STEUCTUEAL EELATIONS IN XENOPAEASITISM 



W. A. CANNON 

 Desert Botanical Laboratory 



At various times normally independent plants have 

 been experimentally caused to grow and develop within 

 the tissues of other independent plants, deriving from 

 this arrangement food and food-materials and organizing 

 tissues and organs. 1 Although in themselves short-lived, 

 the artificial parasites offer interesting suggestions as to 

 the possible conditions under which true parasitism may 

 arise in nature. 2 It is clear, for instance, that the mutual 

 relation of parasite and host is extremely complex, both 

 from a purely physiological point of view and from a 

 structural one. On the one hand, it presupposes suitable 

 osmotic relations and not unfavorable chemical reactions, 

 and on the other, among other things, the fitting and exact 

 adjustment of the tissues of the parasite, and it signifies 

 atrophies as well. 



When we observe the leading structural changes which 

 normally occur in the growth of a haustorium of a habi- 

 tual parasite, such, for example, as the mistletoe, 3 we find 

 a course of development which is full of suggestions. A 

 young haustorium is composed mainly of undifferentiated 

 ground tissue, but there are the beginnings of conductive 

 tissue within, and a protective epidermis without. Upon 

 the commencement of the parasitic relation the most 

 marked changes occur. In the first place epithelial cells 



1 "Artificial Parasitism, etc.," G. J. Peirce, Bot. Gas., 38: 214, 1904. 

 "The Condition of Parasitism in Plants," D. T. MacDougal and W. A. 

 Cannon, Publ. No. 129 Carnegie Inst, of Wash., 1910. "An Attempted 

 Analysis of Parasitism," D. T. MacDougal, Bot. Gas., 52: 249, 1911. 



2 "An Attempted Analysis of Parasitism," D. T. MacDougal, Bot. Gas., 

 52: 249, 1911. 



8 "The Anatomy of Phoradendron villowm," W. A. Cannon, Bull. Ton. 

 Sot. Club, 1901. 



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