Till; ORIGINATION OF PARASITISM. 55 



(ash, 1.15). Echinocactus at 5.72 atmospheres (ash, L.20), and Carnegiea 

 at 6.78 atmospheres (ash, 1.00), for two seasons or more 



It is obvious that the form which showed the greatest depletion of its 

 water-balance would be least liable to afford support for a parasite, and 

 no species has been seen to maintain itself on a host with superior osmotic 

 power, but in consideration of this matter other conditions are to be taken 

 into account. Among - these is the capacity of accommodation displayed 

 by some plants, in which the encounter of the absorbing - organs with a 

 superior concentration of solutions in the medium is followed by an auto- 

 matic increase in the osmotic activity of the cell-sap in the absorbent cells, 

 which might or might not be participated in by the whole body. Detection 

 of this fact in algae and fungi, its demonstration in the root-hairs of salt-marsh 

 plants by Hill (Observations on the Osmotic Properties of the Root-hairs 

 of certain Salt-Marsh Plants, The New Phytologist, vol. vn, p. 133, 1908), 

 the discovery by Livingston that desert plants might sustain themselves 

 for a time in soils which showed an absorbent power for water superior 

 to that of the plant, and the results of Peirce, who determined that peas 

 grown on vines made artificially parasitic on Vicia had a higher osmotic 

 activity than that of peas taken from plants normally nourished, constitute 

 evidence of the greatest interest in this connection. Such automatic vari- 

 ations would undoubtedly be an important factor in periods of extreme 

 desiccation of a host. 



It was thought that acidity of the sap might be a possible factor and that 

 a parasite could not sustain itself on a more highly acid host. The most 

 successful xeno-parasite is Cissus, with an index of 215, which grew on 

 Opuntia (120) and Echinocactus (89) for extended periods, and on Carnegiea 

 (151) with less success. On the other hand, Agave (104) and Opuntia 

 (120) grew on Carnegiea (ranging from 151 to 188). The cylindrical 

 Opuntia (483), much more highly acid than the prickly pears, was suc- 

 cessfully parasitic on Caryiegiea (151 ) and Echinocactus (89). Relative 

 acidity must be reckoned as only a minor factor in view of these facts. 



The insertion of Cissus which proved parasitic, on Opuntia developed 

 adventitious roots which pierced the tissues of the host. Agave likewise 

 developed adventitious roots, which in some cases were so numerous as to 

 destroy great tracts of the host. Opuntia used as a parasite did not de- 

 velop roots, but remained with living epidermal cells in more or less direct 

 contact with the epidermal or cortical tissues of the host, exhibiting much 

 the same behavior as certain plants in grafts. Here, then, are two forms 

 of parasitism, one in which roots were developed and another in which they 

 were not. That it is not necessary to develop root-organs for absorption 

 is shown very clearly by Cuscuta, where the papillar extrusions of epider- 

 mal tissues were seen by Peirce to carry on absorption before the haustoria 

 took form or showed differentiation of tissues. 



