NO. 14 EFFECTS OF INFRA-RED ON PLANTS JOHNSTON I3 



If radiation within certain wave length and intensity limits is 

 necessary for the formation of chlorophyll and if, as appears probable, 

 other radiation limits are destructive either directly or indirectly, 

 then the amount of chlorophyll present in a leaf at a given time is 

 the resultant of these two processes of production and destruction. 

 According to Sayre, as noted above, the effectiveness of the wave 

 lengths apparently increases up to 6,800 A., where it ends abruptly. 

 In earlier experiments it appears that no distinction was made be- 

 tween the near and far infra-red, so that definite conclusions cannot 

 be drawn. From the present experiments it would appear that the 

 near infra-red has a decided destructive action on chlorophyll, even 

 great enough to surpass its rate of formation in the presence of wave- 

 lengths shorter than 6,800 A. It should be remembered, however, 

 that these tentative conclusions are based on the appearance of the 

 leaves. Before definite conclusions can be drawn the experiments 

 should be repeated and chlorophyll determinations made. 



From the experiments of Arthur (i) on the production of pigment 

 in apples it appears that the near infra-red radiation alone or in the 

 presence of visible light has a marked detrimental effect on apples. 

 Under these rays a typical wrinkled, necrotic area soon develops. In 

 his work with tomato plants Arthur found that injury occurred with 

 the use of continuous illumination even as low as 150 foot-candles. 

 The fact that the rate of injury was greatly decreased where half 

 sunlight and half artificial light was used emphasizes the necessity 

 for a more thorough investigation of light sources whose distribu- 

 tions differ from that obtained with the Mazda lamp. 



One point should not be lost sight of, namely, that in the region of 

 the strongest chlorophyll absorption bands the plants grown in the 

 distribution including the infra-red receive some three times greater 

 intensity of radiation. This very likely in large measure accounts for 

 the greater increase in dry weight exhibited by the plants grown 

 under this distribution. It is furthermore likely that the higher 

 internal temperatures produced by the more penetrating near infra- 

 red would account to some extent for other differences exhibited. 

 In a future experiment it is hoped to compare two distributions 

 in which the radiation in this region is approximately equalized. For 

 this purpose it will be necessary to secure heat-absorbing filters which 

 cut off at longer wave lengths. 



CONCLUSIONS 



The tomato plants that received both visible and excessive near 

 infra-red radiation under the artificial conditions of these experiments 



