2. CONDITIONS OF PARASITISM IN PLANTS. 



The mechanical adhesion of the bodies of seed-plants which would make 

 parasitism possible migrht be broug-ht about in three different ways: (1) 

 Roots growing thickly interlaced in the soil might unite or penetrate each 

 other; (2) adventitious roots arising from internodes at any place on the. 

 aerial stems might pierce the bodies of other plants ; (3) seeds lodging 

 in the bark or in the wounds of a plant might germinate and send absorb- 

 ing organs into the tissues of the possible host. 



Of these methods that of incidental root-parasitism seems to bear the 

 greater probability of occurrence and to be illustrated by some very striking 

 examples. Pe^-Laby has recently described a case in which the main 

 root of a plant of Passiflora ccerulea had become attached to a root of 

 Euonymus japonicus, forming a specialized absorptive tissue and under- 

 groingr general atrophy of its own root-system, in a manner suggestive of a 

 highly developed degree of parasitism, although, of course, no hint of this 

 was yet to be seen in the shoot of the Passiflora. (Pe^-Laby, M. E. La 

 Passifiore parasite sur les racines du fusain, Rev. Gen. d. Bot., xvi, 453, 

 1904.) 



Similarly Cannon has discovered that Krameria canescens and K. parvi- 

 folia, desert plants hitherto taken to be autophytic, may fasten upon the roots 

 of a dozen species near which they grow habitually, the structures of the 

 roots being indicative of a stage of modification not far advanced toward 

 complete parasitism. A full description of this matter is given on pages 

 5 to 24 of this paper. 



The germination of seeds on the bodies of other plants might result in 

 mechanical parasitism, the advantage being purely one of position with 

 respect to light, and this feature is illustrated by many hundreds of exam- 

 ples, particularly abundant in tropical forests. The development of any 

 form of nutritive dependence out of such purely mechanical relations has 

 not yet been demonstrated, althougrh various workers, notably Pcirce, have 

 made extensive demonstrations of the possibility of short-lived annuals 

 completing their entire cycle of existence as parasites upon enforced hosts, 

 in the tissues of which their seeds were germinated or were set at an early 

 stage of their existence. (Peirce, G. J., Contribution to Physiology of the 

 Genus Cuscuta, Ann. Bot., vol. xii, 53, 1894; Das Eindring-en von Wur- 

 zeln in lebendige Gewebe, Botan. Ztg., ill, 169-176, 1894; and Artificial 

 Parasitism: A Preliminary Notice, Bot. Gazette, xxxviii, 214-217; 1904.) 



It is also reported that grape-culturists in France insert seeds of the vine 

 in old living stems of the same plant a short distance above the ground, 

 and that the developing plantlets send roots through the tissues which 

 eventually reach the soil and ramify in it. Many of the phenomena con- 

 sequent upon grafting operations are also of interest in this connection. 

 Such unions may be of all degrees of intimacy, varying- from perfect grafts 

 where the vessels unite to other cases in which the cion is separated from 

 the stock by a layer of dead tissue, through which there can be no free 



