vegetable life. The study of the responsive reactions in 

 ])lants must, therefore, be regarded as of fundamental im- 

 portance in the elucidation of various phenomena relating 

 to the irritability of living tissues. 



