INFLUENCE OF ILLUMINATION 31 



be lost by reflection, absorption or transmission. Of the 

 energy absorbed by the leaf, many have shown that a small 

 proportion only, and this in varying quantity, is used in car- 

 bon assimilation. Owing to ignorance of certain factors and 

 the degree of their significance in the sequence of carbon 

 assimilation, it is not possible to give a satisfactory account 

 of the energy relationships of the plant. It may, however, 

 be mentioned that Brown and Escombe,* the first to attempt 

 the drawing of an energy balance sheet of the leaf, concluded 

 that Polygonum Weyrichii used from -42 to 1-66 per cent, of 

 the available radiant energy for carbon assimilation, figures 

 based partly on observation and partly on calculation : 

 Puriewitsch,f on the other hand, found that Polygonum 

 Sacchalinense similarly employed from 2-5 to 77 per cent, of 

 the radiant energy. 



In no natural conditions is the full radiant energy made 

 use of by plants : the photosynthetic value of the noontide 

 sunshine at the summer solstice in these latitudes lies, accord- 

 ing to Blackman and Matthaei,^ between -04 and -05 gram of 

 carbon dioxide per 50 sq. cm. of leaf surface per hour ; the 

 highest assimilation actually measured by these workers was 

 •0290 gram. 



The general statement that carbon assimilation varies with 

 the intensity of the illumination is true only when light is the 

 limiting factor ; other factors, particularly temperature, are 

 intimately associated in the process in nature. For this 

 reason it is impossible to consider the effect of one condition 

 to the exclusion of the other factors, a fact well demonstrated 

 by Blackman and Matthaei.§ In one of their experiments, an 

 abstract of which is given on p. 32, the leaves of Helianthus 

 tuber osus were surrounded by an atmosphere containing on 

 the average 4 per cent, carbon dioxide and the light throughout 

 was diffused and of varying intensity. 



It will be seen from observations 2 to 4 and 6 and 7 that 

 the assimilation was remarkably uniform in conditions of 



* Brown and Escombe : " Proc. Roy. Soc," B, 1905, 76, 29. 

 f Puriewitsch : " Jahrb. wiss. Bot.," 1914, 53, 210. 

 % Blackman and Mattha;i : " Proc. Roy. Soc," B, 1905. 76, 4° 2 - 

 § hoc. cit. 



