403] F. M. Hildebrandt 205 
measured, it is possible to calculate the dry weight of the 
plant approximately, by multiplying the leaf-area by the 
proper constant. The proportionality between the weight of 
the plant and its leaf area is not quite so constant as that 
between leaf area and leaf product, but in the great majority 
of cases the variation in the ratio of dry weight to leaf area, 
from a constant value, is less than 10 per cent. The rela- 
tions given hold over a very wide range of climatic conditions 
and for plants varying in height from 2 or 3 centimeters 
to 18 or 20 centimeters. Since none of the plants in these 
experiments were grown to maturity, it is impossible to say 
whether this relation holds up to that time. 
From the foregoing facts it may be concluded that the dry 
weight and leaf area of soy-beans 4 weeks old from the seed 
can be determined approximately from their leaflet dimen- 
sions. Soy-bean should therefore be very suitable for use as a 
standard plant for the measurement of climate in the man- 
ner suggested by Livingston and McLean, since the rate of 
its growth can be approximately determined from easily ob- 
tained leaf measurements. Also, the properties of soy-bean 
given above should make it a useful plant for any piece of 
physiological research in which it is desired to know approxi- 
mately the dry weight of the plant used, at various stages 
of its development. 
A METHOD FOR APPROXIMATING SUNSHINE INTENSITY 
FROM OCULAR OBSERVATIONS OF CLOUDINESS 
By F. Merritt HILpEBRANDT 
Air temperature, the evaporating power of the air, and 
sunshine intensity may be considered the main climatic con- 
ditions affecting plant growth and one of the first essentials 
in ecological, agricultural, and forestal studies is some means 
by which these may be measured in the field. We are al- 
ready provided with instruments for measuring the first two, 
