478 MacDouGaL: INDUCED AND OCCASIONAL PARASITISM 
which cuttings were inserted in resting potato tubers and also in 
joints of Opuntia Blakeana. Late in May it was found that nearly 
all of the cuttings in potatoes were forming short basal roots with 
still shorter clubshaped branches, and the cavities about them 
were being enlarged rapidly by decay. A few cuttings were send- 
ing out aerial roots from the upper ends. These were all trans- 
ferred to joints of Opuntia, some in the glass houses and some in 
the open, late in May, 1911. All in the glass house were alive, 
and about half of those in the open, in August, rg1r. Theamount 
of leaf development was small and the growth of the shoots 
offered no features not previously described. 
It is notable that in these, as well as in all previous experi- 
ments dealing with this subject, very little evidence of forcible 
penetration of the tissues of the host was seen. Whether by the 
previous action of bacteria or by excretions from the xenopara- 
site, the invading roots never actually bored through masses of 
living cells. In all cases the layer of tissue, one or more cells in 
thickness, nearest the roots was found to be dead and more or 
less disintegrated. Peirce, in his earlier work on this subject, 
cites instances in which roots of Brassica and Sinapis actually 
penetrated among living cells by a comparatively rapid growth. 
(Das Eindringen von Wurzeln in lebendige Gewebe. Bot. Zeit. 
52': 169. 1894.) 
It is notable that the experiments included in this research, 
which has been carried on for four years, were made under ex- 
tremely arid conditions, in which the transpiratory loss was high. 
The plants that were induced to live as parasites were therefore 
under the double burden of securing nutriment from a host, which 
furnished a substratum offering physical conditions widely dif- 
ferent from those which their absorbing organs ordinarily en- 
counter, and of maintaining their own turgidity as a necessary 
condition of growth and other constructive processes. As has 
been amply demonstrated, the change from an autophytic to a 
parasitic condition is one that may be readily made by many 
species. The distributional movements of plants, which are con- 
stantly bringing new pairs into contact, would operate to bring 
eligible parasites and possible hosts together and cases of newly 
originated dependent nutrition may be expected from time to 
time. 
