Quantitative Analysis of Plant Growth. 201 
as an organism be considered, growth appears most broadly 
speaking as a process of increment in dry-weight. Accordingly, in 
line with the practice of previous investigators in this branch of 
plant physiology increase in dry-weight may be adopted as the 
best measure of growth. 
The Study of Growth Bates as opposed to the Study of Final Yields. 
Until recently the idea of rates as applied to the growth of the 
plant as a whole has been only vaguely conceived. Investigators, 
especially those working along economic lines, have been concerned 
mainly with the final yields. A study of plant growth from the 
outlook of rate is, however, more fundamental in so far that it aims 
at disentangling the details of the mechanism of growth and at a 
precise knowledge of the factors limiting growth throughout its 
course. 
A solution of the problems of growth may be regarded primarily 
as the precise determination and evaluation of those factors, 
internal and external, which control the rate of the plant’s increase 
in dry-weight from time to time throughout its life-cycle. 
Secondarily, after these factors are known and their values 
expressed in appropriate units, we may he in a position to state 
constants for the growth-rate of particular plants applicable to the 
whole life-cycle, and such constants will not only he of use from 
the economic point of view in comparing plants, but also will have 
physiological significance. 
A rate is the amount of change per unit time. The actual 
rate of growth is the increase in dry-weight per unit time and 
depends upon various external factors and upon what we may call 
the amount of the “growing material.” 1 
Growth Bate expressed per Unit of Dry Weight. 
It is clear that if we could determine the unit of “ growing 
material ” it would be possible to express the rate of growth per unit 
of “ growing material,” and we should then be in a position 
to evaluate the various external factors and to determine 
specific plant constants. Under constant external conditions the 
rate so expressed would remain constant for a given plant, and 
changes in rate observed would be related to changes in external 
factors when these factors are limiting the rate. Let us take the 
simple case of a population of unicellular organisms growing in a 
medium the composition of which, together with other environmental 
factors, is constant. In such a case the actual rate of growth, as 
1 The term “amount of growing material” is equivalent to the ‘ active 
mass ’ of the reactant in a chemical reaction. 
