44 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. 



plants normally rooted in the soil. The explanation for this action is to 

 be found in the high ash-content, implying- great osmotic activity of the 

 juices of the xeno-parasite. An analysis of material taken from the ter- 

 minal branches of a plant on January 12, 1910, gave an acidity of the sap 

 equivalent to 0.483 gram H^SO^ per 100 cc, the total solids being 12.040 

 grams and the inorganic material or ash 2.052 grams in this quantity. 

 A second determination was made on February 9, 1910, in which the total 

 solids were found to amount to 10.66 grams per 100 cc. of sap, the inor- 

 ganic material or ash being 2.10 grams. The ash did not differ materially 

 in the two analyses, and the difference in the amount of total solids obtained 

 may be attributed to the colloidal nature of the material obtained by press- 

 ure. The average of three determinations of the freezing-point indicated 

 an osmotic activity of 11.98 atmospheres at a temperature of 25° C. This 

 is of importance, as it was the condition existent at the time successful 

 preparations were set up. The concentration doubtless increases with aridity 

 of the dry foresummer and then falls with the summer rains and the growth 

 and expansion of the stems. (For osmotic activity of Carnegiea see p. 35. ) 



The probability of Opuntia becoming at least partially parasitic on Carne- 

 giea led to careful observations in the field on all of the expeditions from the 

 Desert Laboratory. A prickly pear consisting of several healthy joints was 

 seen growing from a cavity made by "carpentarios" in the summit of a 

 Carnegiea about 80 miles west of Tucson in November, 1907 (see plate 9). 

 Neither host nor parasite was disturbed. A plant of Opuntia fiilgida about 

 50 cm. high occupies the soil in a dirt-filled cavity in a stump of Carnegiea 

 on the Yuma Mine road, 9 miles west of Tucson, but here the arrange- 

 ment is hardly parasitic. A small Opuntia blakeana issuing from a knot- 

 hole in the base of a stem of Parkinsonia micfophylla was found in 1906 

 and was dissected for examination. The roots were normally developed, 

 occupying a mass of humus and soil that had fallen in through a second 

 cavity on the opposite side of the small trunk. The proportion of humus 

 was much greater than that of soils ordinarily occupied by this plant, hut 

 nothing more than mechanical parasitism might be attributed to the union. 

 The Opuntia was preserved, being set in the soil at the Desert Laboratory, 

 and showed a vigorous growth when transplanted. 



A similar occurrence of Opuntia on Acacia greggii, 18 miles east of Tuc- 

 son, was found by Dr. Cannon in 1909. (Plate 10, A.) 



EUPHORBIA-OPUNTIA. 



Freshly made cuttings of Euphorbia (an African species with cylindrical 

 stems), which had been growing in a shelter near the Desert Laboratory, 

 were inserted in stems of Opuntia versicolor and O. discata early in March, 

 1908. Free cuttings quickly desiccated, but many of these remained fairly 

 normal for 2 months, one opening 3 flower-buds in succession. No roots 

 were formed, however, and probably the unions were of the nature of 

 grafts. No care was taken to shade the preparations, and no opportunity 



