THE PLANT BODY 27 



faces toward the most abundant light supply. In a similar 

 manner, roots which are growing in soil where the amount of 

 water or food varies on different sides of the root system are 

 able to turn toward the maximum food or water supply, as 

 when roots grow into old wells and water pipes, attracted by 

 the excess of moisture. These turnings in response to stimuli are 

 called tropisms. 



It is not surprising, therefore, that plants, probably on account 

 of their stationary habit, have developed a wonderful sensitive- 

 ness to the forces and agents of their environment, which enables 

 them to adapt themselves and their fixed architectural plan to 

 the variations in their surroundings which might otherwise 

 harm them. 



Investigation has shown that plants are sensitive to gravity, 

 sunlight, moisture, soluble substances in the soil, and various 

 other stimulating forces and materials. Indeed, this ability to 

 adjust their organs is more marked in plants than in most animals. 

 While plants are sensitive to a great variety of stimuli in their 

 surroundings, certain forces and agents are more prominent as 

 directing stimuli than others, and these will therefore receive 

 the most attention in the following discussion. 



Stimulus and response. Plant stimuli, as indicated above, are 

 usually the external forces and materials of the- environment. 

 Any difference in the intensity of such external stimuli or in 

 the direction of their application to a plant organ is capable of 

 bringing forth a response in the form of growth, food building, 

 adjusting movements, or even the death of the organism. We 

 are here concerned only with that form of response to stimuli 

 that results in definite movements which adapt the plant more 

 effectively to the daily and seasonal changes in its surroundings. 

 A moment's consideration will enable the student to realize the 

 great difference between the higher plants and the higher animals 

 as regards both the reception of an external stimulus and the 

 method, or mechanism, by which the two classes of organisms 

 respond to stimuli. The higher animals are furnished with 

 special sense organs, such as the eye for the reception of light 

 and the ear for sound ; in plants the entire surface of a leaf or 



