TROPISMS 



41 



Gravity. Another factor influencing both plants and animals 

 is gravity. The main or top roots of plants, for example, tend to 

 grow downward. Lateral roots, on the other hand, grow in an 

 approximately horizontal direction. Careful experiments in 

 which other forces are substituted for the pull of gravity have 

 proven that gravity is the attractive force. The stem, on the 

 other hand, grows upward. This seems to be a negative response 

 to gravity. Many animals show this response to gravity in very 

 definite ways. The maintenance of one's equilibrium is un- 

 doubtedly a response to gravity, as has been proved in some of 

 the lower animals, such as shrimps and fishes. 



Food or chemical substances. Plants are greatly influenced by 

 the presence or absence of chemical substances in the soil. You 

 have probably noticed the differences between plants that grow near 

 the sea, where salt is found in the soil, and those growing inland. 

 No one who has traveled in a country where the soil is impregnated 

 with alkali can fail to see the differences between vegetation there 



How does the seedling show energy ? How does the man ? 



and in other regions where no alkali exists but where similar condi- 

 tions of temperature and moisture are found. Since the mineral 

 salts of the soil are absorbed by the plant and later built into it, we 

 can easily see that responses of this sort are of the utmost importance. 

 Temperature. Living things are affected by heat or cold. 

 Animals and small plants that are able to move in the water fre- 



