34 PHYSIOLOGY OF NUTRITION 



then dried and their heat of combustion was also determined. Below are given 

 the average values of all the determinations, calculated for an area of i sq. m. 

 of leaf surface exposed to the light. 



Increase in dry weight 3.51 g. 



Increase in carbohydrates 2 .46 g. 



Increase in carbon 1 . 58 g. 



Increase in heat of combustion 15)35° g.-cal. 



Amount of carbon dioxide decomposed 5 . 626 g. 



From the data of this experiment Krasheninnikov calculated that there was 

 an increase of from 2.2 to 3.6. g.-cal. for each gram of carbon dioxide decom- 

 posed." 



It is also desirable to know what proportion of the radiant energy falling 

 upon the leaf is assimilated. The first calculation bearing upon this question 

 was made by Becquerel, 1 with the following results, which represent the yearly 

 amounts of assimilation for three different types of vegetation, per hectare 

 (2.5 acres). 



Kilograms of 

 Carbon Assimilated 

 Kind of Vegetation Per Hectare 



Forest in Central Europe 1800 



Well fertilized meadow 35°° 



Helianthus tuberosus (Jerusalem artichoke) 6000 



From a series of calculations, Becquerel came to the conclusion that, in France, 

 plants assimilate less than 1 per cent, of the radiant energy that reaches them. 

 Timiriazev arrived at the same result, and Brown's 2 more recent determinations 

 give a still smaller value. In the latter case a Helianthus leaf received on 

 a sunny day 600,000 g.-cal. per square meter of leaf surface per hour. 

 In the same time an equal surface of leaf produced 0.8 g. of carbohydrates, for 

 the formation of which 3200 g.-cal. were necessary. Thus the leaf accu- 

 mulated, by the photosynthetic process, barely 0.5 per cent, of the solar energy 

 reaching it; viewed as a machine designed to produce organic compounds, its 

 efficiency is thus seen to be far from high/ 



1 Becquerel, Alexandre E., La lumiere, ses causes et ses effects. Paris, 1867-1868. 



2 Brown, H. T., Recherches sur la fixation du carbone par les feuilles et sur la diffusion de l'acide 

 carbonique. Traduit librement de l'Anglais par M. E. Demoussy. Ann. agron. 27: 428-438. 1901. 

 (The original paper is: Brown, Horace T., Opening address by the President of Section B (Chemistry), 

 Brit. Assoc. Adv. Sci., Nature 60: 474-483. 1899. (See also correction: ibid. 60: 544. 1899.) Also 

 published in: Rept. Brit. Assoc. Adv. Sci. 1899: 664-683. 1900. See also: Brown, H. T., and Escombe 

 F., Static diffusion of gases and liquids in relation to the assimilation of carbon and translocation in 

 plants. Phil, trans. Roy. Soc. London B193 : 223-292. 1900.] 



" On alterations in the areas of leaves when the latter are transferred from shade to sun- 

 light, which may possibly have some influence on the magnitudes of such values as these, 

 see: Thoday, D., Experimental researches on vegetable assimilation and respiration. V. A 

 critical examination of Sachs' method for using increase of dry weight as a measure of 

 carbon dioxide assimilation in leaves. Proc. Roy. Soc. London B82: 1-55. 1909. — Ed. 



v In such calculations as this it is to be noted that the plant does not absorb nearly all the 

 energy reaching it and that all the organic material formed does not appear in the final deter- 

 minations, some of the latter being destroyed in respiration. — Ed. 



