56 THE ORIGINATION OF PARASITISM. 



In the valuation of the arransfements in which some xcno-parasites 

 developed roots and others did not, it is evident that neither would be 

 indicative of greater facility for the assumption of parasitism, since either 

 roots or stems might well give rise to specialized absorptive tissues. The 

 formation of adventitious roots on aerial stems would serve to pierce the 

 tissues of a possible host, but any notable development of these organs 

 would be impossible by reason of the smothering action of the dense tissues, 

 which would yield but little oxygen in comparison with the amount to be 

 obtained in soils or natural waters. Any closely fitting adjustment would 

 necessitate the origination of either new or highly modified structures for 

 absorption from the tissues penetrated. The absorptive structure formed 

 by the single individual of Passijiora which has been seen by Pee-Laby to 

 be parasitic on Euojiymus was formed from the base of a stem, but it is 

 clearly not a root in structure and may be fairly interpreted to be a haus- 

 torium, or new organ, although its discoverer gives it a cauline character. 

 The results of Cannon show that Krameria attaches itself to some of its 

 numerous hosts by means of direct prolongations of the root-tips which 

 penetrate the tissues, while the attachment to others is by lateral forma- 

 tions on roots which have the character of new organs. 



The assumption of a dependent relation by a plant is accompanied by 

 very marked and direct alterations from the normal, which affect every 

 stage of ontogeny of the individual concerned. The first and most appar- 

 ent deviation is one of stature, as individuals parasitically nourished are 

 uniformly smaller than the average independent individuals of the same 

 species. The root-system is less extensive and is sparsely branched; the 

 shoot, likewise, is not so tall nor so densely branched, while a very nota- 

 ble reduction of leaf-surface ensues. Practically all of the organs show 

 measurements less than the normal. The alterations in question fore- 

 shadow, in the first individual subjected to dependent nourishment, the 

 degradation or atrophy which marks the soma of the advanced parasite, 

 and the probability of inheritance of acquired characters is brought before 

 the experimenter with peculiar vividness. No data are at hand to show 

 similar effects of an abundance of organic nutriment in the food-material 

 taken in by ordinary methods of absorption, and it would be unjustifiable 

 to assume such results. 



The comprehensive evidence bearing upon the influence of cion and 

 stock upon each other obtained by Mitosch, Guignard, Rivi6re, Daniel, 

 Bailkache, Timpe, Baur, Edler, Winkler, Voechting, and others (see re- 

 sume by MacCallum, ' 'The Reciprocal Influence of Scion and Stock, ' ' Plant 

 World, XII, p. 281, 1909), shows conclusively that the alterations in par- 

 asites are not the simple direct result of decreased nutrition alone. The 

 dependent member of a couple may undergo changes in form and struc- 

 ture of its organs, alterations in ash or cellulose content, and show variations 

 in the balance of almost any food-material present, and may receive sub- 



