439° BOTANICAL GAZETTE [DECEMBER 
treatment of such changes in physics as well as physiology. The 
two second half-hour periods of sunshine will be termed the first 
and second sun exposures, the second half-hour of shade giving us 
the measure for the shade exposure. All of the sun-shade ratios 
are given in table II. Examples may make the procedure of their 
derivation more evident. The first ratio of test I for Physals 
is 3.0+1.7, or 1.76. The second ratio for the brown atmometer 
in test II is 4.12.5, or 1.64, etc. 
TABLE If 
SUN-SHADE RATIOS OF RATES OF WATER LOSS 
Physal. | Xanth. | Blk. atm. | Brn. atm. | Wht. atm. 
est E evecaha deg 1.76 35 2.05 1.62 1.17 
(canvas) 2d sun exp.....} 3.00 1.88 1.90 1.72 4-49 
est ist sunexp....} 2.68 | 1.22 1.60 1.35 1.35 
(cheesecloth) / 2d sun exp..:..| 1.84 ae 1.64 1.43 1.52 
If both plants were influenced alike and only by the direct 
heating of the sun’s rays, and if the instruments were affected by 
radiant energy just as were the plants, that is, per unit of surface 
exposed, then we should expect all these ratios to be equal. _ In 
so far as they are not equal, they signify a variation in the effect 
produced upon the two plants and upon the three instruments by 
the same alterations in light intensity. Thus, if any one of these 
instruments were used as a basis for light measurement, to predict 
the influence of light changes upon either of these plants, the instru- 
mental result must obviously be corrected. Since it is already 
clear that the two plants do not entirely agree in their sun-shade 
ratios, it will be necessary to find correction coefficients, not simply 
for each instrument, but for each instrument for each plant. From 
the sun-shade ratios of table II have been calculated the correction 
constants, by which the ratio of any instrument for any period is 
to be modified (multiplied) so as to equal the corresponding ratlo 
for either plant, and these coefficients are given in table IT. As an 
example of the method of derivation, the first coefficient of cor- 
rection for the black atmometer in reference to Physalis (test I) is 
