474 MacDovucGaL: INDUCED AND OCCASIONAL PARASITISM 
nutriment. The sole survivors on this plant as host are two 
opuntias, one a cylindrical and the other a flattened form, while 
the failures may be numbered by the score. 
Opuntias with mucilaginous juices also offered unfavorable con- 
ditions for parasites. While this work was being done, estimates 
of the acidity of the sap of various cacti were made of specimens 
taken at random without regard to the time of day, exposure to 
light, or temperature. No connection could be established be- 
tween acidity and parasitism with such data at hand. Recently, 
however, Prof. H. M. Richards has carried out some work on respi- 
ration at the Desert Laboratory, in which it was found that the 
acidity of a cactus is four times as great at sunrise as late in the after- 
noon, and that the amount of acid present is affected in an im- 
portant manner by the rise and fall of the temperature. These 
variations together with the effect of the changing acidity upon 
the absorptive capacity of the mucilages and of other colloids, such 
as those of the cell walls, might account for the entire lot of experi- 
mental cases presented. Indeed, it is not too much to say that 
when all of these factors are properly integrated, the possibility of 
dependent parasitism between two species might be predicted 
with fair certainty. 
After the completion of the last article dealing with this subject 
a few of the experimental parasites still remained active. The 
description of the further history of these arrangements and of some 
unusual conditions of this kind found among native plants consti- 
tute the purpose of the present paper. 
A case of parasitism of Opuntia Blakeana on Carnegiea gigantea 
was discovered in Roble’s pass, 7 miles southwest of the Desert Lab- 
oratory, on March 19, 1911, and a visit was made to it a few days 
later in company with Prof. H. M. Richards for the purpose of 
making a detailed examination and photographs. 
The sahuaro was a tall specimen with four branches, being 
about two hundred years old. The largest branch was about 12 feet 
long and arose from the trunk about 7 feet from the ground. 
Germination of an Opuntia seed had evidently taken place in the 
axil, and the roots had penetrated the corky layers in the angle. 
The growth of the opuntia had resulted in the development of two 
main stems, one consisting of two and the other of three joints, 
