SUPPLEMENT 35 



found to be even higher had the reading been taken after the leaf had been 

 subjected to that temperature for a shorter period, gradually shifted down- 

 wards : 30-5 in curve IV. 



We owe an interesting theoretical interpretation of the curve of assimila- 

 tion to BLACKMAN (1905). KANITZ (1905) independently drew attention to 

 the fact that curve I between o C. and 27 C. agrees with VAN 'T HOFF'S law 

 on the dependence of the rate of reaction of chemical processes on temperature ; 

 BLACKMAN sought to show that this curve is the resultant of two antagonistic 

 processes. CO 2 -assimilation, being a chemical process, must increase with 

 temperature somewhat in the way indicated by the curve AB in Fig. 256, but 

 the temperature has the further effect of rendering the chloroplasts inactive. 

 The higher the temperature the more rapidly does this inactivity approach 

 completeness, and we may express, very roughly, the curve of inactivity by 

 the line CD (Fig. 256). Owing to the combined stimulatory and retarding 

 action of temperature then we arrive at the ' optimum curve ' AE, which, as 

 a matter of fact, corresponds with that obtained by actual observation. 



125, 11. 14-18, delete [Very often . . . experiment.] 



11. 312, for It is only . . . provided read which indeed quantitative 

 chemical methods cannot measure accurately in such a case unless 



1. 35, for entirely when respiration read finally only when this diminution 



1. 47 P. 126, 1. 20, for Further we are . . . environment increases.] read As 

 the light increases in intensity CO 2 -assimilation also increases. When the light 

 is about as intense as ordinary sunlight, however, this relation is not maintained, 

 and that for several reasons. In the first place, intense light, like high tempera- 

 ture, causes the chlorophyll to become inactive. PANTANELLI (1903) found 

 that in Elodea, for example, when the intensity of the light was either i or 

 (sunlight = i) CO 2 -decomposition went on for fifty minutes at a uniform rate, 

 but that it decreased markedly after fifteen minutes' exposure to light of 

 intensity = 4. In the second place, the C0 2 -decomposition may remain con- 

 stant in spite of an increase in light intensity, because the supply of carbon- 

 dioxide is insufficient to render a further increase in assimilation possible. 

 Thirdly, the temperature may exert a limiting effect on it. Owing to the 

 factor first mentioned the curve representing the dependence of assimilation 

 on illumination will take the form of an ' optimum curve ', and its apex will 

 in due course approach those of lower light intensities. The effect of an in- 

 sufficient supply of CO 2 or of a low temperature will express itself in a curve 

 which at the beginning will rise proportionally to the light intensity, and will 

 then proceed horizontally. Fig. 25 c shows these relations diagrammatically. 

 The law of minimum is thus appreciable here also ; the factor operative in 

 minimum degree determines the amount of organic substance formed. 



BLACKMAN and MATTHAEI (1905) have shown that in nature, even in 

 diffuse light, the deficiency in CO 2 never permits of the maximum of assimila- 

 tion being reached. When they artificially increased the percentage of CO 2 in 

 the air, the temperature of the leaves prevented the complete utilization of 

 the sunlight always in direct sunlight, but only on cool days in diffuse day- 

 light. On warm days, on the other hand, the light intensity was insufficient 

 when the light was diffuse to induce the maximum assimilation possible at 

 such a high temperature. 



Legend to Fig. 2$c. The abscissa shows light intensities, the ordinates show amounts 

 of assimilation. Curve I, assimilation with a sufficiency of CO 2 , and high temperature ; 

 II, under the same conditions as I, but a later reading ; III, Assimilation with sufficiently 

 high temperature but a deficiency in CO 2 ; after reaching B the tracing becomes horizontal 

 because higher assimilation values are prevented by deficiency in CO 2 ; IV, assimilation 

 with a sufficiency of CO 2 , but at a lower temperature ; after reaching A, any rise in assimila- 

 tion is rendered impossible by the lowness of the temperature. 



C 2 



