^ CONDITIONS OF PARASITISM IN PLANTS. 



tion of the juices were made. (Beitiage zur chemischen Kenntniss der 

 weissen Mistel {Viscum album); date unknown, but subsequent to 1860.) 



The fact must not be lost sight of that the transfer of a plant from an 

 autophytic to a parasitic condition is an extremely radical one and involves 

 serious disturbances of a group of environmental conditions. Thus, in 

 the insertion of slips of one plant in the body of another the development 

 of the roots would necessarily be carried on in confined wound cavities, in 

 which the only free oxygen available for the growth of these organs would 

 be that coming from the intercellular spaces of the host and by transfusion. 

 The lack of this substance alone would operate to prevent parasitism, while 

 in other cases the speedy formation of oxidases and peroxidases in the 

 wound tissues, together with the ready formation of corky 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 inter- 

 mediate tissues would proceed best when cut off from free aeration. 



In any consideration of this subject the facts presented by nutritive 

 couplings in which the parasitism is still in an elementary stage are of 

 extreme interest. The extensive and long-continued investigations of Dr. 

 W. A. Cannon upon the root-habits of desert plants has resulted in the 

 discovery of the parasitism of Orthocarpus purpurasceiis on various hosts, 

 and also in finding that species of the genus Krameria in the Tucson region 

 sustained a dependent nutritive relation to a number of woody hosts. 



The parasitism of Krameria presents many unusual features, among" 

 which may be mentioned that, while its roots are often devoid of absorbent 

 hairs and it appears to be wholly or very largely dependent upon supplies 

 from its hosts, yet it shows but few of the atrophies or alternations ordina- 

 rily accompanying dependent nutrition. 



