324 PHOTOSYNTHESIS 



The values oi w (energy used in photosynthesis) may be taken as a 

 measure of the efificiency of the photosynthetic process in the leaf. In 

 the results of Brown and Escombe given in Table 36 a number of points 

 should be noted. The quantity of energy incident on the leaf, R, when 

 full insolation was used is not high, about 0.5 calorie per sq. cm. per 

 hour. This is no doubt due to the condition of the London air. Un- 

 fortunately, in the experiments, the results of which are given in Table 

 36, the actual rates of photosynthesis were not determined. But from 

 other experiments in the same publications, it is apparent that an in- 

 crease in the light intensity from about 0.2 calorie per sq. cm. per mi. 

 to 0.5 calorie does not produce a corresponding increase in the rate of 

 photosynthesis. Brown and Escombe cite one case in which the solar 

 radiation (0.5 calorie per sq. cm. per minute) was reduced to one-twelfth 

 its original value (0.04 calorie) before any effect on the rate of photo- 

 synthesis was observed. The rate at this intensity of illumination was 

 0.00034 cc. i)er sq. cm. per minute and they consider that under these 

 conditions "practically the whole of the rays capable of producing photo- 

 synthesis were utilized for that purpose." They estimate the energy used 

 in photosynthesis under these conditions at 0.0017 calorie per sq. cm. 

 per minute, so that with an incident intensity of 0.041 calorie per sq. cm. 

 per minute 4.1 per cent "also represents the percentage of photosynthesiz- 

 ing energy in the original unscreened solar radiation." It is doubtful 

 whether the results of Brown and Escombe can be accepted in the strict 

 quantitative sense. The experiments do show that the incident radiation 

 can be reduced appreciably without afit'ecting the rate of photosynthesis. 

 Weigert," on the basis of Brown and Escombe's results, has calculated 

 the efficiency of the photosynthetic reaction as being 98 per cent of the 

 energy absorbed by the green parts of the leaf converted into chemical 

 work. 



In other words, in these experiments light was not the limiting fac- 

 tor controlling the rate of the reactions. It is apparent that with a de- 

 crease in R, the per cent of incident energy used in photosynthesis in- 

 creases. The leaf was thus receiving a great deal more energy than it 

 could utilize in photosynthesis. The greater portion of the absorbed 

 energy was dissipated in the vaporization of water. In all probability 

 the concentration of carbon dioxide was the limiting factor. Neverthe- 

 less, the experiments at least approximate the conditions existing in 

 nature and go to show that the photosynthetic process, taken as a whole, 

 with solar radiation, is of a low order of efficiency and that the propor- 

 tion of the absorbed energy used in photosynthesis is a variable quantity. 

 The results also indicate that the efficiency will be at a maximum when 

 the illumination is just of sufficient intensity to produce maximal photo- 

 synthetic rate under the given conditions of temperature, carbon dioxide- 

 concentration and chlorophyll-content. The experiments, furthermore, 

 show what an important role, in the utilization of the absorbed energy, 



'Weigert, Zeit. wiss. Phot., 11, 381 (1911). 



