86 PLANT PHYSIOLOGY 



In certain instances it is claimed that parasitism is ad- 

 vantageous not only to the parasite but also to the host. 

 These cases must be examined separately. Pure parasitism 

 is beneficial only to the parasite. Whatever may be our 

 views as to the origin of parasitism, we must admit that 

 parasitic associations are entered into each season only 

 because the parasite is a dependent organism, incapable of 

 elaborating its own complex foods from simple compounds. 

 The parasite may be wholly dependent, taking from its host 

 all the food it needs, or it may be only partly dependent, 

 taking only certain kinds of food. In either case the foods 

 absorbed are worked over, assimilated, incorporated or con- 

 sumed, by the parasite itself. No food is absorbed by the 

 parasite in forms which it can use without modification for 

 building its own body. 



The nutrition of parasites differs from that of other or- 

 ganisms only in certain stages of the process, and not fun- 

 damentally even in these. Instead of absorbing from water, 

 soil, and air the raw materials which must be elaborated 

 into foods, the parasite absorbs from its host matters al- 

 ready elaborated by its host. The means of absorption are 

 the same in parasites as in other organisms (see Chapter 

 IV.). 



Parasitism consists essentially in the absorption from a 

 living organism of more concentrated solutions of more 

 highly elaborated food-materials or foods than can other- 

 wise be obtained. Using the division of the process of nutri- 

 tion proposed on page 41, into, first, the absorption, and 

 second, the combination of food-materials, third, the assimi- 

 lation, and fourth, the incorporation of foods, we see that 

 the parasite differs from the self-sustaining and independent 

 green plant only in omitting the second stage of the process, 

 absorbing from its host the food-materials or foods already 

 elaborated, which an independent plant would elaborate for 

 itself. To make this clearer, let us examine a few instances. 



Among higher plants, perhaps the simplest, in other words, 

 the least complete form of parasitism is that of the Euro- 

 pean and American mistletoes, Viscum album and Phorn- 

 dendron villosun). The mistletoes are green perennials con- 



