Significance of the Efficiency Index of Plant Growth 95 
most cases only the lowest flowers were fully developed. 
The five plants per pot were of medium strength only. 
and were the most mature of all. The flowers were fully 
out, and many on the main stem had begun to form fruit.” 
In the case of the barley “ the thinly seeded plants were better 
developed and much less mature than the more crowded 
specimens.” 
The author gives the dry-weights of the plants at the time of 
harvest and the efficiency indices calculated by Blackman’s 
formula, but she does not give the seed weights. 1 As our previous 
discussion has shown, the average efficiency index for a given 
plant tends to decrease with increasing maturity, and consequently 
it is misleading to compare the dne plant per pot which was 
decidedly less mature, with the other plants (other numbers per 
pot) which were more mature. Possibly, had the whole lot been 
grown to maturity—the five plants per pot would have taken less 
time—the average efficiency indices would have been much closer 
together or might have been the same. 
Again, one may be tempted to compare the mustard with the 
barley. For this purpose the efficiency index is of no use since the 
plants (mustard and barley) were grown for different periods and 
had reached different stages of maturity. 
The efficiency indices as given by Brenchley, if her seed 
weights were the same, are simply the logarithmic functions of 
the final weight divided by time, the time for each set of 
experiments being the same. It is quite clear that from the point 
of view of comparing the effect of different seedings per pot in 
Brenchley’s particular experiments the efficiency index has no 
advantage over the simpler method of giving final dry-weights, 
assuming the seed-weights were the same, but rather the reverse, 
because the former implies some physiological significance of the 
figures given for the efficiency indices. 
Summary. 
A careful consideration of existing data upon plant growth 
does not warrant a rigid application throughout the whole 
life-history of a plant of the compound interest conception of 
plant growth advocated by V. H. Blackman and formulated in the 
equation W 1 =W 0 e rt . The value r regarded by Blackman as a 
1 We have calculated the seed-weights from the efficiency indices and 
dry weights given by Brenchley. Whereas the seed-weights in the case of 
barley are all exactly the same (55 mgs.), in the case of mustard (variety not 
stated) the seed-weights form a descending series starting from the one plant 
per pot (18-3 mgs.) and ending with the five seeds per pot (15*5 mgs.) 
