ABSORPTION OF CARBON 



47 



the actual progress of assimilation, as the resultant of the two 

 curves. 



Blackman's explanation was completely confirmed by the 

 experiments of his coworker, Miss Matthaei. In studying the 

 character of the assimilation curve near the optimal point, Miss 

 Matthaei discovered that the situation at this point depends on 

 the duration of the experiment. The longer the experiment is 

 continued, the lower will be the temperature at which the break 

 in the curve will occur: from 37.5° C. in an experiment lasting for 

 1 hr. this point is moved to 30.5° C. in one that continues for 4 hr. 

 This is quite plausible, as during 

 a longer period the inactivation 

 process becomes more effective. 



In the light of this explana- 

 tion, the conception of the opti- 

 mum does not convey the im- 

 plied meaning. Literally, this 

 term means the best temperature, 

 while in reality at this point 

 the factor of inactivation, or, 

 more generally speaking, of the 

 destruction of the chloroplasts, 

 is already considerable. A 

 somewhat lower temperature 

 should be recognized here as the 

 "best" at which the above factor is as yet almost imperceptible. 



The influence of temperature would show itself most clearly 

 only when the experiments were conducted with sufficiently inten- 

 sive light and with sufficiently high contents of carbon dioxide. 

 Under natural conditions these two factors, especially a sufficient 

 amount of carbon dioxide, are seldom secured. This circumstance 

 complicates the experiment, and the primary influence of tempera- 

 ture on assimilation may be entirely disguised. In studying the 

 simultaneous effect of several environmental factors on assimila- 

 tion, Blackman found that some of them, being in the minimum, 

 limit the influence of the other factors. This dependence of one 

 factor on another is called the "law of limiting factors." 



This law operates distinctly when at least two external factors 

 are acting simultaneously, for instance light and the carbon- 

 dioxide content of the atmosphere. It has been pointed out that 



Fig. 20. — Blackman's scheme, explaining 

 the bend in the temperature curve. 



