Brown .— Studies in the Physiology of Parasitism. I. 345 
discs. What the explanation of this phenomenon is it is at present 
impossible to say. 
As participating in the abnormal retarding effect of bean juice upon the 
fungal extract, we have therefore to consider at least three factors: 
1. The retarding action (properly so called) of chemical substances 
present in the bean juice. 
2. The effect of the precipitate present. 
3. The ‘ hardening ’ effect of the bean juice upon the potato tissue 
employed. 
In comparing the experiments in vitro with what happens when the 
fungus invades the bean plant, it is to be noted that factors 2 and 3, 
which play a considerable part in the former, may not, and probably do not, 
have their counterpart in the latter. 
The experiments above tabulated are very clear on one point—that the 
expressed sap of Fegatella has no special inhibitory action, but behaves in 
a manner quite comparable with the sap of potato, cucumber, &c. The 
marked resistance of the tissue of hepatics to the action of the fungal 
extract and to the fungus itself is not to be ascribed in any way to any 
specific anti-properties of the cell-sap. 
H. Discussion of the Nature of the ‘Lethal Principle\ 
It has been shown in the foregoing that the macerating action of the 
fungal extract can be destroyed in various ways: by heat, by mechanical 
agitation, and by neutralization with alkali. The extracts so deactivated 
possess no lethal activity whatever. From microscopical investigation it is 
known that death of the cells takes place at a late stage in the process of 
disintegration of the cell-walls. The latter process is therefore the deter¬ 
mining factor of the whole action. This dependence of lethal upon 
macerating activity may be explained in either of the two following 
ways: 1 
1. That both actions are due to the same substance or group of 
substances. 
2. That the two actions are due to different substances, but the lethal 
substance is unable to reach the protoplast until the permeability of the 
cell-wall has been sufficiently increased by the action of the macerating 
substance. 
In the absence of an exact knowledge of the diffusive capacity of the 
1 It might be suggested that no toxin exists in the fungal extract, but that it is produced as a result 
of the action of the extract upon the cell-wall. Experiment has shown that a fungal extract in which 
a quantity of well-washed, grated turnip tissue has been digested behaves similarly to standard 
extract as regards deactivation by heat and by neutralization with alkali. In other words, the 
hydrolysis products of cell-wall substance are not. of toxic nature. 
A a 
