1 2 7 
‘ Biologic Forms ’ of the Erysiphaceae. 
cold water, and the water heated slowly to 50 °C. ; the leaves were then 
taken out, dried and inoculated. A marked susceptibility was shown by 
the leaf after this treatment, and in many cases inoculation was followed by 
the appearance of almost full infection. Ascospores as well as conidia 
were used in these experiments. Marked susceptibility, shown by the pro- 
duction of minute powdery Oidium- patches, was also induced by immersing 
leaves for 1 minute in water at the temperature of 49*5° C. or 50° C. 
The same result was obtained when leaves attached to plants growing in 
a pot were immersed for 1 minute in water at the temperature of 50° C. 
We see, then, from the results of the above experiments, that not 
only mechanical injuries, such as wounds from cuts, bruises, attacks by 
slugs, &c., but also injuries due to the action of narcotics and heat, cause 
a leaf to become susceptible to a c biologic form’ of a Fungus to which 
it is normally immune. 
To describe cases where a form of a Fungus which is specialized to 
certain host-plants and confined to them under normal circumstances 
proves able to infect injured parts of a strange host, I propose the terms 
xenoparasite and xenoparasitism 1 . In the case of the specialized Fungus 
when on its proper host under normal conditions the terms oecoparasite and 
oecoparasitism may be used. 
It is obvious that mechanical injuries quite similar to those produced 
by cutting, bruising, &c., described in the above experiments, are constantly 
being inflicted on plants in nature— by animals, and by frost, hail, wind, &c. 
In the case of cereals the agricultural operation of rolling seedling plants 
causes a number of leaves to be torn or bruised. An experiment carried 
out during the present spring has demonstrated that bruised places on 
leaves produced by rolling are rendered susceptible in the same way as 
those caused artificially by pressure in the experiments mentioned above 
(see, also, Part I!, Section e). At the beginning of last May I collected 
from a field of young barley which had just been rolled a number of 
bruised leaves. The soil of the field was stony and c steely,’ and in the 
operation of rolling about 30 °/ o of the leaves had been bruised more or less 
severely. Ten of these injured barley leaves were placed in a Petri dish 
on damp blotting-paper, and each inoculated over the bruised cells with 
conidia taken from wheat leaves. On the fourth day several of the leaves 
bore at the inoculated place numerous small straggling mycelial patches, 
and on the seventh day these patches, in the case of four of the leaves, bore 
several little tufts of a few clustered conidiophores. A number of sapro- 
phytic Fungi were now growing vigorously at the injured places, and 
stopping the further growth of the Oidium. 
1 The same term may be applied to a Fungus when growing on parts of its normal host which 
are immune when uninjured. Thus, the Oidium on Euonymus japonicus is normally unable to 
infect the old leaves of this plant, but proves able to cause infection when sown at an injured place. 
