THE ENERGY REEATIONS IN PHOTOSYNTHE'SIS 325 



is played by transpiration and that, under circumstances, the cooHng effect 

 of the transpiration is such that the leaf absorbs heat from the surrounding 

 air. 



It should perhaps also be pointed out that these experiments of Brown 

 and Escombe constitute the first steps in determining the energy rela- 

 tions of illuminated leaves, that they have served to indicate the various 

 factors which must be considered and the experimental difficulties which 

 are encountered in such an undertaking. The accuracy of the results 

 depends upon a number of determinations, foremost of which are the 

 determinations of R, the total incident radiation, the absorption coeffi- 

 cient of the leaf, the accurate estimation of very small quantities of car- 

 bon dioxide in an air-stream, and the amount of water lost by trans- 

 piration. Due consideration must, of course, also be given to the effect 

 of illumination on the opening and closing of the stomata and the effect 

 of this on the gaseous exchange and on transpiration. Finally, it should 

 be mentioned that the experiments of Brown and Escombe were carried 

 out with detached leaves, and that these show a rate of photosynthesis 

 considerably higher, as measured by the carbon dioxide taken in (about 

 45 per cent) than leaves attached to the plant. 



Puriewitsch * endeavored to determine the energy used in photosyn- 

 thesis from the increase in the heat of combustion of leaves after insola- 

 tion and by calculating the ratio of this increase to the total amount of 

 radiant energy falling on the leaf. For measuring the intensity of the 

 solar radiation a bolometer was used ; this was kept at right angles to the 

 incident light. Photosynthesis was determined with excised leaves ; these 

 were kept in the dark for 24 hours prior to their use. The leaves were 

 cut in half along the mid-rib ; one half was immediately dried and its heat 

 of combustion determined, the other half, with the petiole in water was 

 exposed at right angles to the sunlight for several hours. The area of 

 both halves of the leaves was determined by making a photographic print 

 and measuring the area with a planimeter. The half leaves were then 

 dried and their heats of combustion determined or in some cases the heat 

 of combustion was calculated, though it is not clear how this was done. 

 The results are tabulated in Table 37. In all these experiments ( for con- 

 venience the numbering is not that followed by Puriewitsch) with the 

 exception of 4 and 8 the incident solar radiation fell on the bolometer and 

 the leaves at right angles ; in 4 and 8 they were kept in one position at an 

 angle of 50° with the horizon. In experiments 4, 7 and 10 the heats of 

 combustion were determined, in the others they were calculated. 



In the experiments of Puriewitsch there is a great variation (0.6 to 7.7 

 per cent) in the amount of solar energy utilized. The source of illumina- 

 tion was the sun ; the holographs also indicate great fluctuations in the 

 intensity of illumination during the course of the experiments. Never- 

 theless the light was undoubtedly greatly in excess of the amount the leaves 

 were capable of utilizing. This is also indicated by the fact that roughly 



* Puriewitsch. lahrb. zi'iss. Bot., 53, 229 (1914). 



