PREVIEW 55 



its bark ; while an examination of the soil around its roots would 

 show us many other living forms such as the pupae and nymphs of 

 insects. Perhaps our tree might have queer looking growths called 

 galls on the leaves or stems. These, if examined, would be found 

 to be the homes of tiny insects and bacteria. If the tree had 

 flowers we should be sure to see numerous insects on them. 



Each insect has its own favorite food plant or plants, and in 

 many cases the eggs are laid on the plant so that the young may 

 have food close at hand. Some insects like the rotted wood of 

 trees. An American zoologist, Packard, has listed 462 species 

 of insects that live upon oak trees alone. Everywhere insects are 

 engaged in taking their nourishment from plants, and millions of 

 dollars of damage is done every year to gardens, fruits, and cereal 

 crops by these animals. Insects in turn are the food of birds ; 

 cats and dogs may kill birds ; lions and tigers live on large defense- 

 less animals such as deer or cattle ; and finally, man eats the 

 bodies of both plants and animals. But if we reduce this search 

 for food to its final limit, we see that green plants provide all the 

 food for animals. For the lion or tiger eats the deer which feeds 

 upon grass or green shoots of young trees, and the cat eats the bird 

 that fives on weed seeds or on insects that eat plants. Green 

 plants supply the food of the world. 



On a field trip no one can fail to observe that plants often give 

 animals a home. The grass shelters grasshoppers and smaller 

 insects. Some insects, such as the tent caterpillar, build their 

 homes in the trees or bushes on which they feed, while others 

 tunnel through the wood, making homes there. Spiders build 

 webs on plants, often using the leaves for shelter. Birds nest in 

 trees, and many wild animals use the forest as their home. Man 

 has learned to use many kinds of plant products to aid him in 

 making his home, wood and various fibers being the most important 

 of these products. 



So far it has seemed as if green plants benefited animals and 

 received nothing in return. We shall see later that plants and 

 animals together form a balance of life on the earth and that each 

 is necessary for the other. Certain substances found in the 

 body wastes of animals are necessary to the fife of a green plant. 



