THE CONDITIONS OF PARASITISM IN PLANTS. 



DEPENDENT NUTRITION IN SEED-PLANTS. 



The number of seed-plants which obtain water and dissolved substances 

 from the bodies of other living- org-anisms is large, althottgh the species 

 exhibiting- dependent nutrition are confined to a few families. In anj-- 

 nutritive couple, whether in parasitic arrangement or in mycorrhizal sym- 

 biosis, the plant deriving greatest advantage from the association usually 

 displays a more or less, marked lack of differentiation of the tissues, which" 

 may extend even to the seed and embryonic structures and is also charac- 

 terized by a lessened production of chlorophyll. The capacity to construct 

 this important material has been entirely lost by some forms. The easily 

 made assumption that these morphogenic effects are due to the availability 

 of organic food-material is not borne out by experimental cultures of plants 

 in which substances of this kind were furnished in ample proportions. 

 The effects in question seem to be produced only when two organisms 

 assume a direct physical contact by which the ready passage of material 

 from the body of one to the other is made possible, and this generalization 

 also appears to be capable of strict application to animals. 



The studies of the general anatomical features presented by the families, 

 the members of which exhibit parasitism, have so far failed to yield any 

 conclusions as to the morphological features which might be favorable to 

 such arrangements. The specialization of tissue which ensues when a 

 seed-plant becomes parasitic fortuitously is far more striking than any 

 simple anatomical character which might be interpreted to indicate a pre- 

 disposition to the dependent habit of nutrition in autophytes. 



Theoretical considerations lead to the belief that it is to purely physio- 

 logical features and to the habits of green plants that we must look for 

 the conditions favorable to the organization of parasitism. Evidence to 

 the effect that a number of green plants may take in organic compounds 

 through their membranes is increasing, and this capacity would facilitate 

 parasitism as at present understood. The course of development of the 

 absorbing organs, their mechanical relations to the bodies of other plants, 

 and the concentration of the fluids in the cells of the possible parasite would 

 be features to which attention would naturally be directed in any attempt to 

 ascertain the method of origin of this method of nutrition. Some results 

 of importance with respect to this matter are presented in the following 

 pages. 



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