242 Priestley and Pearsall. — Growth Studies. II. 
with the increase in number of yeast cells, until finally cell-division practi- 
cally ceases. 
It is recognized that this brief statement must necessarily understate 
the complexity of the conditions prevailing, but it seems so essential that 
this picture of the mechanism should be clear, that it may be restated 
with the help of Fig. 2. In this figure successive generations of yeast are 
pictured as arising at equal intervals of time represented by the horizontal 
TIME 
2 4 & 12 16 18 19 /9 
a b 
Fig. 2. Diagrammatic representation of the growth 
of yeast in a limited amount of nutrient solution. 
the curve is shown to depend upon the limiting supply of oxygen. Growth 
under aerobic conditions must certainly be based on two general types of 
reversible reaction that are mutually interdependent: 
(1) the type characteristic of synthetic metabolism 
A + B C + H 2 0 . 
(2) the type providing for the release of energy 
x+o 2 — y+co 2 . 
A limiting supply of oxygen will affect the progress of the second type 
of reaction and hence the rate at which Y is formed and energy released. 
Since we assume synthetic metabolism to depend on the presence of a source 
of energy, the first type of reaction will also slow down and C will be 
produced more slowly. The reactions of the first type have to be 
considered as proceeding in a long and complex chain (as in the synthesis 
of complex proteins from amino-acids), where C,the product of one reaction, 
forms a starting-point for the next reaction in the chain. It is improbable 
distances. Then in section a 
each cell gives rise to two 
cells at every time interval. In 
section b a constant number of 
cells gives rise to new cells, the 
remaining cells passing over 
into a resting stage. In region 
c the number of dividing cells 
decreases until all are in the 
resting stage and no growth 
ensues. 
This simple statement ne- 
glects the possibility of varia- 
tion in the rate of growth of 
individual cells and simply 
assumes a sharp contrast be- 
tween the dividing and the 
resting condition of the cells. 
It will obviously support 
further analysis. The transi- 
tion from the a to b regions of 
