PLANT PROPAGATION 



117 



to the light and air, and those that are most vigorous usually get 

 above and shade the others. Again, the supply of water and mineral 

 food in the soil of a given area is limited ; hence, plants that cannot 

 get what they need are dwarfed and finally starved to death. In 



FIG. 54. Charles Darwin. 



the third place, injurious insects destroy an enormous amount of 

 vegetation, the loss of cultivated crops alone from this cause being 

 estimated at $700,000,000 annually. Frosts, dry seasons, heavy 

 rains, and fungous diseases are other important factors in the life 

 of many plants. And so if we were able to see what is actually 

 going on in each square foot of the earth's surface, whether of forest, 



