QUALITY OF RADIATION 199 



leaves will be developed, although the period of exposure is 

 much too short for the development of chlorophyll, and the 

 internodes will become shorter. The quantity of light re- 

 quired is very small and the change effected is independent 

 of the products of photosynthesis. Further, if the plant be 

 exposed to light after growth has been stopped by low tem- 

 perature or by the action of anaesthetics, the morphological 

 change still takes place when growth is resumed, which indi- 

 cates that the action is not exerted on the " living metabolic 

 machinery." Priestley considers that the response is due to 

 a photocatalytic action on some metabolic product, a view 

 which is supported by Gregory.* 



This work introduces questions relating to the irritability 

 of the plant in response to the action of light, on which there 

 is a voluminous literature, a problem which is outside our 

 present survey. f 



From these investigations it is clear that the reaction of the 

 plant to light is very complex ; that the total effect observed 

 is the balance of the measures of the accelerating and depres- 

 sing influences of light ; that the magnitude of the effect 

 depends upon the intensity and the duration of light ; and 

 the possibility of a photocatalytic reaction. 



The above observations relate to ordinary white light ; 

 the quality of radiation affects the growth of plants according 

 to the wave-length and to the physiological peculiarity of the 

 species. 



Schanz, J in continuance of his earlier work,§ grew Begonia, 

 Cucumis, Lobelia, Petunia, and other plants in frames arranged 

 in a series of eight ; by means of glass of different opacities, 

 all but the first, which was uncovered, were illuminated by 

 light from which certain rays had been abstracted :— 



1. Unaltered daylight. 5. Rays longer than 560 /x/x. 



2. Rays longer than 320 /x/x. 6. Yellow light. 



3. Rays longer than 380 /x/x. 7. Green light. 



4. Rays longer than 420 it/x. 8. Blue-violet light. 



* Gregory : " Ann. Bot.," 1928, 42, 469. 



f See Priestley : " New Phvt.," 1926, 25, 227, and the literature there 

 cited. i Schanz : " Ber. deut. bot. Ges.," 1919, 37. 43°. 



§ Id.. 1918, 36, 619. 



