293 
Vines. — The Proteases of Plants. 
The main conclusion at which Bokorny arrives is that the acid reaction 
is essential to the digestive activity of yeast, and that the degree of acidity 
has an important influence upon the character of the digestive process 
as indicated by the products: thus, when the acidity is less than 0-5 °/ o , 
only a little albumose is formed, but a relatively large quantity of substances 
that are not precipitated by zinc sulphate or ammonia sulphate. 
It can hardly be said that the paper adds material facts to existing 
knowledge of digestion by yeast, nor can the conclusion as to the relation 
between acidity and proteolysis be regarded as convincing. In the first 
place, the objection may be raised that no antiseptic was used ; and though 
it may be urged that in many experiments the amount of acid present 
(0*5-1 °/J was sufficient to prevent bacterial action, yet in those cases 
where the acidity was less strong, and the digestion prolonged (24-48 
hours), the possibility of such action is obvious : in one case, indeed 
(Expt. 1), an offensive odour was noted and the development of mould. 
In the second place, no account is taken, in estimating the digestive 
products, of the proteids contained in the yeast itself. I have found 
that a watery extract of dried yeast, after boiling, filtering, and concen- 
trating, yielded a mainly proteid precipitate with alcohol amounting to 
something like 20 °/ o of the original weight (see p. 298) : hence, when 
it is borne in mind that in Bokorny’s experiments the weight of yeast 
employed amounted to to, 20, or even 4o°/ o of the proteid supplied for 
digestion, it is clear that the omission to take the yeast itself into account 
is a serious one. Finally, it is doubtful if any material amount of proteo- 
lysis was effected when the proportion of added acid was 0*5 °/ o or more : 
for, as Hahn and Geret have pointed out, and as I have myself found, 
the digestive activity of yeast rapidly diminishes with increasing proportions 
of added acid (see p. 302). 
I give now a selection of my experiments to illustrate the digestive 
activity of yeast under various conditions. I have employed fresh brewers’ 
yeast, also yeast that I myself preserved in the dry state, but chiefly the 
dried yeast that is now obtainable as an article of commerce (prepared 
by the Granular Yeast Company Limited, London, E.C.), which is con- 
venient to use, with the great advantage that it is possible to make a 
number of experiments with uniform material. The experiments include 
observations on self-digestion (autolysis), on the peptolysis of Witte- 
peptone, and on the proteolysis of fibrin. The test applied in the autolytic 
and peptolytic experiments was that for tryptophane, the presence of this 
substance being taken as evidence of peptolysis. When the experiments 
were comparative, the test had to be applied with certain precautions. 
Thus, in each set of observations, it was necessary to ascertain in some one 
case what quantity of chlorine-water had to be added to a given quantity 
of the digested yeast-liquid in order to produce a tryptophane-reaction 
