Carbon Assimilation. 



Actual determinations of the heat of combustion of the 

 material produced in assimilation have been made by Krasheninnikoff 

 (1901) and by Puriewitsch (1914). They measured the increase in 

 dry weight per unit area per hour by Sachs' dry weight method 

 and also the increase in the heat cf combustion per unit area per 

 hour. The increase in heat of combustion per unit increase in dry 

 \teight gives the heat of combustion per gram of the products of 

 assimilation. Krasheninnikoff obtained an average value for this of 

 4-4 X 10' gram-calories. From Puriewitsch's data we have calculated 

 the values for different species set out in the following table. It 

 will be observed that the values agree well with the number 

 obtained by Krasheninnikoff, but not with the value assumed by 

 Brown and Escombe (3-76 X 10'). 



Table XL. 

 Heats of Combustion of the Products of Assimilation. 



D. The Quantitative Measurement of the Radiant Energy 



Incibent on the Leaf and the UriusATiON 



OF this Energy. 



A full discussion of the methods used and principles involved 

 in the measurement of radiant energy would be out of place here. 

 The instruments generally employed are of four kinds, the 

 thermopile, the bolometer, the radiometer and radiomicrometer. 

 For a description of these instruments the reader is referred to 

 physical text books, and for a more complete discussion on the 

 relative merits of the various methods, to Kayser's Spectroscopic 

 Baly's Spectroscopy, and Coblentz's " Instruments and Methods 

 used in Radiometry " (1908). Generally speaking, the radiant 

 energy is absorbed and transformed to heat in the measuring 

 instrument, and thus a measure of the total energy obtained, but 

 if by a suitable method a spectrum of the source of light is produced, 

 the same method can be used for measuring the distribution of 

 energy in the different parts of the spectrum. 



