297 
Vines. — The Proteases of Plants . 
important to determine whether or not the results given by such a mixture 
apply equally to others containing less yeast, and it was found that they 
do not apply. 
The following experiments were made with mixtures containing 2 °/ o 
dried yeast : — 
Experiment 5. Acid. 50 cc. of the mixture were placed in each of 3 bottles, 
Nos. 1, 2, and 3, acidified respectively with o-i %, 0-2%, and 0-5% HC 1 . After 
3 hours in the incubator none gave any tryptophane-reaction; after 24 hours the 
reaction was faint in No. 1, none in either of the others. 
Alkaline. In a similar experiment, in which the contents of the 3 bottles 
had been rendered alkaline by the addition of 1 %, 2 %, and 3 % Na 2 C 0 3 respectively, 
no tryptophane-reaction was obtained after digestion for 4 hours, or for 24 hours. 
Here proteolytic activity was destroyed by o- 2 °/ o HC 1 , as also by 
1 °/ o Na 2 C 0 3 , degrees of acidity and alkalinity which produced no such 
effect in mixtures containing 5 °/ o yeast. It may be concluded that there 
is a definite ratio between the amount of yeast present in a mixture and 
the amount of acid or alkali necessary to prevent autolysis. 
On the evidence of the tryptophane-reaction, it results from the fore- 
going observations that autolysis is very active at the natural acidity of the 
yeast-mixture : anything more than a slight addition of either acid or alkali 
tends to diminish it. The acidity of yeast is partly due to the presence 
of organic acids ; but not chiefly, for I have observed that it is impossible 
to neutralize a mixture or extract of yeast by adding excess of chalk. In 
view of the large proportion of phosphoric acid (about 50 °/ o ) and of potash 
(about 35%) in the ash of yeast, it may be concluded that the acidity 
is mainly due to the presence of acid phosphate of potash. Naegeli ( 9 ) 
has in fact suggested that the cell-sap contains KH 2 P 0 4 and K 2 HP 0 4 . 
Repeated digestions of mixtures to which excess of chalk had been added 
(see Experiment 3) have shown me that autolysis is even more active when 
the free organic acid present has thus been neutralized than at natural 
acidity. The conclusion to be drawn is that the most favourable degree of 
acidity is that afforded by the acid phosphates, a conclusion agreeing with 
that of Weis (see p. 291) in the case of malt. The influence of added acid 
on autolysis would appear to be, in accordance with the views of Fernbach 
and Hubert with regard to malt, that it is favourable so long as it merely 
suffices to convert the dibasic (K 2 HP 0 4 ) into monobasic (KH 2 P 0 4 ) phos- 
phates, but unfavourable when free acid begins to accumulate. Similarly, 
the action of added alkali is favourable so long as it merely neutralizes any 
free acid present, but unfavourable when it begins to convert the monobasic 
into dibasic phosphates. 
The study of autolysis necessarily involves the consideration of the 
proteids contained in the yeast-cell. I am not aware of any more recent 
