SUMMARY 237 



The efficiency is modified to some extent by the physio- 

 logical condition of the plant. The different values for the 

 efficiency did not, however, vary to any great extent, the 

 lowest efficiency observed being 5-8 and the highest 7-4 per 

 cent. 



It is interesting to compare the efficiency of transforma- 

 tion in an ordinary steam-engine with that of the photo- 

 synthetic organ. In the former the potential energy of coal 

 is transformed into the kinetic energy of motion ; in the 

 latter the kinetic energy of radiation is transformed into the 

 potential energy of complex chemical compounds. The 

 efficiency of the photosynthetic organ may be taken as 

 about half that of an ordinary steam-engine. 



Summary 



Previous determinations of photosynthetic efficiency 

 suffer from the disadvantage of the absence of adequate 

 means for the exact measurement of the incident energy, 

 of the various indeterminate losses to which that energy 

 was subject, and of the energy stored by the plant. 



In the experimental determination of the photosynthetic 

 efficiency of the leaves of Hydrilla described in the present 

 chapter, the losses were practically eliminated by the fact 

 that the plant was immersed in water. 



The energy absorbed by the leaves was very accurately 

 determined by a sensitive Calorimeter, in which the rise of 

 temperature was indicated by a thermo-electric couple. The 

 loss of heat by radiation was eliminated. 



The accuracy of the calorimetric determination was 

 tested by the independent method of the Magnetic Radio- 

 meter. Similar values for coefficients of transmission and 

 of absorption were obtained by the calorimetric and the 

 radiometric methods. 



The method is so sensitive that a simultaneous deter- 

 mination of the absorption and the storage of energy was 

 obtained in a time as short as three minutes. 



