480 RELATION TO ENVIRONMENT. 



vigor of many plants. Birds and other animals aid in distribu- 

 tion by eating fruit and seeds, many of the seeds being evacu- 

 ated unharmed, while in some seeds their germinating power is 

 increased by passage through the alimentary canal of animals. 

 Squirrels bury nuts, many of which are forgotten, and are thus 

 in a condition to germinate. Some animals carry in their hair 

 or fur seeds which are provided with grappling appendages. 

 The various mechanical adaptations for pollination and seed 

 distribution, the various means for protection, might also be 

 mentioned. 



938. Factors changing the soil. Burrowing animals bring 

 about a rude sort of accidental culture, perhaps giving rise in the 

 past to certain weed types. Certain ants cultivate grains for 

 food. Of the greatest importance to vegetation is the burrowing 

 work of earthworms, since they loosen up heavy soils, permit 

 access of air, of humus, and often make it possible for roots to 

 penetrate into compact soils. When they come to the surface 

 their earthy excrement covers leaves and assists in decay, while 

 the alkaline excretions of their bodies neutralize to some extent 

 the humic acid in the woods. Man in his cultural operations 

 has profoundly changed the face of nature, produces immense 

 crops of certain useful plants, and, by taking advantage of a 

 knowledge of laws of evolution, is turning his attention to guiding 

 evolution of more useful plants. 



939. The active factor in plants. Plants themselves are im- 

 portant biotic factors influencing vegetation. The shade of the 

 forest, or of other layered plant societies, protects vast numbers 

 of plants, gives protection to vast numbers of climbers, epiphytes, 

 etc., while all are apt to harbor parasites when living, but cer- 

 tainly are the prey of the scavenger members when dead. These 

 immensely important members of all plant societies reduce dead 

 vegetation to humus, and eventually to a condition in which it is 

 again available for food for the higher plants. In this phase of 

 the work the lower organisms often join forces with the roots of 

 the higher plants in symbiosis or mutualism. Then there are 

 the bacteria which fix nitrogen, the legume tubercle organism, 



