Some Studies on Yeast. 
BY 
HORACE T. BROWN, F.R.S. 
With eight Figures in the Text. 
PART I. 
The Relation of Cell-reproduction to the Supply 
of Free Oxygen. 
T HE rate of increase of living cells undergoing division in an environ- 
ment suited to their growth is influenced by so many factors that any 
attempt to analyse them, and to demonstrate the limitations which they 
severally impose on cell -increase, becomes exceedingly difficult if attention 
is confined to multicellular organisms. 
The problem can be much simplified if we consider a unicellular 
organism such as ordinary yeast, for in such a case we have not only the 
power of controlling the external conditions with comparative ease, but also 
a ready means of estimating the actual cell-increase by direct counting of 
the cells. Moreover, there is the further advantage that the technical 
applications of yeast in the fermentation industries have stimulated inquiry 
to so large an extent that we have a considerable accumulation of facts 
which can be utilized in any such investigation. 
When active yeast-cells are suspended in a well aerated nutrient liquid, 
such as a malt-wort of suitable concentration, and containing an excess of all 
the organic and mineral substances requisite for their complete nutrition, we 
should expect a uniform rate of reproduction to be maintained as long as 
the temperature is invariable and the cells are so sparsely distributed as to 
avoid mutual interference by crowding ; always provided that no restrictive 
influence is exerted by the products of growth or fermentation. Under 
these ideal conditions, if, at stated intervals of time, a census were taken of 
the number of yeast-cells present in unit volume of the liquid we should 
expect to find, on plotting the ‘ density of population’ against the time, that 
the resulting curve was a logarithmic one, and any departure from such an 
experimental rate of increase would necessarily connote the existence of 
some factor or factors tending to restrict the free reproduction of the cells. 
[Annals of Botany, Vol. XXVIII. No. CX. April, 1914.] 
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