306 
Vines . — The Proteases of Plants. 
the incubator. After 20 hours’ digestion the fibrin in the watery extract was unaltered, 
whilst that in the NaCl extract was much diminished, and 4 hours later had dis- 
appeared. The fibrin in the watery extract had not disappeared after digestion 
for 4 days longer. 
In another experiment in which extraction was prolonged for several 
hours, the activity of stronger aqueous and 2°/ 0 NaCl extracts of yeast was 
compared. It was found that 30 cc. of a 10 °/ o yeast NaCl extract digested 
o-2 grm. fibrin within 18 hours, and the same quantity of 5 °/ o yeast NaCl 
extract digested the fibrin in 24 hours ; whereas the times of digestion 
by the corresponding aqueous extracts were 46 and 66 hours respectively. 
The object of the next experiment was to ascertain in what way 
NaCl affects digestion. Does it directly promote it, or does it do so 
indirectly by dissolving out of the yeast something that distilled water fails 
to extract or extracts less completely ? 
Experiment 5. 3 grms. of dried yeast were extracted with 60 cc. of 2 % NaCl 
solution (toluol 1%); 6 grms. were also extracted with 120 cc. of toluol- water : 
extraction and filtration occupied about an hour and a half. 40 cc. of the NaCl 
extract were put into a bottle (No. 1) with 0-2 grm. fibrin : of the aqueous extract, 
40 cc. were put into a second bottle (No. 2), and other 40 cc. into a third bottle 
(No. 3), to which NaCl to 2 % was added, as also 0-2 grm. fibrin to both 2 and 3. 
After 18 hours in the incubator the fibrin in No. 1 had disappeared: this was 
not the case in either No. 2 or No. 3, nor had it disappeared after digestion for 
24 hours. 
This result, in the first instance, confirms the conclusions as to the 
superior activity of NaCl extracts as compared with aqueous extracts ; 
and, in the second place, it gives an explicit answer to the question pro- 
pounded above. It clearly shows that the presence of NaCl is of 
importance in the process of extraction rather than in the process of diges- 
tion ; and it may be inferred that the NaCl solution dissolved out of the 
yeast something, no doubt a protease, that water alone failed to extract. 
An experiment was made to test the action of a NaCl extract on 
boiled fibrin. It was found that 60 cc. of a 20°/ o yeast-extract with 
NaCl digested 0-2 grm. fibrin in 3 days: the action was slow, but it 
was more rapid than that of a watery extract of the same strength (see 
P . 304). 
Summary of Experiments on Yeast. 
Evidence has been adduced to prove that yeast can effect both 
peptolysis and peptonization. The fact that these processes can be carried 
on by filtered extracts makes it clear that they are not due to the yeast- 
plant as a living organism, but to one or, perhaps, more substances that 
can be dissolved out of it ; and there can be no doubt that the active 
substance is, in any case, a protease. An important issue is thus raised 
