Priestley and P ear sal L — Growth Studies. II. 241 
of the occurrence of an S growth-curve will require individual analysis 
before profitable generalizations can be developed. 
The Rate of Growth of Yeast. 
It is of interest to note that if all the data available for the growth of 
yeast are collated, from the time that a few cells are seeded into a definite 
quantity of suitable nutrient medium until the time that growth comes 
to a standstill, they give a curve of the general type shown in Fig. 1. 
Moreover, these data, though appearing in the work of different observers, 
show that this curve can be divided into three distinct regions, a , and c, as 
shown in Fig. 1. In the region a increase in mass is an exponential 
function of time, in region 
b the rate of growth is 
directly proportional to time, 
and in region c there is a 
rapid fall of growth-rate. 
The nature of the earlier 
part of the curve (a) was 
established by Slator ( 17 ), 
who has shown that when 
the nutrient liquid is inocu- 
lated withverysmall amounts 
of yeast (1,360 to 90,000 
cells per c.c.), a logarithmic 
rate of increase follows. 
This may be regarded as 
the natural rate of increase in 
yeast, when every cell is actively growing, and at equal intervals of time one 
cell gives rise to two, two to four, four to eight, and so on. This ex- 
ponential rate of increase is even more readily recognized when growth 
is by fission, as in bacteria. 
When the rate of growth becomes proportional to time, Horace Brown 
(6) supplies valuable evidence showing that the amount of oxygen available 
is the limiting factor, under the normal conditions of the culture medium. 
The concentration of oxygen above the nutrient fluid is constant, and 
hence in this part of the curve the number of cell-divisions in each unit of 
time also remains constant. 
In the last part of the curve, it appears from Adrian Brown’s work ( 5 ) 
that the growing crowding of the cells in the nutrient fluid causes 
the available oxygen supply for an individual cell to fall below the 
minimum quantity essential for cell-reproduction. This effect increases 
