774 BIOLOGICAL EFFECTS OF RADIATION 



valid today. Deherain (9), whose work did not convince Sachs, con- 

 cluded that light, not heat, determined transpiration, and that the 

 optically brightest part of the sun's spectrum, that is, the red and yellow 

 portions, caused the greatest rate of transpiration. Flammarion (12) 

 obtained maximum transpiration in the orange-yellow region and mini- 

 mum rate in the violet. On the other hand, Wiesner (54) advanced the 

 idea that the visible portions of the spectrum which have the greatest 

 influence on transpiration are those corresponding to the absorption 

 bands of chlorophyll and that the most effective wave-lengths, therefore, 

 are not the optically brightest ones, but the blue and red ones. The blue 

 region was more effective than the red. Yellow, and especially green 

 light, he found to be least effective. Moreover, he attributed the 

 influence of light on transpiration principally to its heating effect. 

 Wiesner's conclusions were essentially upheld by a number of investi- 

 gators who followed, including Henslow (14). In none of these investiga- 

 tions were light intensities satisfactorily equalized. In many cases the 

 filters used were not spectroscopically pure. Precautions were not taken 

 to avoid heating effects independent of the radiation under consideration. 

 The sunlight source generally used was a variable quantity. 



A more recent investigation in which an attempt was made to 

 correct some of the errors in method and assumption of the earliest 

 workers, particularly those of Wiesner and his supporters, is that 

 of Iwanoff and Thielmann (19). In most of their experiments they 

 used an arc light source behind a ferrous-sulfate solution, used to absorb 

 infra-red radiation, and two color filters. One of these, a copper ammo- 

 nium sulfate solution transmitted up to wave-length 5350 A ; the other, a 

 potassium bichromate solution, transmitted down to 5500 A. Since 

 they had no means of measuring the light energy actually absorbed by 

 their experimental objects, the authors determined by means of a thermo- 

 pile and galvonometer the relative total incident energy and equalized 

 the total intensities of the light energy falling on the leaves by regulating 

 the distance from the light source to them. A great many experiments 

 were performed with detached leaves of Cyperus alternifolius, Libertia 

 formosa, and Bromus inermis. Potted plants of Cyperus were also used. 

 Transpiration rates were determined by taking several readings (loss 

 in weight) at 10-, 15-, or 30-min. intervals for each specimen under each 

 condition. The readings represented amounts transpired for short 

 periods only, of about 10 min. duration, since the rate was found to 

 decrease more or less under constant external conditions, as time went 

 on. 



The results showed that when leaves or potted plants were transferred 

 from red-yellow to blue-violet light of approximately equal intensity, 

 they always showed a more or less marked increase in rate of transpira- 

 tion, while if they were transferred from blue-violet to red-yellow light, 



