136 RESPIRATION 



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 intensity 

 of respiration sometimes observed in green plants in bright 

 sunshine f is due, at any rate in part, to the copious supply 

 of sugar. In 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. 



Lundegardh $ has shown that the respiration of shade 

 plants, e.g. Oxalis, is lower than in sun plants, e.g. Nasturtium 

 and Atriplex. In the former an equilibrium between respira- 

 tion and carbon assimilation, the compensation point (see 

 p. 7), occurs in normal air at a light intensity of 1/120 to 1/140 

 that of direct sunshine, whilst in sun plants the equilibrium 

 is reached in a light intensity of 1/40 to 1/60. In order that 

 the carbon assimilation of shade plants may compensate the 

 respiration during the night, an average intensity of illumin- 

 ation of 1/93 that of sunshine at a temperature of 18 C. is 

 necessary and an increase of this light intensity is requisite 

 for the maintenance of growth. Of shade plants in general, 

 Oxalis has a great power of carbon assimilation and this helps 

 to explain why it can grow in places where the light intensity 

 is very low. 



TI IE MECHANISM OF OXIDATION, AND THE ENZYME 



SYSTEMS INVOLVED. 



The catabolic processes of plants may be directly referable 

 to specific enzymes, zymase for instance in alcoholic fermenta- 

 tion ; but in the respiratory activities of higher plants, their role 

 is not defined with that precision and degree of completeness 



* Matthaei : loc. cit. 



f See Rose : " Rev. gen. Bot.," 1910, 22, 385. 



% Lundegardh : " Svensk. bot. Tidskrift.," 1921, 15, 46. 



