212 The Plant World. 



full)' parasitic on Carnegiea, and on Echinocactus. Rooted 

 cuttings of agave, or the common century plant, maintained 

 themselves for extended periods upon Carnegiea and Opuntia, 

 as it had a pressure of about 1 1 atmospheres. 



A careful consideration of all the detail gathered from the 

 experiments showed that one plant might become parasitic 

 upon another only when it possessed a sap having a higher 

 osmotic activity, and hence being capable of withdrawing sap 

 from the body of the possible host. Other causes might inter- 

 vene to prohibit parasitism even when the relative osmotic 

 pressures were favorable, but such inequality was a necessity 

 in all cases. It is to be noted that the pressure of the sap changes 

 unequallv in different species with the seasons and that a plant 

 which might have a higher osmotic pressure at one season would 

 be incapable of acting as a parasite during other seasons, with 

 respect to any given host. The plants used in the tests were 

 natives of arid areas and accumulate a relatively great amount 

 of surplus water at times, thus lowering the osmotic 

 activity of the sap, while during the drier seasons water is lost 

 in such quantity as to increase the osmotic pressure. In none 

 of the analvses of desert plants hitherto made, has the sap been 

 found to show a higher activity than that of the common lilac and 

 other plants which have been found by Dixon to display an ac- 

 tivity equivalent to 25 atmospheres and even higher. It is to 

 be noted also that many plants show a capacity for automatically 

 increasing the osmotic activity of the sap when the solution in 

 the substratum becomes more concentrated, and this may play 

 a very important part in the origination and development of 

 parasites. 



The relative acidity of the sap of two plants appeared to be 

 of no importance in the determination of their capacity to enter 

 into parasitic relations, quite contrary to a common supposition. 

 The ready formation of wound tissue, or the exudation of 

 enzymatic products, would act as a deterrent to a possible para- 

 site, while the capacity for formation of adventitious, or of ab- 

 sorptive organs from the surface of stems would be an opposing 

 factor in the development of parasitic relations. The more im- 

 portant features of the specialization by which parasitism en- 

 sues may thus be put in the form of an algebraic equation, the 



