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. MERRILL HILDEBRANDT 



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, 



