2 Carbon Assimilation. 



e.g., those concerned in growth and irritabih'ty. However, the 

 subject is likely to prove an extremely difficult one in respect to 

 plant physiological problems, more especially because it is so 

 difficult in a complex series of reactions to distinguish between 

 purely photochemical reactions and the chemical actions coupled 

 with them. 



Consequently the usual characteristic of a photochemical 

 reaction of a temperature coefficient close to unity is one of which 

 cautious use must be made in drawing conclusions as to reactions in 

 the living organism where a complex of physical and chemical 

 processes may between them produce any possible temperature 

 coefficient. Yet the fairly high temperature coefficient, between 2 

 and 3, obtained by F. F Blackman for carbon assimilation is fairly 

 clear evidence of the complexity of the processes of carbon 

 assimilation, as it indicates that the photochemical process must be 

 coupled with actions of another kind. 



Similar difficulties present themselves when we attempt to 

 analyse the energy relations of the green leaf. The radiant energy 

 absorbed by the leaf is partly transformed into heat energy which 

 results in raising the temperature of the leaf, and partly into 

 chemical energy, but it is extremely difficult to distinguish between 

 the amount of energy transformed into heat and that transformed 

 into chemical energy, 



Yet this transference of radiant energy into chemical energy is 

 of immense importance in the world, for upon it all life depends. 

 Without it of course the organic world as we know it would be non- 

 existent. Also our present well-developed industrial civilisation 

 can be directly traced to this transformation of radiant energy into 

 chemical energy ages ago. It is of course owing to the sun's energy 

 stored in coal, the result of carbon assimilation in past periods, that 

 industrial development occupies its present position. It has been 

 well shown by A. H. Gibson (1913) how comparatively inadequate 

 are other natural sources of energy to replace that of coal when the 

 world's coal supply becomes exhausted, and it becomes evident that 

 some means of utilising the sun's energy such as th» plant is able to 

 do, will have to be developed if our civilisation is to continue. 

 Whether this can be secured by growing plants which produce 

 material of high calorific value, or whether the energy of sunlight 

 can be better stored directly by some photochemical reaction, 

 cannot even be guessed at present. 



