RESUME OF EXPERIMENTAL RESULTS. 49 



g-idity in the outer parenchymatous tissues of the stems, which undergo 

 changes simulating decortication. The zones of curvatures producing the 

 movements moved up and down the stems from base to apex in a period 

 of about 72 hours. 



Successful xeno-parasitism is dependent in the first place upon the 

 superior osmotic activity of the parasitic member of the nutritive couple, 

 although not all pairs of plants sustaining such inequality are capable of 

 becoming host and dependent, and other features act as limiting factors of 

 minor importance. The proportion of salts dissolved in the sap of the 

 experimental plants and the osmotic activity as indicated by freezing-point 

 tests undergo wide seasonal variations, as a result of which a xeno-parasite 

 may maintain itself upon a host during the period of greatest turgidity 

 of the latter and be unable to withdraw material from it during the drier 

 season, when the sap of the host is of a relatively greater concentration. 

 (See Drabble and Lake, "The Osmotic Strength of Cell-sap in Plants 

 growing under Different Conditions," The New Phytologist, vol. iv, 1905, 

 and "The Relation between the Osmotic Strength of Cell-sap in Plants 

 and their Physical Evironment," Bio. Chem. Jour., vol. iv, 1907.) 



The relative acidity of the sap of two plants appeared to be of no impor- 

 tance in the determination of their capacity to form a nutritive couple. 

 Such plants as Carnegiea undergo rapid oxidation on injured surfaces and 

 form wound-cork so rapidly as to inhibit parasitism, except by species 

 with extremely high osmotic activity, which suffer depletion of their own 

 water-balance very slowly and which take solutions from an enforced host 

 against great resistance. Agave as a xeno-parasite forms roots so profusely 

 as to destroy the tissues of the host. Experimental arrangements of xeno- 

 parasites were most successful when regenerated cuttings were inserted 

 in the bodies of the host in a resting condition, in the colder season, with 

 the concentration of the sap increasing, but before the osmotic activity had 

 reached its maximum. 





