SPECIAL SENSES 17 



accounts for the popular notion that roots 

 seek rich soil. 



Moisture also exerts a directive influence 

 upon roots, and sometimes upon other parts 

 of the higher plants, as well as upon the or- 

 gans of many lower plants. Usually the 

 movement is toward the moister side. It is 

 evidently beneficial to the plant in keeping its 

 absorbing organs bathed in the greatest 

 available supply of fluid. 



Plants are thus seen to react sensitively to 

 gravity, light, solutions, moisture and con- 

 tact. Each is a special kind of sensitiveness, 

 having its own method of reaction. Two or 

 more kinds of sensitiveness may reside in the 

 same organ, when its position will be a re- 

 sultant of the several forces. There are no 

 exclusive organs of sense in plants, although 

 there is more or less localization in certain 

 parts ; and there are no nerves although the 

 motor impulse may be transmitted some dis- 

 tance, even as far as seventy centimeters or 

 more in very vigorous sensitive plants, for 

 example in mimosa. To complete the com- 

 parison I should say there are no muscles in 

 plants, although they execute movements of 

 very considerable amplitude. Their motor 

 mechanism is operated by devices having no 

 counterpart in the animal organization, but 

 is the outcome of specific adaptation. 



Moisture sense 



