MacDouGa: -INDUCED AND OCCASIONAL PARASITISM 475 
which showed some atrophy. The season of 1910 had been very 
dry and a small cylindrical branch had arisen from the basal cylin- 
drical part of the stem, the terminal section of which had died 
back. (See PLATE 22.) 
The large branch, which was over a foot in diameter at its base, 
was cut away and the roots of the opuntia dissected out, a task of 
some difficulty. Some of the rootlets reached a distance of a foot 
from the base of the opuntia, and while many of the branches 
partly encircled the base of the branch of the sahuaro, yet one main 
root and its branches had penétrated directly inward into the 
tissues of the greater cactus to a depth of over six inches, being 
completely submerged and cut off from the air. The advance of 
the root had been followed by the death of the cortical cells of the 
host and by the formation of scar tissue enclosing the parasitic 
roots, and then secondary root formation had followed, which 
resulted in a dense mesh of fibrils, none of which were in actual 
contact with living tissue. (See PLATE 23.) 
The contact thus made with the Carnegiea was undoubtedly 
the source from which the chief supply of solutions was obtained. 
The remainder of the root systems was in such position that the 
moisture collected in the sinus of the branch and the stem might 
be absorbed, but as the amount of this liquid would be small and 
would be available for only a few hours during the entire year, it is 
all but a negligible quantity in the nutrition of the opuntia. A 
transverse section of a portion of the cortex enclosing a penetrating 
root would show this organ surrounded by a flattened tube of 
corky tissue derived from the cortex of the host. The folds of this 
tube extended for several millimeters from the root. 
The parasitic opuntia was brought away intact and set in the 
soil in the terrace of the Desert Laboratory to allow observation of 
its further development, and under autophytic conditions. Similar 
experience with a plant taken from a Parkinsonia showed an abrupt 
alteration in the amount, rate of growth, and form of the flattened 
stems. [Bot. Gaz. 52:—. 1911. (In press.)] So great was the 
difference that the identity of the species was mistaken previous to 
its cultivation in the ground under the customary conditions. 
An Opuntia discata growing from the trunk of Acacia was 
reported by Dr. W. A. Cannon in 1909 and a special excursion was 
