tgit] MACDOUGAL—PARASITISM 255 
by Dr. W. A. Cannon, in which a flat-jointed Opuntia was found 
established in a cavity in the trunk of an Acacia Greggii. One or 
more species of Opuntia native to the Tehuacan region find a foot- 
hold in the crevices of tile roofs,.and stone and adobe walls, in a 
very noticeable manner; and it was in this vicinity that one of 
these plants and a native grape were found rooted in the sinus of 
a forked tree Yucca (fig. 2). 
The conditions of such association seem favorable for the slow 
extraction of solutions from the host plant through non-living 
tissues without the actual contact of the living cells, an approxi- 
mation to the initial conditions of parasitism. It is obvious that 
the crowded root systems of a wide physiological variety of plants 
in the soil furnish numberless duplications of these conditions, and 
it seems entirely reasonable to suppose that such contiguity of 
absorbent and succulent roots may acccount, in part, for the 
greater number of root parasites. 
It is to be noted that among the higher plants the part played 
by destructive secretions is at a minimum. The activity or 
absence of such substances in seed parasites has been variously 
described. In no instance, however, are there such abundant 
and disintegrating effects as may be seen resulting from bacterial 
and fungal parasites of plants and animals. 
The parasitic arrangements described above, in which the host 
furnishes lodgment and a slowly yielded supply of solution, are 
characterized by a slow growth of the parasite, in which the amount 
of development is limited, the members being reduced. The 
Opuntia parasitic on Parkinsonia, which was described in Pub- 
lication No. 129, Carnegie Inst. of Washington, rg1o (see pl. 10), 
was taken from the host early in 1910 and set in the adobe soil of 
the terraces in the courtyard of the Desert Laboratory. In the 
course of the growing season of that year, it made three new joints, 
each of which was three or four times the bulk of those previously 
formed, the total growth in the previous 7 or 8 years. F urther- 
more, in this autophytic growth it developed characters which 
demonstrated that it properly belonged to Opuntia Toumeyi instead 
of O. Blakeana, with which it was first identified, because of its small 
joints and atrophied spines (fig. 5). 
