680 



THE AMERICAN NATURALIST 



[Vol. XLVI 



Tissues of the Host 



The structure of the flat stems of Opuntia, broadly 

 speaking, consists of thin-walled, large parenchyma, 

 through which there course strands of conductive tissue. 

 Protection of the stem is afforded by a heavy cuticular- 

 ized epidermis. 



When the parasitic relation is entered into, wound 

 tissue, with heavy outer walls in certain cells similar to 

 those of the cork, is formed about the injury caused by 

 the introduction of the cutting. The cutting sends out 

 adventitious roots which penetrate the parenchymatous 

 tissue of the host, and sooner or later these roots are 

 surrounded by wound tissue which the host promptly 

 organizes as a result of the unusual stimulation. By 

 this formation the water-storing ground tissue of the 

 host is separated from the living cells of the parasite. 



Tissue Relations or Pakasite and Host 

 In rapidly growing roots, contact is made with the 

 living parenchyma of the cactus, and the parasite is in 

 physical position to absorb foods and food materials. In 

 instances, however, where root growth is slow, wound 

 tissue is formed by the cactus, and the parasitic relation 

 is not favorable for absorption. Following the forma- 

 tion of wound tissue cork is organized by the parasite, 

 so that the cushion of non-living material separating host 

 and parasite in the older portions of the culture, comes 

 to be derived from both species. 



When one compares the structural relations of a haus- 

 torium of a habitual parasite with the analogous absorb- 

 ing organ of such a xenoparasite as Cissus, several sug- 

 gestive inferences may be derived. The relation may be 

 presented briefly in the following parallel : 



No special digestive cells. Epithelium developed. 



Root-hairs suppressed. No root-hairs formed. 



