HOW PLANTS AND ANIMALS REACT 17 



Food or Chemical Substances. We shall see later that plants are 

 greatly influenced by the presence or absence of chemical sub- 

 stances in the soil. Since such substances are absorbed by the 

 plant and later built into the organism, we can easily see that 

 responses of this sort are of the utmost importance. As we well 

 know, animals, including man, are much influenced by some kinds 

 of chemical substances that we call foods, and may be seriously 

 affected by other combinations of chemicals called poisons. Re- 

 sponse to chemical substances is called chemot'ropism. 



Temperature. Living things are affected by heat or the absence 

 of it. Animals and small plants that are able to move in the 

 water frequently go from a cooler to a warmer part of the fluid, 

 or away from a temperature that becomes unfavorable to their ex- 

 istence. They are thus ssiid to show thermotropism. In cold weather 

 green plants either die or temporarily suspend their life activities, 

 becoming dormant. Likewise, small animals, such as insects, may 

 be killed by cold or may hibernate under stones or boards. Their 

 life activities are slowed down until the coming of warm weather. 

 Bears and some other large animals go to sleep during the winter and 

 awake, thin and hungry, on the approach of warm weather. Ani- 

 mals and plants used to certain temperatures are killed if removed 

 from them. Even man, one of the most adaptable of all animals, 

 cannot stand great changes without discomfort and sometimes 

 death. He heats his houses in winter and sometimes cools them 

 in summer so as to have the amount of heat most acceptable to 

 him, i.e., about 70° Fahrenheit. 



The Value of Tropisms. — A study of hundreds of experiments 

 with plants and animals shows us that tropisms are of the greatest 

 use to them. Response to a favorable stimulus results in placing 

 the living plant or animal where it can better succeed in the world. 

 And in general, tropisms bring the organism into adjustment with 

 its environment. 



The Environment determines the Kinds of Animals and Plants 

 within it. — In our study of geography we learned that certain 

 luxuriant growths of trees and climbing plants are characteristic 

 of the tropics, with their moist, warm climate. The tropical jungle 

 is often a tangle of long climbing plants, the leaves festooned over 

 the trunks of tall trees, while the jungle floor is covered with a 



