MEASURING PHOTOSYNTHETIC ACTIVITY 253 



determined. This gain in weight does not. of course, represent the total 

 amount of material synthesized; an important correction must be applied. 

 Sachs found that there was a greater gain in dry weight in leaves which 

 had been detached from the plant and illuminated than in leaves which 

 were still attached to the plant and he attributed the lower gain in the 

 latter case to the migration of the products of photosynthesis from the 

 leaf into the stem of the plant. Therefore, he figured, that by adding 

 the rate of loss in weight of leaves during the period of darkness to the 

 rate of gain during periods of insolation he would obtain a value which 

 would represent a close approximation of the true rate of photosynthesis 

 during illumination. 



Brown and Escombe ^* subjected the method of Sachs to a careful 

 investigation. They compared values of the rate of photosynthesis ob- 

 tained by the half leaf method with those obtained by the measurement 

 of the actual intake of carbon dioxide by the leaf. The two methods gave 

 widely difl^erent results. First of all they found that the rate of photo- 

 synthesis of leaves attached to the plant as measured by the absorption 

 of carbon dioxide is about 45 per cent less than that of detached leaves. 

 Sachs was therefore not justified in making a correction of the diurnal 

 depletion of material synthesized. According to the results of Brown and 

 Escombe the method of Sachs gives values which are far in excess of that 

 indicated by the direct measurement of the absorption of carbon dioxide. 

 This fact is illustrated in the results tabulated in Table 31 taken from 

 the work of Brown and Escombe of the rate of photosynthesis of Catalpa 

 bignonioidcs. The results by the two methods were obtained on the same 

 leaves. 



TABLE 31 



Rate of Photosynthesis as Determined by the Half Leaf ^Method and by 



Estimation of the Amount of Carbon Dioxide Absorbed. The 



Values Are Calculated per Square Decimeter per Hour. 



(From Brown and Escombe.) 



Carbohydrate 

 Observed Formed Cal- 



Increase in Dry CO2 Absorbed by culated from COa 



No. Weight in mg. Leaf, cc. Absorbed, mg. 



1 9.83 L41 1.76 



2 7.14 1.43 1.79 



3 2.60 2.35 2.94 



4 7.22 2.33 2.92 



Mean 6.69 2.35 



From Table 31 it is evident that the method of determining the rate 

 of photosynthesis from the increase in dry weight yields results which 

 are very much higher than does the direct method of determining the 

 amount of carbon dioxide absorbed. Brown and Escombe suggest that 

 this discrepancy may in part be due to "changes in the power of retention 



■^ Brown and Escombe, Proc. Roy. Soc, B 76, 49 (1905). Thoday, ibid., B 82, 

 1 (1909). 



