100 Significance of the Efficiency Index of Plant Growth. 
fluctuations in the efficiency index, on which the three authors 
rightly lay stress, clearly indicates that caution must be exercised 
in making deductions as to a plant’s general economy of working 
from the efficiency index calculated over short periods, but the 
efficiency index does nevertheless give a measure of the plant’s 
average efficiency during any particular period. 
As was pointed out in the original paper we know nothing of 
the experimental errors of Gressler’s results and he admits that the 
number of plants used for the dry weight determinations was 
sometimes very small so that the results cannot with assurance 
be subjected to a very close analysis. Up to the 7^-8th week 
the dwarfness of H. cucumerifolius nanus is certainly due to a low 
efficiency index as well as to a small seed weight, as the table 
provided by Kidd, West and Briggs shows, for the index of this 
plant is markedly lower than that of any of the other four forms; 
and even up to the 12J—13th week the index of this particular 
species is still the lowest. It is true that when the last 
determination is made the indices of both H. uniflorus giganteus 
and H. macrophyllus giganteus fall below that of H. cucumerifolius 
nanus. It is to be noted, however, that during the last period of 
growth shown in the table the dwarf form more than doubles in 
weight, while the two other forms increase only 18% and 38% 
respectively, in spite of the fact that the period in question is longer. 
It is obvious that the average efficiency index over the whole 
period of growth has been reduced, both in the case of H. uniflorus 
and H. macrophyllus , by a long period of slow growth in the last 
stage of development. Whether this period is absent in H. cucumeri¬ 
folius nanus or the plants were cut down too early to exhibit it is not 
clear; the latter would seem more probable. 
The three authors have also criticised Dr. Brenchley’s 
use of the efficiency index in exhibiting some of her results. In 
the case of the barley plants she investigated the plants differed in 
maturity in relation to harvesting, but it is clear that they had 
practically reached their full size, and in Dr. Brenchley’s 
opinion it is probable that in all of them increase of dry weight 
had ceased. The mustard plants if allowed to continue growth 
would certainly have increased considerably in dry weight, but 
as already stated the efficiency indices do give a measure of the 
economy of working of the various sets under the given conditions 
and for the given period, and so exhibit the effect of external 
conditions more accurately than do the final dry weights alone. 
