CHAPTER XXV 



EFFICIENCY OF THE PHOTOSYNTHETIC ORGAN IN 

 STORAGE OF SOLAR ENERGY 



Different determinations of the efficiency of the photosynthetic organ — 

 Error arising from incorrect values of carbohydrate-factor and of 

 heat of combustion of the photosynthetic product — Unsatisfactory 

 determination of losses by reflection, by emission and by transpiration 

 — Elimination of these losses in water-plants — Radiometric and 

 calorimetric determinations of energy — Determination of the co- 

 efficients of transmission and of absorption by the Radiometer and 

 the Calorimeter — Simultaneous determination of energy absorbed and 

 of energy stored — Typical examples — Comparatively high efficiency 

 of the photosynthetic storage of energy. 



The economic life of the present age may be said to be 

 dependent to a great extent on the utilisation of the solar 

 energy that had been stored in past ages by vegetable life. 

 What is the efficiency of the physiological mechanism by 

 which this was accomplished ? From Boussingault's result 

 that a square metre of actively assimilating leaf-surface of 

 Nerium Oleander forms 0-000538 grm. of starch in one 

 second, Pfeffer (1871) attempted to form an approximate 

 estimate of the efficiency of the plant as a transformer of 

 energy. Assuming the heat of combustion of one gramme of 

 starch to be 41,000 calories, he found 2-2 calories per square 

 metre per second to be the amount of energy conserved by 

 the leaf. Taking the energy of sunlight from Pouillet's 

 results to be 333 heat-units per second per square metre, 

 he estimated that the energy stored is about o-6 per cent, 

 of the incident radiation. It is obvious that the amount 

 of solar energy received is not constant, but varies with the 

 time of the day, the inclination of the leaves, the hygro- 

 metric state of the air, the altitude of the place, and so on. 

 The result is therefore to be taken as only an approximation. 



