90 Franklin Kidd , Cyril West and G . £. Briggs. 
If the compound interest law be applicable to the growth of a 
plant, the rate of interest 1 from week to week, calculated upon 
the assumption that the plant is working at continuous, compound 
interest, will be constant. If this rate is plotted against time the 
result it will be a line parallel to the time axis. These graphs 
show that the rate of interest so far from being constant 
throughout the life-cycle of the plant, goes through a definite series 
F icj ure Z. 
of changes which are correlated with progressive stages in the 
morphological development 2 . Moreover the indications are that 
as a general rule the rate does not remain approximately constant 
even for short portions of the life-cycle of the plant. 
1 The rate of interest is calculated according to the following formula 
Loge W 2 —Logc W,_ m W 2 =Dry weight at the end of the week. 
t r 
W,—Dry weight at the beginning of the week, t=l if the rate is weekly, 
7 if daily., e is the base of the natural logarithms, and r is the rate of interest. 
2 For a more complete discussion of this question the reader is referred 
to (3). 
