204 
Brown . — Some Studies on Yeast . 
This alternative method of determining the cell-increase gives results 
which are quite comparable in accuracy with those obtained by direct 
counting. 
Before proceeding to the consideration of the quantitative relations 
between cell-reproduction and available oxygen, we must discuss the second 
factor which may, under certain conditions, have an important influence in 
restricting growth — viz. the alcohol which the yeast produces in the course 
of an experiment. 
Adrian Brown found that additions of alcohol at the commencement 
had but little effect on the final cell-reproduction if these additions did not 
exceed about 3 per cent., but that above this amount cell-increase fell off 
rapidly, although it was not completely inhibited by the addition of 
8«4 per cent. 
Since the yeast was producing alcohol during its period of increase, the 
amount of alcohol capable of producing any distinct inhibiting effect must 
have exceeded 3 per cent. 
Instead of merely noting the influence of definite amounts of alcohol 
on maximal yeast-increase, I have repeated these experiments in such 
a manner as to be able to follow the whole course of cell-reproduction, 
taking care that the conditions of oxygenation of the liquid were the same 
in each case, and that all other conditions were as far as possible identical. 
The results of these experiments are given in the series of curves of Fig. 2, 
the amounts of alcohol added at the commencement being respectively 0*5, 
i*o, 3-0, 4-0, 6-o, and 8*o grm. per 100 c.c. The figures in small circles 
at various points of the curves indicate the actual amounts of alcohol 
present — that is to say, the amount added plus that produced by fermenta- 
tion. For purposes of comparison there is included the curve of an 
experiment made under identical conditions but without the previous 
addition of any alcohol. The general results indicate that when oxygen 
supply is limited to that afforded by complete aeration of the liquid at the 
commencement, a sensible effect on the cell-reproduction of the particular 
class of yeast used 1 begins to be noticeable when the alcoholic content 
reaches about 2-5 per cent., and that amounts of about 8*o per cent, almost 
entirely inhibit reproduction under these conditions. 
If we turn once more to Fig. 1, where the alcohol percentages are 
indicated on Curve B by the figures within small circles, we see that the sudden 
falling off of cell-reproduction, as indicated by the sharp bend of the curve, 
cannot have been due to the inhibitory effect of the alcohol produced, since 
the quantities are too small to have had any such effect. But, although 
under these special conditions of limited supply of oxygen the gradually 
increasing alcohol is not a factor in producing, at a certain point, the rapid 
diminution of cell-reproduction which is so characteristic of all such 
1 The yeast used was an English high-fermentation yeast. 
