Cnapter 5 



THE INTER-RELATIONS OF PLANTS 

 AND MAN 



Very few species of plants or animals live in complete bio- 

 logic isolation, carrying on the maintenance and reproductive 

 activities described in preceding chapters uninfluenced by the 

 presence of other living things. In fact, in any habitable area the 

 plant and animal populations form a maze of inter-relationships 

 in which one species is inextricably caught in a web of life which 

 involves many others. It is almost impossible to destroy or remove 

 a single species from a locality, or introduce a new one, without 

 producing far reaching eff'ects upon numerous and often totally 

 unrelated groups of organisms. 



Some of these relationships involve only plants, such as that of 

 the semi-parasitic mistletoe deriving its existence while attached 

 to the limb of a sycamore, or that of the forest trees forming a 

 shaded floor where mosses and ferns can thrive in the damp 

 twilight. Other relationships involve only animals as is the case of 

 the food-chains linking carnivorous animals and their prey, or 

 parasitic tapeworms with the mammals whose intestinal tract 

 they are inhabiting. Such relations, of which there are many, 

 often result in profound structural and physiological changes 

 during the evolution of the organisms involved. But the inter- 

 relations which occupy our attention in this chapter are primarily 

 those showing the mutual inter-dependence of plants and animals, 

 and plants and man. Since man, in his biologic heritage, is a 

 member of the animal kingdom, it is relevant to discover the 

 fundamental relations of plants to animals, and thus realize why 

 certain relationships exist between plants and man. 



There are three fundamental ways in which the plant and 

 animal kingdoms influence each other. First and most obvious, is 

 the dependence of all animal life upon green plants as a basic 



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