Induced and occasional parasitism 
D. T. MacDOouGAL 
(WITH PLATES 22-25) 
The results of some experiments in which regenerated cuttings 
of several succulent seed plants were made to become xenopara- 
sites on various hosts have been published by the author during 
the last two years.* 
Regenerated cuttings of vines and of cacti were inserted in 
cavities prepared for them in the bodies of other plants used as 
hosts, being held in place by plaster of Paris seals, and the course 
of growth followed for extended periods. A consideration of the 
morphological and chemical features of the parasite and host of 
all such nutritive couples was then made, to ascertain what factors 
made it possible for one plant to become dependent or parasitic 
on another. It was notable that outside of the single anatomical 
feature of the possession of a water balance or accumulated supply 
of fluid, no morphological characters might be taken as indispen- 
sable in possible parasitism. It might be seen, however, without 
recourse to experimental results, that certain habits of growth, of 
rapid formation of wound tissue and of excretions might well pre- 
vent a plant from being fastened upon by a parasite. The chem- 
ical examinations showed that when one form was induced to 
become parasitic upon another, the host in every case had a sap 
with a lower osmotic activity than that of the xenoparasite. At 
the same time a cutting of a species with high osmotic pressure 
might fail to establish itself on another plant of lower activity. 
This was especially true of the sahuaro, or great tree cactus (Car- 
negiea). Its succulent stems, with soft subepidermal structures, 
offered most inviting conditions to the experimentalist, but the 
acrid secretions poured into wounded cavities generally prevented 
inserted cuttings from withdrawing solutions that might serve as 
* See The condition of parasitism in plants. Carnegie Inst. of Washington. 
Pub. No. 129. 1910; The making of parasites. Plant World 13: 207. 1910; and An 
attempted analysis of parasitism. Bot. Gaz. 52: ——. _tIg11. (In press 
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