INFLUENCE OF LIGHT 93 



hydrogen peroxide show an increase in the evolution of carbon 

 dioxide on the addition of acid but not on the addition of 

 alkali.* Witzemann f also has demonstrated the oxidation of 

 sugar by hydrogen peroxide in the presence of disodium 

 hydrogen phosphate. Whether the phosphate here plays any 

 part in the formation of a hexose phosphate such as is known 

 to occur in alcoholic fermentation + has yet to be demon- 

 strated. 



LIGHT. That plants respire both by day and by night is 

 a well-known fact from which it would appear that radiant 

 energy as such is not a conditioning factor in respiratory 

 activity ; its action is indirect in providing through its photo- 

 synthetic activity a supply of respirable material. Thus an 

 isolated green leaf in darkness shows a continuous fall in 

 respiration ; in the light, on the other hand, the respiratory 

 values during a carbon assimilation experiment are continually 

 changing by virtue of the carbon assimilation and may be 

 doubled by an exposure of four or five hours to light and 

 carbon dioxide. This is possibly due to the production of new 

 carbohydrate although the observed increase is not proportional 

 to the amount of carbon dioxide decomposed. The high in- 

 tensity of respiration sometimes observed in green plants in 

 bright sunshine || is due, at any rate in part, to the copious 

 supply of sugar. Under such conditions, however, other factors 

 are operative : the higher temperature, for instance, would in- 

 crease the respiration intensity ; whilst if the illumination, the 

 temperature and humidity conditions of the atmosphere were 

 operative all in the same direction to cause undue loss of 

 water, the flaccid leaves would ultimately show a respiratory 

 depression. 



In addition to this indirect action of light, there appears 

 to be some direct action, for Spoehr 1T finds that the respiration 

 of the plant is regularly higher under conditions which vary 

 only as regards daytime air and night-time air. The differ- 

 ences observed are, however, not considerable ; in the case of 



* Gustavson : " Journ. Gen. Physiol.," 1920, 2, 617 ; 3, 35. 



t Witzemann : " Journ. Biol. Chem.," 1920, 45, i. 



Vol. I., p. 379. Matthaei : loc. cit. 



II See Rose" : " Rev. ge"n. Bot.," 1910, 22, 385. 



IT Spoehr : " Bot. Gaz.," 1915, 59, 366. 



