ORIGIN OF PARASITISM 239 



of the tissues of the " host " cactus. Opuntia remained alive 

 for a long time, absorbing water through its parenchymatous 

 cells, but forming no roots. But Peirce's pea plants were 

 stunted. Slow growth and poor root formation were 

 characteristic of MacDougal's plants. MacDougal (191 1) 

 lays stress on the necessity of an osmotic pressure superior 

 in the parasite to that of the host. Senn and Hagler 

 (quoted from Tubeuf, 1923) have shown that Viscum, 

 Thesium, Euphrasia, Orobanche, and Pedicularis have in 

 fact osmotic pressures from 0*025 to 0*25 atmos. higher 

 than those of their hosts. Harris and Lawrence (191 6) 

 found that the Loranthaceae of Jamaica had in almost all 

 cases osmotic pressures higher than those of the hosts. 

 It is of interest to note that Harris (1918) found the 

 osmotic pressure of Jamaican epiphytes to be always 

 very much lower than that of the trees on which they 

 grow. 



It is tempting, too, to institute a comparison between 

 the behaviour of a plant grafted on another and the relation 

 of parasite to host. The stock is of course normally treated 

 so that it bears no foliage, but a scion may quite well be 

 grown on a leafy stock, and so reproduce the external 

 features of a mistletoe and an apple-tree. As we have 

 seen, however, the inter-relations are quite different. 



It is extremely doubtful whether the behaviour of the 

 scion grafted on a stock, or even such experiments as those 

 of MacDougal, throw any light on parasitism or its origin. 

 Grafts are successful only within strictly limited bounds of 

 relationship ; the graft is really a regeneration or wound- 

 healing phenomenon, which results in the formation of 

 what is, from the standpoint of nutritive physiology, a 

 single organism — the scion supplying organic compounds, 

 the stock water and salts. Parasitism is normally a relation 

 between two plants which do not stand close to each other 

 phylogenetically, and which may easily stand at the opposite 

 poles of the plant system — e.g. a Viscum on a gymnosperm. 

 It is essentially an attack of one organism on another, an 

 active incursion into its system ; and the union, however 



