INFLUENCE OF ILLUMINATION 25 



falls off regularly at a rate dependent on the temperature ; the 

 higher the temperature the quicker the fall, which rapidity, 

 however, is not maintained. Thus there is a time factor for 

 the higher temperatures. Fig. 5 summarizes the results ob- 

 tained by Matthaei ; it will be seen that the turning point is 

 37 '5 C. which was found to be within a few degrees of the 

 temperature fatal to the leaf. 



The results obtained for temperatures below 25 C. con- 

 formed to Van't Hoff s Law, the coefficient of increase in the 

 rate of carbon assimilation for a rise of 10 C. being 2-1. In 

 subsequent investigations Blackman and Matthaei found the 

 assimilation coefficient for the leaves of Helianthus tuber osus to 

 be K IO =2*3, whilst for Elodea, K IO = 2-05.* The culminative 

 point of the assimilatory curve (Fig. 5) in respect to increas- 

 ing temperature is important and is paralleled in enzyme 

 action ; in view of the results obtained by Willstatter and Stoll, 

 it is not unlikely that the inhibition or destruction of an 

 enzyme at these higher temperatures may be a limiting factor. 



With regard to the internal temperature of assimilating 

 leaves, this will vary considerably in accordance with such 

 conditions as the intensity and quality of the light, the 

 character of the leaf surface and so on ; Blackman and 

 Matthaei f demonstrated, by thermoelectric means, an excess 

 of 7 C. to 1 6 C. in the leaves of Prunus laurocerasus in bright 

 sunlight above the adjacent shade temperature. 



ILLUMINATION. 



Of the light falling on a leaf, a portion only is available 

 for the chloroplasts since varying amounts, according to the 

 characteristics of the leaf surface, such as the presence of cuticle 

 and of hairs, the thickness of the leaf and so on, will 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 carbon 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 relation- 

 ships of the plant. It may, however, be mentioned that Brown 



* Blackman and Smith : " Proc. Roy. Soc." Lond., B. 1911, 83, 389. 

 f Blackman and Matthaei : id., 1905, 76, 402. 



