THE INTER-RELATIONS OF PLANTS AND MAN 111 



source of food. Secondly is the dependence of green plants (and 

 therefore indirectly all animals) upon colorless plants for a 

 continual supply of the inorganic materials needed for protoplasm 

 building. And thirdly is another obvious relationship as a result 

 of which both green plants and animals are preyed upon by 

 parasitic heterotrophic plants. 



Green Plants as a Food Supply 



One of the most vital of inter-relations in the web of life is that 

 in which one species acts as food for another species. As we have 

 seen in Chapter 2, green plants because of their autotrophic 

 habits are independent of the animal kingdom. It is not the same 

 for animals, however. They must secure their food in organic 

 form — as carbohydrates, fats or proteins — ^from the bodies of 

 other organisms. This means an ultimate dependence upon some 

 autotrophic species as a food supply. The very existence of the 

 entire animal kingdom therefore depends upon the presence of 

 green plants! In lakes and oceans the algae synthesize carbohy- 

 drates and proteins from the water and the salts dissolved in it. 

 Protozoa and minute invertebrates feed upon the algae; fishes eat 

 the protozoa and tiny invertebrate animals, only in turn to be 

 eaten by larger fishes. Thus a food chain links the lowly algae with 

 sharks and other aquatic vertebrates. Vegetation is also necessary 

 to assure the existence of land animals, the herbivorous forms 

 feeding upon plant life, subsequently to be eaten by the car- 

 nivorous species. There the sequence of food-linkages may 

 lead from the mighty lion to the insignificant grasses which he 

 tramples under foot. The primate group is predominantly 

 vegetarian, feeding upon succulent stems, storage roots, fruits 

 and seeds. When man diverged from his primate relatives he 

 bore with him this appetite for plant foods, in search of which 

 he ranged the woods and fields. Civilized man improved upon 

 this method of getting his food by developing the science of 

 agriculture. 



Green Plants and Our Oxygen Supply 



Supplies of the various substances necessary for life are not 

 unlimited ; animals consume great quantities of oxygen during the 



