Power to Adjust Parts to Surroundings 253 



the environment. Again, the chmbing roots of some Ivies, and 

 the sucking roots of some parasites, grow out at those places 

 where the stimulus of contact is felt, and therefore exactly at the 

 places where they can best serve their uses; and the places of 

 origin of even ordinary roots are largely controlled by the stimulus 

 of especially abundant moisture or minerals, which explains why 

 roots branch so profusely upon entering drains. Then there are 

 stimuli which start particular stages of growth. Thus it is a 

 stimulus given by some phase of fertilization which starts the 

 formation of the fruit in the higher plants. The advantage is 

 clear, since the fruit would be wasted, and its formation a useless 

 drain on the plant, if no fertile seed were produced; for the dis- 

 persal of the seed is the function for which the fruit exists. Stimuli 

 can also serve as signals to produce a cessation of growth, as in 

 the case of the leaves of the water-plants already considered; 

 and there are plenty of other cases w^here stimuli regulate growth 

 and development in various ways, the further consideration of 

 which we may postpone to the chapters which deal with those 

 subjects, where also we may consider the correlation and Unking 

 of stimuli, with their very important consequences. 



There is one other phase of responsiveness to stimuli which we 

 must consider at this place. It is a familiar fact about organisms 

 that they have a certain power of adjusting themselves, or be- 

 coming toned, as it were, so as to work their best under the pre- 

 vailing conditions to which they are exposed; and when they are 

 thus working in full harmony with those conditions they are said 

 to be in tone. We have a familiar illustration thereof in our 

 human affairs in the way we become accustomed to certain pe- 

 culiarities of food, temperature, fresh air, occupations, etc., to 

 such a degree that we become uneasy when exposed to any others, 

 and hasten back with reUef to the congenial conditions. Thus, 

 most of us work our best at about 70° Fahrenheit and become 

 very uncomfortable when the temperature rises to above 90°, 

 though this is still much less than the natural heat of our bodies. 



