1git] MACDOUGAL—PARASITISM 257 
members during the following two years. Some thickening of 
the cylindrical basal segment, however, was noticeable. 
The work described in this and previous papers has been suc- 
cessful in the demonstration of certain physical conditions which 
make parasitism possible, and has led to the suggestion of physi- 
ological activities which limit or facilitate the adhesion of two seed 
plants in a dependent 
nutritive combination. 
Wider observations 
would doubtless increase 
the known parasitic 
combinations, while it 
may be safely assumed 
that present conditions 
are as favorable for their 
making as at any time in 
the history of the plant 
world. New parasites 
may be expected to be 
brought to our attention 
from time to time. 
The assumption of a 
mutualistic or depend- 
ent role, in fact any de- 
parture from a purely 
autophytic condition by 
a green plant, is inevi- 
tably followed by reduc- 
tions or atrophies. Such 
Fic. 4—Same plant as in fig. 3, March rotr, with 
enlarged roots and stem, and with but one surviv- 
ing branch. 
combinations are displayed by a number of seed plants, not far short 
of half the existing species. It is of interest to note that the parasiti- 
cal consequences have not yet been seen in green plants furnished 
With food material including organic compounds. The total 
reaction is complex, and the exciting ‘causes are probably not 
simple. Whatever they may be, they are furnished only by the 
living or decaying bodies of other organisms. The part played by 
the pathological effects and physiologic reactions of parasites in 
