16 THE SYNTHESIS OF CARBOHYDRATES 



The doctrine of limiting factors, now well known, is due 

 to F. F. Black man * who enunciated the axiom that when 

 a process is conditioned as to its rapidity by a number of 

 separate factors, the rate of the process is limited by the pace 

 of the slowest factor. The limiting factor in any definite 

 instance may be identified by the experimental application 

 of the principle that " when the magnitude of a function is 

 limited by one of a set of possible factors, increase of that 

 factor, and of that one alone, will be found to bring about 

 an increase of the function." j- 



The principle may be illustrated by one of Matthaei's J 

 many experiments on the effect of temperature on carbon 

 assimilation under conditions constant except for temperature 

 and illumination. In the case of Prunus laurocerasus there was 

 a gradual increase in the assimilation as the temperature was 

 raised; at about 11 C. a maximal assimilation of 22 mg. 

 of carbon dioxide per 50 square cm. per hour obtained and 

 was not increased even by raising the temperature to 25 C. 

 By doubling the light intensity, however, the maximal as- 

 similation was equivalent to 37-5 mg. of carbon dioxide 

 per 50 square cm. per hour and again there was no increase 

 on raising the temperature. This means that light intensity 

 was a limiting factor and only by its increase could a greater 

 carbon assimilation be obtained. 



The factors which condition carbon assimilation are ex- 

 ternal and internal : the external factors are capable of 

 control whilst the internal are much less amenable to experi- 

 ment and thus are less understood. Of the external factors, 

 the raw materials, temperature, and illumination are the 

 most conspicuous ; and of the internal factors, chlorophyll 

 and the products of carbon assimilation are the best under- 

 stood. 



*Blackman: "Ann. Bot.," 1905, 19, 281. 



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



Matthaei: " Phil. Trans. Roy. Soc.," Lond., B. 1904, 197, 47. For the 

 application of the principle to the growth of field crops see Balls and Holton : 

 "Phil. Trans. Roy. Soc.," Lond., B. 1915, 206, 103, 403; and Balls: Id., 

 1917, 208, 157. 



