42 WHAT IS BEING ALIVE? 



quently go away from a temperature that becomes unfavorable 

 to their existence. In cold weather, green plants either die or 

 temporarily suspend their life activities. They become dormant. 

 Likewise, small animals, such as insects, which might be killed 

 by cold, usually 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. 

 Animals and plants used to certain temperatures frequently die if 

 they are put in a colder or hotter climate. 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 favorable to his health ; namely, about 68° Fahrenheit. 

 The value of tropisms. A study of hundreds of experiments 

 with plants and animals shows us that their instinctive responses or 

 tropisms are of the greatest use to them. Response to a favorable 

 stimulus results in placing the living plant or animal where it can get 

 food, light, or more moisture and thus better succeed in the world. 

 In general, tropisms bring the plants or animals into adjustment 

 with their environment so that they may obtain what they need in 

 order to succeed in the surroundings in which they must live. 



Practical Exercise. Make a list of as many responses to stimuli in the plant 

 world as you can find and classify them under the headings given above. Do 

 the same thing for animals. Then make a list of your own responses and 

 classify them in the same manner. Do you differ markedly in your responses 

 from plants? From lower animals? If so, how? 



Self-Testing Exercise 



Tropisms are the (1) (2) of plants and animals to 



certain (3) in their (4) . Roots of plants react 



(5) to gravity while the stems react (6). Roots grow 



toward (7), and leaves usually turn toward the (8). 



Earthworms will move (9) the light. Tropisms help bring plants 



and animals into (10) with their (11), so that they 



can (12) there. Plants are affected by the (13) 



(14) found in the soil. Living things are affected by 



(15) and (16). 



