332 Brown.—Shidies in the Physiology of Parasitism . /. 
(£) Detailed Account of certain Cases . 
As has been stated, the action of the fungal extract on a tissue is 
of two kinds, a macerating and a lethal. The following account refers 
to an examination of the course of the action which was carried out with 
the object of establishing the time relationships of the two manifestations. 1 
The criterion of maceration was as usual diminution of coherence. The 
criterion of lethal activity was the failure of hypertonic solutions to cause 
plasmolysis. 
It will be noted that of the two phenomena to be studied, the one 
entails microscopic, the other (in the main) macroscopic observation. 
In order to make correlation possible it is obvious that homogeneity 
of tissue is all-important. We are thus limited to fleshy parenchymatous 
tissue. Even here the ideal tissue is not obtainable. Thus in turnip tissue 
homogeneity is disturbed by the presence of small vascular bundles, the 
small cells in the neighbourhood of which are more slowly acted upon than 
the larger cells of the ground-mass. In other cases—e. g. in the stem (pith) 
of Senecio articulata —there is a gradation from the large, more sensitive 
cells in the centre to smaller, more slowly reacting cells towards the 
periphery. The difficulty, however, is not so great as might appear. The 
coherence of a disc of any particular tissue is no greater than that of 
its weakest part, and we have accordingly to compare the progress of the 
macerating action, as shown by the loss of coherence , with the plasmolytic 
features of the more sensitive portions. Thus in turnip tissue we correlate 
the condition with respect to coherence with the condition as regards 
plasmolysis of the large cells of the ground-mass ; and similarly for the case 
of the large central cells of the pith of Senecio articulata. 
The tissues which have been found useful in this connexion are as 
follows : 2 
Turnip {white) : nearly full grown; must not be old and vacuolated; discs cut 
from an axial cylinder in the neighbourhood of the centre. 
Swede : possesses the great advantage that uniform discs of mm. thickness can 
be cut. 
Cucumber : discs from axial cylinder in the region of the basal contracted 
portion. 
1 Van Hall (l.c., p. 135), from microscopic examination of the tissue lying between the sound 
and the decomposed tissue, states ‘ that it is sometimes possible to observe one or two layers of 
tissue, the cells of which still hang together, although, as can be seen from their contracted proto¬ 
plasts, they are already dead \ From this he concluded that the cells are first killed and then 
isolated from each other. 
2 The potato, which has proved of great value in this investigation, is useless for the present 
purpose on account of the large amount of solid cell contents present, which makes a plasmolytic 
study impracticable. 
