NO. l6 CARBON DIOXIDE ASSIMILATION — HOOVER ET AL. 5 



value is not exactly the humidity in the growth chamber, since mois- 

 ture is added by transpiration of the plants and the recirculated air 

 is mixed with the air supply at the bottom of tube C. The error, 

 however, is small since the temperature of tube C may be raised to 

 within half a degree Centigrade of the temperature of G without 

 moisture collecting in tube G. 



The roots of the plants are kept at a fairly constant temperature 

 by passing the water from tube C through a coil immersed in water 

 surrounding the flask of nutrient solution. 



Illumination. — The plants were illuminated by means of eight 500- 

 watt Mazda lamps arranged in two planes perpendicular to the axis 

 of the growth chamber. The two planes were 30 cm apart, and the 

 lights in each plane were equally spaced around the growth chamber. 

 The light intensity was varied by varying the distance of the lamps 

 from the plants. A thermocouple with a cylindrical receiver, con- 

 structed in this laboratory, was used to measure the intensity. In- 

 tensity measurements were made by a thermocouple placed within the 

 growth chamber. For each position of the lights determinations were 

 made at several points along that portion of the tube which was oc- 

 cupied by the plants during observation. A mean of these values was 

 taken, and these mean values were assumed to be proportional to the 

 average intensities on the surface of the leaves. During an experiment 

 the thermocouple was kept at a fixed position in the tube in order to 

 control the illumination. 



To express the intensity of radiation in watts per cm- the thermo- 

 couple was calibrated against a standard radiation lamp yielding a 

 calibration factor, 3.56x10'*. Thus, reducing the light intensities 

 given in Table 2 to absolute units we get for the maximum and mini- 

 mum .0689 and .0058 watt per cm- respectively, or in terms of calories, 

 .985 and .083 calorie per cm- per minute. The intensity as determined 

 by the thermocouple is not the true intensity at the surface of a single 

 leaf, for a single leaf does not receive energy from all the lights. Since 

 the lights are equally spaced around the plants a leaf with its surface 

 perpendicular to the incident beam from one of the lights would re- 

 ceive energy from only one light in each plane. For any other posi- 

 tions of the leaf it would receive energy from two lights in each plane, 

 but since the radiation impinges on the surface at an angle the 

 energy received from the two lights is practically equal to that which 

 would be received from one at normal incidence. Thus we may take 

 34 of the above values, or .246 and .021 calorie per cm- per minute as 

 the approximate limits of the radiation intensity on the surface of 

 the leaves. 



