1912] REYNOLDS—PARASITIZED LEAF TISSUE 387 
In some of the diseases described, in which the tissue is killed 
and badly disorganized, few if any cytologic changes are to be 
noticed. The reason for this seems to be that the virulence of the 
fungus is relatively so great that the cells of the host are killed 
without having time to react to any stimulus. In such cases; as 
in the Castanea, all tissues seem equally affected, and there are no 
transition cells from the healthy to the dead portion. Another 
factor also which may operate is that the host tissue is old, and has 
not the power of reacting rapidly. This is in contrast with the 
young, highly resistant tissues, which, if parasitized, are able to 
react rapidly, and so present abnormal nuclear and other cytologic 
phenomena. Smilax also is an example of a diseased tissue which 
shows no cytologic changes. 
A question constantly before the student of pathology is whether 
there is any way of judging the degree of susceptibility of a plant 
to disease. First of all it must be remembered that a plant may 
be easily attacked and injured by one disease-producing organism, 
and be quite immune to the attacks of another. Leaving this phase 
of the question, however, we ask, ‘‘Is the amount of hypertrophy 
a true indication of the degree of parasitism?” If we imply in the 
word ‘‘parasitism’’ some influence derogatory to the welfare of the 
plant, we are almost forced to answer the question in the negative, 
for in cases of symbiosis there is often great hypertrophy. 
It is difficult at present to draw a sharp line between parasitism 
and symbiosis, since similar effects upon the host are noted in both. 
Thus, in some of the most noted instances of symbiosis, the myco- 
rhizas, there is often considerable hypertrophy. Moreover, many 
kinds of galls do not seem to injure the host beyond the local 
tissue, and here again there is great hypertrophy. Hence, taken 
alone, an excessive enlarging of a portion of a plant is not sufficient 
evidence of the degree of parasitism. But, taken in conjunction 
with the cytologic changes induced in the host, hypertrophy 
becomes a valuable diagnostic feature. It must not be forgotten, 
however, that even in such cases the parasitic nature of the organ- 
ism is not proved, for cytologic changes in mycorhizas have been 
reported, as noted heretofore. Evidence has previously been 
given in regard to the diagnostic value of the cytologic changes 
