476 MacDouGaL: INDUCED AND OCCASIONAL PARASITISM 
made to examine the arrangement in April 1911. The original 
photograph, from which a drawing has been made, showed a 
heavy basal stem of the opuntia issuing from a knot hole in the 
trunk of the tree and bearing two branches, one consisting of one 
joint and the other of three. (PLATE 24.) The size of the stem 
of the opuntia would indicate that it was perhaps fifteen or twenty 
years old, but the limited supply of food material to be obtained 
from the host had operated to the loss of other flat joints that may 
have been formed from time to time. The more recent observa- 
tion showed a main horizontal stem of four joints, from which 
were arising a number of new joints. A basal branch of three 
joints was dead and was all but detached from the plant. A single 
upright joint bore a small young joint rapidly enlarging. Another 
upright joint and the two terminal joints of the horizontal stem 
were in a dying condition. It seems probable that the plant 
yearly gives rise to more branches than may be maintained by 
the supply of solution derived from the host and that the arid 
after-summer of 1911 will be the occasion for the death of some 
of the joints that are being formed in the growing season of the 
earlier part of the year. (PLATE 25.) 
It was not deemed advisable to dissect this arrangement until 
further observations had been made, but it is probable that the 
development of the roots of the opuntia follow the formation of a 
cavity in the trunk of the acacia by the decay of the wood, and 
that the roots of the intruding cactus operate to cause or hasten 
this integration. A similar case of Opuntia on Parkinsonia has 
been previously described. (See Publication 129, Carnegie Inst. 
Washington, 1910.) 
A plant of Opuntia Blakeana set in the side of a trunk of 
Carnegiea early in 1909 has continued a fairly even existence 
although no growth in the way of formation of new joints has been 
seen. The plant consisted of a basal cylindrical section and a. 
terminal oval joint. The plaster, which was used to hold the 
xenoparasite in place when first arranged, has gradually crumbled 
away, but the parasite is held firmly in place by its own roots.. 
As has been previously noted, some thickening of the basal portion 
of the stem is noticeable. The amount of growth to be expected 
from parasites is always less than that of autophytic individuals. 
