VOLUME LII NUMBER 4 
g 12s 
BOTANICAL (CAZETTE 
OCT OBER Igri 
AN ATTEMPTED ANALYSIS OF PARASITISM 
D.T. MacDoUGAtL 
(WITH SIX FIGURES) 
According to a recently published estimate made by the author, 
about half of the total number of seed plants use complex food 
material derived from other organisms by mycorhizal or parasitic 
arrangements. So far as our observations go, dependent species, 
which are advantaged by contact or association with other species, 
undergo direct somatic modifications, consisting chiefly of atro- 
phies or reductions of the shoot and root system; and in plants of 
fixed parasitic habit, these reductions may be such as to include the 
total disappearance of the roots and to bring the shoot down to 
simple, unbranched, chlorophylless stems, upon which the leaves 
are represented by colorless bracts. The fruits and seeds may 
show various specializations. 
During the course of an extensive investigation of the condi- 
tions under which two species may enter into the relation of host 
and parasite, regenerated cuttings of a large number of species 
were attached to the stems of desert succulents and xerophytes. 
In some cases the attached plants formed roots; and in others 
the epidermal cells performed the function of absorption. 
The ruling factor was in all cases the osmotic ratio between the 
sap of the two plants; one plant may not become parasitic upon 
another except by the aid of a superior osmotic pressure which 
withdraws solutions from the tissues of the enforced host. Many 
causes, however, may operate to prevent a potential parasite 
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