XENO-PARASITISM: THE EXPERIMENTAL INDUCTION OF 

 DEPENDENT NUTRITION. 



MATERIAL AND METHODS. 



The material chosen for experimentation in the present work included 

 chiefly several opuntias which were used as both host and parasite, nor- 

 mal and in etiolated condition; Carnegiea gigantea exclusively as host; 

 Echinocachts wUHzenii as host; Fotiquieria splendens as both host and para- 

 site; Cissies digitata and C. laciniata from southern Mexico; Agave amen- 

 avici, Cotyledon macrantha, and Tradescantia as parasites in addition to a 

 number of other forms, of which but slight use was made. After some 

 preliminary tests the arrangement of the material in the dependent relation 

 was begun seriously in January, 1908, and many hundreds of preparations 

 have been set up since then. 



For the sake of brevity, the preparations are characterized by the names 

 of the two plants used, separated by a hyphen, the xeno-parasitc being 

 placed first. Thus Cissus-Opuntia designates an experiment in which slips 

 of Cissits were inserted in the stems or joints of Opuntia. 



In addition to the preparations described in the following pages, some 

 interesting examples of mechanical parasitism, or pseudo-parasitism, were 

 observed among the cacti . Such occurrences are by no means common in 

 the Sonoran desert, although abundant in southern Mexico, especially in 

 the Tehuacan district. 



The manipulation of the living material was carried on solely by Dr. 

 D. T. MacDougal, who set up and dissected all of the arrangements. The 

 chemical analyses were made by Dr. W. H. Ross, of the Arizona Agricultural 

 Experiment Station at Tucson, who furnished data which bear upon the 

 composition of the sap of the plants tested. Beckman's apparatus for the 

 determination of freezing-point was used in obtaining osmotic pressures, 

 and Dr. Ross gives the following description of his methods in estimating 

 the content and acidity of the sap. 



TOTAL AND INORGANIC SOLIDS. 



When making a determination of the total solids, 25 c.c. of juice or 75 c.c. 

 in those cases in which the juice had been diluted 1:2, were measured 

 into a 250 c.c. flask, which was then filled to the mark with water; 50 c.c. 

 of this solution were evaporated in a shallow platinum dish on the water- 

 bath and dried to constant weight in a vacuum-oven at a temperature of 

 75° and pressure of about 50 cm., in order to guard against any decompo- 

 sition of the organic matter. More satisfactory results were obtained in 



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