The Making of Parasites. 211 



substance alone would operate to prevent parasitism, while in 

 other cases, the speedy formation of oxidases and peroxidases 

 in the wood tissues, together with the ready formation of corkv 

 layers and mucus, would hinder root formation. None of these 

 factors would be detrimental to actual grafting, since in this 

 case the union of living cells and the intermediate tissues would 

 proceed best when free aeration was prevented. 



Some of these experimentally arranged parasites have en- 

 dured for more than two whole seasons, forming roots which 

 penetrated the soft bodies of the hosts in some instances, while 

 other species absorbed juices directly through the epidermal sur- 

 faces of the healed bases of the slips inserted. 



The spread and surface of leaves and branches developed 

 from the inserted parasitic slips were, in all cases, less than that 

 of similar parts which still formed a part of the original plant 

 from which cuttings were made. The atrophies or reductions 

 characteristic of parasites were thus seen as a direct response of 

 the individual to dependent nutrition, foreshadowing the degra- 

 dation or atrophy which reaches advanced stages in species 

 which have followed a long course of parasitism. 



After a great number of experimental trials had been made, 

 in which the failures proved no less interesting than the successes, 

 a series of analyses were made to determine the concentration 

 or osmotic activity of the sap of the species used in the tests. 



The results of these analyses and of the general observations 

 show that the Mexican grape vine with an osmotic activity of 

 12 atmospheres was successfully parasitic on a small prickly 

 pear (Optintia blakeana) in which the mucilaginous juice had a 

 pressure of 9 atmospheres, and on the barrel cactus {Echino- 

 cactus) at a pressure of 6 atmospheres, but not on the giant tree 

 cactus {Carnegiea gigantea) at 7 atmospheres by reason of the 

 copious gummy exudation and fermentation products which fol- 

 low wounds in making the necessary incision in the body of the 

 host. Rooted joints of the prickly pear were parasitic on the 

 barrel cactus (see above), especially when the slender cylindrical 

 joints produced by the prickly pear grown in darkness were used 

 as parasites. The cylindrical prickly pear {Opuntia versicolor) 

 with a juice showing a pressure of 12 atmospheres was success- 



