ECOLOGICAL 147 



like body, thus showing some marks of the degeneration so often 

 associated with thoroughgoing parasitism; but it seems hardly 

 justifiable to rank the numerous leaf-miners and stem-borers as 

 parasitic in the strict sense. They have adopted an internal predatory 

 mode of life. Similarly it seems doubtful if a species of Trypanosome 

 that lives in Euphorbias is really a parasite. It is obvious, yet worth 

 noting, that a plant, from its very nature, is capable of surviving 

 interned and external injuries which would be very serious if the 

 host or victim were an animal. 



Of plants parasitic on plants there are multitudinous instances 

 with great diversity, though not approaching that illustrated by 

 animals parasitic on animals. At the one pole are parasitic moulds, 

 mildews, and rusts, etc. ; at the other pole the dodder on the clover 

 or the toothwort on the roots of the hazel. 



Very clear among parasitic plants is the contrast between partial 

 and complete parasitism. Thus the mistletoe is a partial parasite, 

 for it is usually believed to take nothing but water and salts from 

 the tree on which it grows. Its own green leaves are capable of 

 normal photosynthesis. Yet there is no hint of reciprocity on its 

 part, as has been experimentally corroborated by defoliating the 

 bearer. In contrast to the mistletoe, the leafless and chlorophyll-less 

 dodder (Cuscuta) is a familiar illustration of complete parasitism, 

 for it depends on its host not only for water and salts, but for its 

 organic food as well, in the form of carbohydrates and proteins. 

 The common and pretty Eyebright and Yellow Rattle (Euphrasia 

 and Rhinanthus) are good instances of partial parasites; for while 

 they have green leaves and absorb soil-water, they get on better if 

 their roots come into organic continuity with the roots of neighbour- 

 ing plants, such as grasses. The related cow- wheat (Melampyrum), 

 though green, cannot survive without external aid. In the Alpine 

 Tozzia, belonging to the same family, the whole of the first year is 

 spent as a complete parasite underground; but in the second year 

 there rises a flowering shoot with yellow-green leaves. In the broom- 

 rape (Orobanche) the parasitic dependence has gone still further, 

 for the seeds will not germinate unless they are in contact with a 

 suitable host, such as the broom; there is no chlorophyll, and apart 

 from the underground absorbing roots and stock, there is only a 

 flower-stalk with a few scales. The extreme simplification of the 

 vegetative system is seen in the Rafflesias, where "the whole vege- 

 tative body of the parasite may live inside the host plant, reduced 

 to a spreading weft of undifferentiated filaments— root, stem, and 

 leaf alike lost". From this degenerate vegetative body, none the 

 less effectively adapted for absorption, there burst forth strange 

 and gaudy blossoms. One of them, Rafflesia Arnoldii of Sumatra, 

 is the largest of known flowers, with the immense diameter of a 

 yard. 



