DIFFICULTIES IN ESTIMATING EFFICIENCY 225 



the leaf, by means of differential platinum thermometers, 

 one of which was black and the other bright. The glass 

 chamber did not at first contain the leaf when the full 

 intensity of radiation I was measured ; the plant was then 

 placed in the chamber, and the intensity of transmitted 

 radiation T observed. 



The losses by reflection (r), by emission (e), and by 

 transpiration (t r ), are incapable of exact measurement, as 

 will be seen from considerations to be presently given. The 

 determination of the energy absorbed for photosynthesis 

 is thus subject to considerable errors, since the absorbed 

 energy has to be indirectly estimated by subtracting the 

 various more or less indefinite losses from the energy of the 

 incident radiation. 



Loss by reflection. — Brown and Escombe regard the 

 loss by reflection as negligible ; but this assumption is not 

 fully justified. The loss would have been negligible had 

 the index of refraction of the substance of the leaf been 

 the same as that of air ; but this is far from being the 

 case. 



Loss by emission. — This depends on the rise of tempera- 

 ture produced in the leaf by the absorption of light. It is 

 extremely difficult to measure the actual rise of temperature 

 of the substance of the leaf, and the loss of energy by re- 

 radiation and in convection-currents of air cannot be found 

 with any degree of accuracy. 



Loss of energy in transpiration. — From the heat for 

 vaporisation of water the loss of energy could be estimated 

 if we knew the exact quantity of water transpired by the 

 leaf ; this is not at all an easy matter, the difficulty being 

 accentuated by the fact that transpiration does not remain 

 constant, but undergoes change with the unavoidable varia- 

 tion of temperature and of light during the course of 

 prolonged experimentation. 



The following tabular statement gives the results obtained 

 by Brown and Escombe in their experiments on Polygonum 

 Weyrichii. 



Q 



