Reed—Infection Experiments. 
141 
assumed as proven that there are physiological differences be¬ 
tween the mildews of one morphological species by which it is 
split up into a number of forms, each limited to one or few host 
plants. In what these differences consist is by no means yet de¬ 
termined. Salmon (13), during the summer of 1903, carried on a 
series of experiments to determine the behavior of these special¬ 
ized forms under specially controlled conditions. He cut out of 
the leaf to be inoculated some of the epidermal cells on one sur¬ 
face and most of the mesopliyll tissue. The epidermal cells on 
the other surface were left intact. The spores were sown upon 
these uninjured epidermal cells and the leaf was then placed 
with the cut surface downwards in a petri dish. Salmon found 
that when the leaves were injured in this way that they were 
able to bridge the fungus over to other host piants although the 
healthy leaves of these same plants are immune to the attacks 
of the fungus. For example, under these conditions spores from 
wheat will infect bar ley, oats and Ilordeum sylvaticum ; spores 
from barley will infect wheat, oats, Ilordeum murinum and H. 
sylvaticum; spores from Bromus commut'atus will infect B. race- 
mosus, B. asper and Ilordeum sylvaticum; spores from Bromus 
secalinus will infect B. asper and barley; spores from Arena 
stmgosa will infect barley and wheat. He further found that 
spores from wheat will infect T. monococcum and T. dicoccum. 
All of these are cases in which no infection would occur on unin¬ 
jured leaves. 
Salmon also sowed the spores produced on these injured leaves 
on other injured leaves of the same plants and found that they 
were able to produce infection. In this way the viability of the 
spores formed on the injured leaves was proven. It was further 
discovered that- spores produced on injured leaves of Ilordeum 
sylvaticum by inoculating with conidia from wheat were in turn 
able to infect healthy leaves of H. sylvaiicum. 
These investigations indicate that although there are definite 
physiological species, normally restricted to one or a few host 
plants, yet injured host plants of one physiological species may 
be infected by spores of another physiological species and, in 
this way, the mildew may be bridged over from one host plant 
to another. 
