490 Brown. — Studies in the Physiology of Parasitism. IV. 
with the variation of the length of the period allowed for development, is 
merely an experimental method by which the ratio of actively growing to 
non-growing mycelium can readily be altered while at the same time 
ensuring economy of space and labour. With this understanding, factors 
(i) and (2) will be considered as independent factors during the account 
of the experimental part of the work. Their exact bearing on the 
phenomena concerned will be set forth in the general discussion of results. 
Factors influencing Activity of a given Enzymic Solution. 
It was shown in the first paper of this series that the activity of an 
extract of germ tubes increased up to a certain limit with the amount 
of enzyme present and was reduced by the presence of certain other sub- 
stances, e. g. salts, acids, &c. (except, in the case of acids, in the immediate 
neighbourhood of the neutral point). As therefore the enzymic activity is 
the resultant of two antagonistic factors, amount of enzyme and amount of 
retarding substances, it is possible for two extracts of equal activity to 
contain widely different amounts of enzyme. If we suppose, as seems 
legitimate, that deactivation of an extract by heating to 65° C. merely 
affects the enzyme (and it may be, some other colloidal substances which, 
however, are not likely to be of importance in this connexion), then it is 
possible to separate the two factors. The method of separation is as 
follows : 
Suppose two extracts x and y are to be compared. Portions of # and 
y are heated to 65° ; such deactivated extracts may be termed x and y 
respectively. If now 1 c.c. of x be mixed with 1 c.c. of y — which mixture 
may be represented by the symbol xy — and if similarly we prepare the 
mixture x'y, it is plain that the retarding factors (salts, acids, &c.) in both 
mixtures are equal in amount. We can therefore judge from any 
differences in the activities of xy' and x'y as to whether ;r or y is richer in 
enzyme. 
The method of estimating the activity of the extracts has been de- 
scribed in the paper already referred to (No. 1, p. 324). The discs employed 
were of 0*5 mm. thickness, and were of potato, turnip, or swede. 
Notation. In the following account, estimations are made of the 
activities of preparations obtained by extracting the washed and dried 
hyphal mass of the fungus, and of preparations consisting of the medium in 
which the fungus has been grown. For convenience in description these 
will be referred to as H (hypha) and M (medium) preparations respectively. 
Furthermore, as preparations of different ages will be compared with one 
another, a suffix will be added to indicate the age (in days) of the prepara- 
tion under consideration. Thus by H 2 is meant an extract of hyphae of two 
