iO LIVING THINGS AND THE ENVIRONMENT 



things need water, in order to sustain life. The roots of green 

 plants grow toward a source of water. Some animals appear to 

 be stimulated to move toward water, whereas others move away 

 from moisture. In the words of science, they show hydrot'ropism, 

 and are positively or negatively hydrotrop'ic. Water is of so much 

 importance to man that from the time of the Caesars until now" he 

 has spent enormous sums of money to bring pure water to his cities. 

 The United States government has spent millions of dollars on 

 irrigation to bring the water needed to support life in the western 

 desert lands. 



Light. Light is another important factor of the environment. 

 A study of the leaves on any green plant growing near a window 



will convince one that the 

 stems of such plants grow 

 toward the light, and that 

 the leaves are held in such 

 positions that they get a 

 maximum amount of sun- 

 light. All green plants are 

 thus influenced by the sun. 

 Other plants which are not 

 green seem either indiffer- 

 ent or negatively influ- 

 enced by the stimulus of light. The direction, as well as the 

 intensity of light, is an important factor. Animals may or may 

 not be attracted by light. A moth, for example, will fly toward a 

 flame ; an earthworm will move away from light. Movements 

 toward or away from light are known as positive or negative 

 heliot'ropism (Gr. helios, sun) or photofropism. Some animals 

 prefer a moderate or weak intensity of light and live in shady 

 forests or jungles, prowling about at night. ■ Others seem to need 

 strong light. Man himself is most comfortable and works most 

 efficiently in a moderate intensity of light. 



Gravity. Another factor influencing both plants and animals 

 is gravity. Roots of plants, for example, grow downward and are 

 thus said to be positively geotrop^ic. The stem, on the other hand, 

 grows upward; it is negatively geotropic. Many animals show this 

 response to gravity — geot'ropism — in very definite ways. 



The effect of light upon a growing plant. 



