LIMITING FACTORS 145 



or by illumination. Whichever of these three is least 

 favourable will determine the rate of assimilation. No 

 matter how bright the hght may be, or how warm the day, 

 the plant cannot assimilate more rapidly than is possible 

 with the supply of carbon dioxide available . We may further 

 note that that factor is limiting, an increase in which leads 

 to an increase in assimilation rate (Fig. 13). 



U Id. 



60 

 50 

 40 

 30 

 20 

 10 



' - ' ' ■ ■ I ' 



-(0* O" -^JO" 20" v30 



Fig. 13. — Light, temperature and assimilation; each graph repre- 

 *^nts the course of assimilation (mgs. CO2 per 50 sq. cm. per hour) 

 ^ith rising temperature for a given light intensity (L = i, 2, 4, 13) : in 

 ^ach, assimilation rate rises till light becomes limiting, and then the 

 graph takes a horizontal course : the exact point of inflection for L = 4 

 is conjectural, and this part of the graph is dotted : light = 13 is not 

 limiting for the highest temperature (15° C.) for which a value is given. 

 (After Matthaei, modified.) 



These conceptions of Blackman's have the breadth and 

 simplicity characteristic of so many important generalisa- 

 tions. They have been the subject of some criticism which 

 has not shaken their position. Recently two pieces of work 



L 



