216 Variations in Growth-Rates [414 
graphically they give only comparatively slight variations 
from a smooth curve and the actual graph may readily be. 
smoothed to give such a curve. After this has been done the 
ordinates of the smoothed curve, corresponding to the various 
culture periods, may be measured, and the series of graphi- 
cally derived values thus obtained may be taken as a tentative 
scale to indicate approximately the relative growth-rates to be 
expected for this plant in these general surroundings. Of 
course, the seasonal march of the climatic conditions in this 
particular greenhouse must be expected to vary from year to 
year, and it surely varies from greenhouse to greenhouse ; 
nevertheless, the tentative scale derived as just described may 
be of value in several ways. 
For the first sixteen four-week periods of the present 
study, beginning with Feb. 14, as given in the table pre- 
sented above, these relative seasonal indices of growth-rate 
(by either dry weight or leaf area, which appear to be propor- 
tional, or by their average) are respectively as follows: 61, 71, 
79, 86, 91, 96, 99, 100, 99, 96, 92, 87, 81, 75, 68, 61. In this 
scale of growth-rate values the maximum (100) occurs for 
the period ending June 19, and it represents actual average 
growth-rates, as obtained in this study, of 1.24 g. of dry 
weight and 209 sq. cm. of leaf area (one surface only), per 
plant, per period of 28 days. While these derived results 
are extremely tentative and probably only very roughly ap- 
proximate, it is clear that we have here a new kind of descrip- 
tion of the climatic conditions of this greenhouse for the 
spring, summer and autumn of 1916, these conditions and 
their seasonal march being described in terms of their ability 
to produce dry material and leaf surface in the standard plant 
here employed. 
By such a method as this the climatic plant-producing 
power for any four-week period may be directly compared 
with that of any other similar period, no matter when or 
where these periods occur, the standard plant being used as an 
automatically integrating instrument for the measurement of 
