ACIDITY 135 



of pH 9-4 — 7-6 the process comes to a standstill.* In Penicil- 

 Hum chrysogenum variations in the value of pH between 4 and 

 8 do not affect the normal rate of respiration, that is, the rate 

 at neutrality, pH = 7 ; an increase of the value to 8-8 results 

 in the respiration decreasing to 60 per cent, of the normal, at 

 which level it remains ; a decrease, on the other hand, in the 

 value of pH to 2-65 causes a gradual rise in the respiration rate 

 followed by a gradual fall to the normal. At pU i-io to 1-95 

 the preliminary rise, amounting to 20 per cent., is followed by 

 a fall to below normal. The depression brought about by a 

 concentration pH = 1-95 or less is irreversible, whilst the simi- 

 lar decrease effected by a pW value of 8-8 is reversible, the 

 respiration rate returning to normal after the plant is placed 

 in a neutral solution. In acid solutions there is an increase 

 in the production of carbon dioxide and a decrease in alkaline 

 solutions, a phenomenon which may be paralleled in the test 

 tube : a neutral solution of dextrose and hydrogen peroxide 

 shows an increase in the evolution of carbon dioxide on the 

 addition of acid but not on the addition of alkali. f Witze- 

 mann % also has demonstrated the oxidation of sugar by hy- 

 drogen peroxide in the presence of disodium hydrogen phos- 

 phate. 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 demonstrated. 



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 photosynthetic 

 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 



* Meyerhof : loc. cit. 



f Gustafson : " Journ. Gen. Physiol.," 1920, 2, 617 ; 3, 35. 



% Witzemann : " Journ. Biol. Chem.," 1920, 45, 1. 



§ Vol. I., p. 494- 



