150 Science of Plant Life 



It is estimated that grasshoppers and leaf-hoppers often eat 

 as much of the grass in a pasture as do the farm animals. 

 Plant lice and scale insects remove the sap from the cells of 

 the tender growing parts and may kill the entire plant. Other 

 animals, like the earthworm, favor the growth of plants by 

 loosening the soil and promoting the change of fallen leaves 

 to humus. Herbivorous (Latin : herba, herb, and vorare, to 

 eat) wild animals, like the rabbits, squirrels, and deer, markedly 

 affect natural vegetation, while the domesticated cattle, sheep, 

 and hogs to a large extent determine what plants can survive 

 in pastures and grazing lands. Man, more than all other 

 animals put together, has modified the natural vegetation of 

 the earth. In some cases he has destroyed it ; in other cases 

 he has encouraged and protected it. Most of all, he has 

 selected certain plants and made of them the food supply of 

 the world. 



Other plants as an environmental factor. Other plants, 

 such as weeds growing among cultivated crops, may modify 

 the environment of plants by shading them, by removing water 

 from the soil, and possibly by producing poisonous substances 

 in the soil. Or a plant may directly affect another plant 

 by growing on it and taking its nourishment from it. For 

 example, the mistletoe grows on trees and injures them. Corn 

 smut and wheat rust live on corn and wheat and decrease 

 or prevent the production of grain. 



The complexity of the environment. The environment of 

 plants is made up of many factors, and the factors them- 

 selves are more or less dependent upon each other. Conse- 

 quently it is often difficult to determine definitely the cause 

 of a particular effect that is undoubtedly produced by some- 

 thing in the environment. But as our knowledge of botany 



