MacDoucGaL: INDUCED AND OCCASIONAL PARASITISM 477 
That some actual parasitism or derivation of nutriment takes place, 
is evident from the fact that similar small plants separated from 
the soil and exposed to the same climate desiccate within a single 
season, while this preparation has survived three hot summers. 
A small individual of O. versicolor set in the trunk of a Carnegiea 
in the spring of 1909 is also robust and thriving, having undergone 
considerable thickening of the stem, one branch, and the exposed 
portion of the roots. Both of the above preparations give every 
appearance of being permanent during the ordinary life cycle of 
the individuals. 
Some of the most important experimental results were obtained 
by using regenerated cuttings of a grape (Cissus laciniata), from 
the region of Tehuacan, Mexico, as a parasite, and some new 
arrangements were set up early in 1911. This plant undergoes 
secondary thickenings of the stems in such manner that portions 
near the ground, or partially imbedded in it, attain a diameter of 
several inches, and contain large balances of water and food 
material. Oftentimes the thickening will take place in portions 
of the climbing stem many feet from the ground. The thinner 
portions of the stem die upon the cessation of active growth, with 
the consequence that sections of thickened stem may be supported 
by the dead tendrils high above the substratum. In some in- 
stances such thickened sections will be held by a second vine, 
clasped by the tendrils. The beginning of activity in these sus- 
pended stems usually consists in the formation of long adventi- 
tious roots which hang down and are capable of making a length 
of one or two yards. If the ground is reached, new supplies ascend 
through the aerial roots and the plant survives. Many of the 
thickened sections are held at such elevation that the supply of 
water and material does not suffice for the construction of roots 
of a length that may reach the ground, and consequently the 
isolated members perish. Similar behavior was exhibited by 
plants in the glass house at the Desert Laboratory. It is evident 
that such suspended sections offer some chance of contact or 
penetration of other living plants, thus setting up parasitism, and 
the previous experiments showed that this grape might be made 
parasitic on Opuntia, Echinocactus, and sometimes on Carnegiea. 
A number of new preparations were made in April, 1911, in 
