444). If there were only green plants in the world, all the carbon dioxide would 

 at last be used up. Any plant that died would permanently retain its carbon 

 compounds and so keep carbon out of circulation, since under such conditions 

 nothing would decay. 



The Food Cycle^ Under the sod, where it is too dark for green plants 

 to make new carbohydrates, we find hundreds of species of bacteria, fungi, 

 larvae of various insects, snails, moles, ant colonies, many kinds of "worms" 

 and perhaps snakes. Some of these organisms live on the roots of plants that 

 hold their crowns or leaves above the ground. The larger or the more active 

 of the animals move out of their burrows and gather food abo\'e the ground. 

 The ants, for example, forage on leaves, on various bits of dead organic matter, 

 and on plant lice. Sometimes a swarm of ants will attack a living caterpillar 

 or other insect that is not too active. 



Earthworms live on dead leaves and other plant parts, and on dead organic 

 particles in the soil. A worm swallows masses of earth and digests the organic 

 contents in the food tract. Snakes come out for their prey, as do ants. Through 

 the processes of decay, bacteria and fungi release the proteins, fats and car- 

 bohydrates locked up in dead plants and animals. As a result, these organic 

 compounds break down into carbon dioxide, water, urea, ammonia salts, and 

 other nitrogenous compounds. 



Many of the inhabitants of the soil are parasitic on others. And all plants 

 and animals discharge into the soil some of the products of metabolism, or 

 wastes. As a result, carbon, nitrogen, sulfur, and other materials return to the 

 air and water and soil, and become again and again incorporated in living 

 bodies, taking part for longer or shorter periods in "being alive" (see illus- 

 trations, pp. 150, 151). 



Each organism that is not a food-maker gets food from others, and in turn 

 supplies food to others. Plants and animals thus stand in a sort of continuous 

 food "chain". This is not exactly a friendly give-and-take, since it seems to 

 run in one direction only. Beginning with the simplest chlorophyl- bearing 

 plants, the species in a food chain become generally larger and larger. 



There is, however, a limit to this chain. This does not mean that the largest 

 trees or the largest animals are free of all enemies. It means merely that there 

 are other ways of getting food besides that of destroying smaller neighbors. 

 As practically everybody knows, small fleas "have smaller still to bite 'em; 

 And so proceed ad infinitum^ Parasites are also links in the iood chain. In 

 this series the plants and animals become smaller and smaller, although, as 

 with the main food chain, there are exceptions at many points. It does not 

 follow, for example, that flesh-eating "cats" are larger than those vegetarian 

 deer upon which they prey. A lion can successfully attack a giraffe. 



Flesh-eating animals that travel in packs or work in gangs often have ad- 



^See Nos. 1, 2, and 3, p. 576. 

 560 



