Significance of the E fficiency Index of Plant Growth . 91 
It is clear that the growth of a plant obviously does not 
follow the compound interest law in any exact sense as implied by 
the equation of Blackman. It is, of course, possible to calculate 
the rate of interest from week to week as if the plant did follow 
this law, as has been done in the case of the above figures. The 
index r then is not a constant but a function of time or of the stage of 
morphological development. 
Blackman says that “ The rate of interest (r of the equation) 
is clearly a very important physiological constant. It represents 
the efficiency of the plant as a producer of new material , and 
gives a measure of the plant’s economy in working. The rate of 
interest, r , may be termed the efficiency index of dry weight 
production.. It may also be termed the ‘ economy constant ' 
of the plant; it is of course comparable to the velocity constant of 
a chemical reaction.” The impression given by this paragraph and 
indeed throughout the paper is that the “efficiency index” has a 
physiological significance and is a constant which provides us with an 
important standard of comparison between plants of different species 
and between plants of the same species grown under different 
conditions. 
The value of r can be determined over the whole or any part 
of the life-cycle. Limitations of experimental method make it 
impossible to observe results over short enough periods to allow 
the values of r to be an absolutely correct representation of actual 
rates, but the shorter the period the more closely do the values 
of r approximate to these. If a long period is taken the value of 
r does not represent any actual rate of increase, but merely the 
rate at which the plant would have had to accumulate dry material 
if it had increased at a constant rate of continuous compound 
interest throughout the period. 
As indicated by Fig. 1 and as made clear by Table 1 the 
average value of the efficiency index for any period depends upon 
the period taken, and moreover in comparing' one plant with 
another the ratio of these averages varies with the period taken. 
This shows that an average is not a constant for a plant nor is it 
a standard for comparison with other plants. 
We will now consider whether the rate of interest at any 
point in the life-cycle calculated for the shortest possible time, 
gives any indication of the efficiency of the plant when comparing 
it with other plants for which the rate of interest is similarly 
calculated. The answer is in the negative, for the reason is that any 
