CHAPTER XI 



ENERGY AVAILABLE FOR RAISING THE SAP 



Energy available for secretion. It has been 

 pointed out earlier that, under normal conditions of 

 transpiration, water is probably extracted from the 

 mesophyll cells and exposed on the outside of these cells 

 to evaporation by a secretory action. Hence the lifting 

 force of the transpiration current in these cases may be 

 attributed to the expenditure of energy by the proto- 

 plasm of the leaf- cells. For this the energy entering the 

 cells at the moment, and that stored as energetic com- 

 pounds in the protoplasm, are available. 



Energy entering the leaf. With regard to the 

 former the data determined experimentally by Brown and 

 Escombe are applicable. 



By use of a Callendar radiometer they found that the 

 maximum amount of energy incident on leaves of plants 

 in Kew in full sunshine amounted to TO to 0*5 cal. per 

 sq. cm. per minute. Had the observations been made 

 in a higher position, and in one free from the veil of smoke 

 hanging over Kew, this amount would have been greater. 

 The coefficient of absorption of the leaves experimented 

 upon averaged about 0*7. Consequently each square 

 centimetre should absorb at least 0'5 x 0*7 cal. per 

 minute of radiant energy. Of this it was found about 

 0*25 cal. might be required for the vaporisation of the water 

 given off (allowing 42 x 10" 5 gr. to be transpired per sq. 



