Igi2] REY NOLDS—PARASITIZED LEAF TISSUE 385 
The effects of the rust upon the leaf tissue are similar to those 
of gall-producing insects. The nuclei are enlarged, the proto- 
plasm is often increased, and there is considerable hypertrophy. 
In other words, the rust seems to stimulate the tissue rather than 
to retard its growth. The witch brooms are evidences of such an 
influence, and Moritarp (49) describes such effects caused by 
insects in several hosts. 
The effects of the fungous invasion upon the protoplast are 
various. At times the nuclei may entirely or almost entirely 
disappear from the diseased areas, leaving the cells, thus deprived 
of the nucleus, in a dead condition. This was noted in Potentilla 
and Gaylussacia. In other cases there is an unusual activity in the 
nuclear divisions, resulting in several nuclei in one cell. Viola 
and Panicum have already been cited as examples of this. The 
fact has already been given that the nuclei may also become larger 
than normal. The chromatin content of the diseased nuclei seems 
at times to become greater also. Such a condition is seen in 
Panicum, in which the nuclei of the diseased cells stain more 
intensely with hematoxylin than those of the normal cells. This 
may not be due to an actual increase in the chromatin, but either to 
a relative increase in the nuclear acid, or to a physical rearrange- 
ment of the chromatin. The chloroplasts may be reduced in 
size and finally disappear, as in Potentilla, or they. may persist, 
as in Viola, until the final disintegration of the cell. The differ- 
ences in the reactions of the cytoplasm and of the cell walls have 
already been cited in other connections. 
With the attacks of parasitic fungi on leaves the effects are 
varied, depending upon the species of the host and of fungus. 
The virulence of the parasite and the degree of resistance of the 
host are the chief factors involved. If the leaf, while in actively 
growing condition, is attacked by the fungus, the changes in the 
host are often profound, if the fungus is able to maintain itself 
against the protective measures put in operation by the host. The 
cytologic changes already described are likely to occur. The leaf 
curl of peach caused by Exoascus deformans is an example of this 
type of change. Miss KNowLes (34) has examined the leaf struc- 
ture in this disease and has found that the tissue is greatly altered 
