Factors Affecting Growth and Reproduction 223 



W. A.Orton. U.S. D. A. 



Figs. 127 and 128. A healthy potato plant and one showing the mosaic disease. The 

 organisms that produce disease are a most important factor in the environment of both 

 wild and cultivated plants. 



Man, more than all other animals put together, has modified 

 the natural vegetation of the earth. In some cases he has de- 

 stroyed 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. If he understands the interrelations 

 of the processes occurring in plants and how these processes are 

 affected by the various factors of the environment, he may secure 

 desirable modifications of both the vegetative and reproductive 

 structures of the plant. 



Other plants as an environmental factor. Other plants, such 

 as weeds growing among cultivated crops, may modify the en- 

 vironment of plants by shading them and by removing water and 

 soluble salts from 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. These are only two out of 



