V. H . Blackman. 
97 
THE SIGNIFICANCE OF THE EFFICIENCY INDEX 
OF PLANT GROWTH. 
By V. H. Blackman. 
B Y the courtesy of the authors, Messrs. Kidd, West and Briggs, 
I have seen in typescript the paper, which appears in this 
number of the New Phytologist, entitled “What is the Significance 
of the Efficiency Index of Plant Growth ? ” 
The main point of the paper on The Compound Interest 
Law and Plant Growth, which recently appeared in the 
Annals of Botany (XXXIII, 353, 1919) was to stress the view 
that no conception of the growth of an annual plant was satisfactory 
which did not take into account the fact that the capacity for dry 
weight production increased with time on the compound interest 
principle. Accordingly it was suggested that the best way of 
comparing the activity in dry weight production of two plants was 
to compare the rate at which the material already present could 
produce new material. On the assumption that the material was 
added continuously—an assumption which is, of course, only an 
approximation to the truth except in the case of plants grown in 
continuous light—a formula was put forward by which this activity, 
or the “efficiency index” as it was termed, could be calculated. 
In their paper Kidd, West and Briggs call in question 
the value of the efficiency index as a measure of a plant’s activity 
on the ground that the index is very variable. That the efficiency 
index is a variable quantity is, of course, obvious, for it must be 
markedly affected by external conditions ; by light deficiency, for 
example, it can be reduced to zero or even to a minus quantity. 
Also Gressler’s results, which were quoted, showed clearly that the 
efficiency index fell off markedly in later periods of growth. 
The marked fluctuations in the index which have been demon¬ 
strated by Kidd, West and Briggs for Maize, Mustard and Sun¬ 
flower, and by Brenchley for the Pea (in a paper in the forthcoming 
number of the Annals of Applied Biology , Vol. VI, No. 4), are of 
great value in that they provide a deeper analysis of the growth- 
relations of the plant at various stages. These fluctuations in the 
efficiency of the plant do not, however, affect the value of the 
efficiency index, for it is obvious, and it is indeed clearly stated in 
the original paper (pp. 357 and 358), that all that the calculation gives 
is the average efficiency index. Kidd, West and Briggs state that 
the efficiency index if a long period is taken “ does not represent 
any actual rate of interest, but merely the rate at which the plant 
would have had to accumulate dry material if it had increased at a 
