CHArTER VII 



RECIPROCAL NUTRITIVE DISJUNCTIVE SYMBIOSIS 



In reciprocal nutritive disjunctive symbiosis one or more, but not 

 necessarily all, of the organisms concerned obtain food from one or 

 more of the others, but those that lose food and get none in return 

 are nevertheless benefited by the relationship in some other way so 

 that the symbiosis is reciprocal so far as actual benefit derived from 

 it is concerned. The outstanding examples are those concerned with 

 pollination by means of animals, especially insects, and with dis- 

 semination through the aid of animals, especially birds and ants. 

 The subject of dissemination, like that of pollination, falls under the 

 general term symbiosis only in part. Furthermore it is closely 

 related to the development of plant communities; a subject to be 

 dealt with at length in later chapters. For these reasons it will be 

 more convenient and also conducive to clearness to postpone all 

 discussion of dissemination until we are ready to discuss vegetational 

 development. The present chapter, therefore, will deal entirely 

 with pollination by animals. 



46. Insect Pollination.— Insects visit flowers, ordinarily, for the 

 purpose of obtaining food, either nectar or pollen, and while accom- 

 plishing this purpose they incidentally effect pollination. These 

 simple symbiotic relations between insects and the flowers they visit 

 involve some of the most remarkable phenomena in the whole realm 

 of Nature. No evolutionary facts are more astounding or more 

 nearly inexplicable than that the flowers of many species of plants 

 remain unpollinated unless they are visited by insects; in some cases 

 by a particular kind of insect. 



The great majority of insect-pollinated, or entomophilous, flowers 

 are monoclinous. This is probably a distinct advantage since it 

 makes possible twice as many acts of pollination for a given number 

 of insect visits than if the flowers were diclinous. That is, in the 

 case of diclinous flowers it is necessary for the insect to visit two 

 flowers, a staminate and a pistillate, in order to accomplish one act 

 of pollination, while in monoclinous flowers it is possible in each visit 

 after the first one to deposit pollen on the stigma and at the same 

 time collect more pollen from the stamens. Furthermore insects 

 which were bent only on gathering pollen would, of course, not visit 

 flowers that were only pistillate at all. 



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