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On the Physiology of Parasitism. 
excreted by the fungus played a part in the killing action. Later 
investigators confirmed the presence of an enzyme in connection 
with the action and little doubt remained that the action manifested 
on the cell wall of the host was due to this substance. As regards 
the killing action of the fungus on the host cell, there has been a 
progression from the view of de Bary, according to which soluble 
oxalates may play a part in the effect, to that of Behrens (2), who 
states that the toxic action is not due to a substance of volatile or 
enzymatic nature, and finally to that of Smith (3), who ascribes the 
whole toxic action of the fungus to soluble oxalates. 
Prom a perusal of the literature, it appeared that previous 
investigators had employed, for the study of the active principle 
concerned, extracts obtained from comparatively old cultures or 
from host tissue which had been parasitised by the fungus. Now 
the actively invading portion of the fungus is essentially of the 
nature of a young and fresh culture. It may therefore be objected 
to the former method that extracts of old mycelia do not necessarily 
bear any close relationship to those of a young vigorous culture, and 
that in particular the former extracts would contain various products 
such as the so-called “staling” products which would affect the 
experiments in vitro while they would play no direct part in the 
phenomenon of parasitic attack. To the extracts obtained from 
parasitised host tissue, there is, in addition to the foregoing, the 
further objection that such extracts contain substances derived from 
the host and not from the fungus at all. It was argued that the 
presence of the latter would render the experimental results 
difficult of interpretation, if not entirely nugatory. 
I. Nature of Active Principle of Fungus. 
Preparation of Extract. The aim of the investigation was thus 
to prepare in the first place an extract from young hyphae alone, 
and it was hoped that this extract might prove to be of a sufficiently 
powerful nature to recommend it for use in this connection. The 
method which was finally adopted for this purpose is described 
in detail elsewhere. It consisted essentially in the sowing of a large 
quantity of spores in a suitable nutrient on horizontal glass plates. 
A short germination period (ca. 24 hours) was allowed, after which 
the spores were washed, dried, and ground to a fine powder. The 
latter was extracted in water, and a clear extract obtained by 
centrifuging off the debris. 
