316 Brown.—Studies in the Physiology of Parasitism. I. 
consideration. On the question as to the presence of the toxin these 
authors were unable to make any notable contribution. 
Criticism . Viewed generally, the methods of preparing the fungal 
extract which have been adopted by previous workers fall under the 
two heads: 
1. Extraction from the old mycelium. This is the method of de Bary, 
Marshall Ward, Kissling, Smith, and Nordhausen. 
With respect to this method it can reasonably be objected that the 
secretions of an old mycelium do not necessarily bear any close relation¬ 
ship to those of a young vigorous culture. It is to be borne in mind 
that the active invading portion of the fungus is essentially of the nature 
of a young and fresh culture. In order therefore to study the nature of the 
active principle of the fungus, it is necessary to examine the secretions of 
young and vigorously growing hyphae. In this way only can the complica¬ 
tions due to the presence of waste products (‘ staling ’ products) be avoided. 
2. Extraction from plant organs (fruits, tubers, &c.) which have been 
overrun by the fungus. This method was employed by de Bary, and more 
particularly by Behrens. 
The time required for the complete invasion of a compact structure 
such as a good-sized fruit may be considerable; in one case, for instance, 
Behrens employed a three months’ old culture of Penicillium luteum on 
apple. It will thus be seen that the objections relative to old and (in 
the main) stale cultures put forward under the previous heading apply here 
with equal force. Furthermore, this method presents additional complica¬ 
tions. In extracts of this sort there are present substances derived both 
from the fungus and from the host, in varying amount according to the 
degree of invasion at the time of extraction. Now when we bear in mind 
the plasmolysing effect of plant and especially of fruit juices, and further¬ 
more that plasmolysis normally induces death of living tissue, it is easy 
to see that such extracts as those of de Bary and Behrens can give 
no certain indication of the primary toxic principle concerned. It will 
be shown later that the plasmolysing effect of a plant juice is greatly 
reduced by the growth in it of a fungus. Nevertheless the presence of the 
unknown remainder constitutes in all cases a complication which certainly 
should be avoided. 
A similar criticism would also apply to the bacteriological investigations 
above mentioned. Here, however, the active solutions obtained were so 
weak that little was attempted in the way of studying the question of 
toxicity. 
The work of Smith calls for special criticism. His method was to 
compare the action of mycelial extract with that of various strengths of 
oxalic acid, and from certain similarities he concluded that this acid played 
a great part in the changes concerned. Other acids he found behaved 
