92 Franklin Kidd , Cyril West and G. E. Briggs. 
definite time the rate of interest depends not only upon the species 
of plant and upon the external conditions but also upon the 
stage of morphological development reached and upon the previous 
history of the plant. 
The only way in which plants can be compared is by a 
comparison of the whole series of efficiency indices throughout 
their life-cycle. From such comparison useful deductions can be 
drawn, as the present authors have shown (3), and in a broad way 
the general efficiency of plants can be roughly compared. 
From an economic point of view it may be suggested that r 
would be a useful comparison if calculated over the complete life-cycle, 
but here the essential idea of the “ Substanzquotient” put forward by 
Noll and his pupils is far preferable, as it simply states the ratio of 
final weight to seed weight divided by time, and makes no 
assumption as to the rate at which the addition to dry material has 
been made. Unless time is of importance the ratio of final weight 
to seed weight is sufficient. 
In order to show definitely the uselessness of r averaged over 
an arbitrary period or of the “ Substanzquotient” for that period as 
a standard of comparison of the efficiency of different species we 
have taken the same results as those quoted partially by Blackman 
and have tabulated them in full (Table 1). The weights of the 
seedlings on germination, the weights of the plants after 
approximately equal periods and the efficiency indices calculated 
for various periods are given. The efficiency index from week to 
week for Helianthus macrophyllus gigantens and for Helianthus 
cucumerifolius nanus have also been calculated and are reproduced 
graphically in Fig. 1. 
Column 4 shows that the average efficiency index for the first 
four and a half to five weeks is practically the same for four of the 
species but much smaller for H. cucumerifolius nanus ; column 8 
shows that the same four species after about 8 weeks still agree in 
having roughly the same efficiency index, and that at the same time 
H. cucumerifolius nanus is less, but whereas H. cucumerifolius 
nanus has maintained, and even slightly increased its r as 
compared with the first period, the other four species have all 
fallen off. Column 12 gives the average efficiency index up to the 
time of the final record of dry-weight. Whether the experiment 
was terminated at that point because the plants had completed 
their growth or whether for other reasons cannot be stated with 
certainty. Since the dates of the final record vary considerably, 
