676 



THE AMERICAN NATURALIST 



[Vol. XLVI 



are formed directly from parenchyma, and then after 

 penetrating the host, such of the periphery of the haus- 

 torium as touches non-living cortical host cells, organizes 

 cork. Finally, upon the attainment by the haustorium of 

 the woody cylinder the conductive tissue of the hausto- 

 rium opposes cell for cell the conductive tissue of the host, 

 and in such parasites as possess sieve-tubes, the sieve- 

 tubes hold a similar relation to the sieve-tubes of the 

 host. 4 It happens therefore in habitual parasites that a 

 portion of the development of the haustoria occurs after 

 the parasitic relation has been entered into, so that the 

 direction of the development of much of the tissue of the 

 haustorium is fortuitous, depending in part on the posi- 

 tion occupied by the tissues of the host. 



Duration of the Xenoparasite Eelation 

 Although induced parasitism means naturally a limited 

 period during which the artificial relation can be con- 

 tinued, this period varies greatly with the different nutri- 

 tive couples. A review of this phase of the subject will 

 not be given here, as it is completely presented in the 

 papers referred to above, but two or three of the most 

 pertinent parasitic relations will be cited. Peirce grew 

 Pi* inn sativum on Vicia Faba to maturity (Peirce, "Arti- 

 ficial Parasitism," I.e.). MaeDougal (see above) records 

 many experiments of which the following may be given: 

 Cassus laciniata was grown on Opuntia blakeana from 

 February 1, 1908, until April 19, 1909, and another cul- 

 ture, which is especially treated in this paper, lasted from 

 early autumn, 1911, to June 10, 1912. In the instances 

 where Cissus was employed roots were freely formed, the 

 stem attained considerable length and organized tendrils 

 and leaves. From these facts a large capacitv for adjust- 

 ment on the part of the induced parasites is exhibited, 

 and also a degree of physiological adaptabilitv is shown 

 which reveals something of the plasticity of such plants 

 and argues a fair suitability for the dependent relation. 



'-On the Structure of the Haustoria of Some Phaneoreamic Parasites," 



