Reed—Infection Experiments with Mildew . 531 
sativa, TJlmus alata and Fagus ferruginea in North America, 
on Hippophae rhamnoides in Europe, and on Adesmia sp. in 
Argentine. A second variety, called rigida, instead of the 
typical thin-walled, flaccid conidiophore, has a rigid, thick- 
walled conidiophore. It occurs on Parmentiera alata from 
Mexico. The third variety, subspiralis ■, has a conidiophore 
which is especially twisted in its basal portion and is found 
on Dalbergia Sissoo from India. It still remains to he de¬ 
termined whether each of these morphological varieties con¬ 
stitutes a single physiological species, or whether they are 
split up into forms limited to definite hosts. 
All of these results indicate that these various physiological 
species of mildews have definitely limited powers of infection. 
Under normal conditions they are restricted to certain defi¬ 
nite hosts. If a host plant is injured in any way, it loses its 
power of resistance to other specialized forms and so becomes 
infected. This infection is not due to any variation in the 
infecting power of the mildew, hut to a change in the capac¬ 
ity of the host to resist infection. The immunity which it 
naturally enjoys is destroyed. Furthermore, although the 
fungus can develop upon an unusual host which is injured, it 
does not change in its power of infection. It does not acquire 
the capacity of infecting healthy plants like the injured one, 
nor does it lose its power of infecting its original host. 
Burma’ the past year I have carried on infection experi¬ 
ments with the mildew on the cucurbits in order to determine 
whether specialization has also occurred in this mildew. 
Early in October 1904, plants of the Hubbard squash were 
found infected with mildew. Some of the infected leaves 
were brought into the greenhouse and placed in a moist cham¬ 
ber. In a short time conidia were produced abundantly. 
These were used to inoculate leaves of various cucurbits grow¬ 
ing in the greenhouse. About ten days later numerous patches 
of mycelium producing conidia abundantly were observed on 
one vine. The mildew spread rapidly over this plant, im¬ 
mense quantities of conidia being produced. The vine subse¬ 
quently fruited and was thus proved to be a Hubbard squash. 
