326 



PHOTOSYNTHESIS 



TABLE 2>7 

 Utilization of Solar Energy by Various Leaves. (From Puriewitsch.) 



there is an inverse ratio between the amount of energy used and the total 

 energy falling on the leaves. The experiments were carried out in the open 

 air during the summer in Kiew, so that the concentration of carbon 

 dioxide was probably limiting the rate of photosynthesis, although the 

 temperatures are not given. The temperature, which is an exceedingly 

 important factor, would vary with the rate of transpiration of the leaf 

 which in turn is dependent upon the "evaporating power of the air," a 

 function of temperature, air movement and relative humidity. These 

 experiments serve to demonstrate the extreme difficulty of determining the 

 photosynthetic efficiency of a plant in the field and the necessity of taking 

 into consideration many metereological factors in order to interpret the 

 results obtained. 



The wide variation in the results obtained by Puriewitsch are also 

 noticeable in experiments with the same species. Thus Polygonum 

 sacchalinense showed a utilization in L33 hours of 7.7 per cent, in 2.33 

 hours of Z.7 per cent and in 5 hours LI and 2.5 per cent. This Purie- 

 witsch considers as indicating a decreasing utilization with longer periods' 

 of time due to an accumulation of the products of photosynthesis. It has, 

 in fact, frequently been observed that the rate of photosynthesis decreases 

 as the products of photosynthesis accumulate ; this phenomenon has been 

 described under the discussion of the Time Factor. 



The experiments carried out by Puriewitsch, it must be recognized, 

 are not sufficiently extensive to permit drawing very satisfactory con- 

 clusions. They constitute an advance over the methods of Krascheninni- 

 kofif ^ who also determined the increase in the heat of combustion of leaves 

 that had undergone photosynthesis, in relation to the amount of carbon 

 dioxide absorbed. Krascheninnikofif's results indicate that carbohydrates 



" Krascheninnikofif, "Accumulation of Solar Energy in Plants" (Russian), Mos- 

 cow, 1901. 



