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V . H. Blackman . 
constant rate of continuous compound interest.” This is of course 
perfectly true and state the nature of the index very clearly. The 
efficiency-index-like averages in general is only an abstraction, but 
it is none the less valuable as a basis for comparison. 
The three authors further insist that the only way in which 
plants can be compared is by a comparison of the whole series of 
efficiency indices throughput their life-cycle. This is rather a large 
claim. Surely in the absence of such full data the comparison of 
the average efficiency for larger periods or for the whole life-cycle 
is of value. And even if the daily or weekly efficiency indices are 
available for the whole life-cycle it is still useful for purposes of 
comparison to sum up the activity of the plant for longer periods 
by calculating the average efficiency index on the supposition 
that it is constant throughout those periods. 
The efficiency index was described as an “ important physio¬ 
logical constant.” The word “ constant ” might have been put in 
inverted commas in the original paper, for the only real constants 
of a living organism are, no doubt, those of the physical and 
chemical processes of metabolism. There is, however, a precedent 
for the use of the term for the rate of an important physiological 
process, for Dugger, in his hook “ Plant Physiology ” (p. 255), uses 
the term for the average rates of such physiological processes as 
photosynthesis, transpiration, etc. It should have been clear that 
the use of this term “ constant ” in relation to the efficiency index 
did not, and could not, imply that the rate of addition of new 
material was an invariable quantity. 
In supposing that the index would he the more accurately 
determined the shorter the period of time under consideration, the 
three authors are pressing too closely the analogy with a physical 
constant, and exhibit a failure to realise—and this seems to lie 
at the basis of their criticism—that the index is almost necessarily 
only an average of a number of widely differing rates. 
Data obtained from daily periods would not necessarily be any 
more valuable than those for longer periods, nor hourly data any 
more valuable than daily ones. A little consideration will show 
that, except for plants grown under external conditions which are 
kept artificially constant, the rate of increase of material must wax 
and wane with the rising and setting of the sun, and that during 
the night the rate becomes a negative quantity. The efficiency index 
when applied to plants growing under natural conditions is thus 
necessarily only an average or conventional measure of the plant’s 
