388 BOTANICAL GAZETTE [MAY 
in such tissues as the leaves of Pyrus and Xanthium. Here the 
hypertrophy is coincident with the changes in the constitution of 
the cell, and it indicates a rather strong parasitism. On the other 
hand, as in Psedera and Gaylussacia, the virulence is so great that 
the cytologic changes are simply the fleeting features of a rapid 
degeneration, and of course no hypertrophy could take place. 
Here again the condition indicates a strong parasitism. TUBEUF 
(80) points out that the degree of susceptibility of the host, or in 
other words the degree or strength of parasitism, is indicated by the 
amount and kind of deformation, as has just been indicated. He 
draws his illustration from the Uredineae. He says, “If the host 
suits the fungus only in a limited degree, then no hypertrophy will 
result, and the latter will attain only to the formation of sperma- 
gonia. Let the host, however, be the one best suited to the fungus, 
then hypertrophy will result and aecidia be developed.” He gives 
as evidence a series of experiments upon Gymmnosporangium, in 
which the fungus developed to different degrees varying with the 
host used. FENSTLING (22), in discussing the effects of rust upon 
their hosts, comes to the same conclusion regarding the relation of 
the degree of change to the time of the fungus attack. 
In the leaf-inhabiting fungi, so far as studied, the mode of 
attack seems to be through the aid of some substance injurious or 
stimulative to the host cells. Two lines of evidence are at hand. 
The protoplasm of cells outside of the tissue directly in contact 
with the fungus often becomes killed, and the cell walls become 
brown. It is difficult to see how such a condition could exist if 
there is no toxic substance produced. In the second place, most of 
the cells of the host tissue examined by the writer contained no 
trace of the fungus, yet, as already shown, the nuclei are often 
enlarged or become numerous, and the chloroplasts also are reduced 
in size. Here again, there must be a substance which is diffusible 
through the cell walls, which is stimulative to the nucleus or 
poisonous to the chloroplasts. No indications were to be found as 
to the origin of such a substance. It is possible that the host 
cells may have produced it as a defensive measure, and that in 
turn certain of the cells were killed by this toxic substance. On 
the other hand, it is possible that the fungus may produce such a 
