294 LIVES OF PLANTS AND ANIMALS 



and worms. Many animals that prey on other animals are noc= 

 turnal in their habits. On the other hand, most animals can 

 be shown by experiment to respond to light by definite turning 

 movements. The well-known flight of the moth to death in the 

 flame of a candle is an example. Experiments on the tropisms of 

 insects show that a mechanical turning to the source of light is a 

 very general reaction made by all insects. 



Gravity. The roots of plants respond positively to gravity by 

 growing toward the center of the earth, while the stems respond 

 negatively by growing away from the center of the earth. If a 

 plant stem which usually grows erect is placed in a horizontal 

 position, it will soon erect itself. This response is readily seen in 

 trees and grasses which have been beaten down by wind and rain. 



If boxes containing germinating seeds are fixed on the rim of a 

 horizontally placed wheel which is rotated rapidly, a force stronger 

 than gravity is introduced and the growing stems will tend to grow 

 toward the center of the wheel and the roots will grow toward the 

 circumference. 



Water. We need only to look at the luxuriant growth of plants 

 along a stream or irrigation ditch to realize the part water plays 

 in plant life. To anyone who has visited the Imperial Valley of 

 California, where water has made the desert " blossom as the rose," 

 the role of water is evident. But an oversupply of water kills plants, 

 as we can see along the shores of any artificial lake where the trees 

 standing in the water are killed. The drying up of lakes has been 

 responsible for the extermination of many fish, just as the bringing 

 of water to new localities may mean new animal life in that locality. 



We have seen in the balanced aquarium some of the adaptations 

 necessary for life there. Plants which live entirely in the water 

 often have slender parts with finely divided leaves. Their roots 

 are apt to be short and stout. The interior of such a plant is made 

 up of spongy tissues which allow the air dissolved in the water to 

 reach all parts of the plant. If the plant has floating leaves, as in 

 the pond lily, the stomata are all in the upper side of the leaf. 



Animal life is also restricted to those forms which can easily | 

 move, feed, and breathe in water. In the case of insect larvae, 

 as the mosquito, we often find adaptations which enable them to 



