ASSIMILATION OF CARBON BY PLANTS 



205 



are accelerated with increase of temperature, more or less in 

 accordance with the rule of van't Hoff but inducing opposite 

 effects in the leaf. If one of them is the process of decomposition 

 of carbon dioxide and the other the process of the '^nactivation," 

 or loss of faculty, of the action of the chloroplasts, then with 

 comparatively low temperatures the process of inactivation is 

 almost imperceptible, and assimilation is subject to van't Hoff's 

 law. But with a temperature of 20 to 25°C., the inactivation 

 begins to increase in effect. It proceeds at a rapidly increasing 

 rate, and at 30 to 35° it has overbalanced the process of assimila- 

 tion and soon stops it. These relationships are shown diagram- 

 matically in Fig. 63, where the 

 curve AB shows the theoretical 

 progress of assimilation as con- 

 trolled by temperature; the 

 curve CD shows the increase of 

 the inactivation phase; and the 

 curve AE indicates the actual 

 progress of assimilation, as the 

 resultant of the two curves. 



Blackman's explanation was 

 completely confirmed by the 

 experiments of his cow^orker, 

 Miss Matthaei. In studying 

 the character of the assimila- 

 tion 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 explanation, the conception of the optimum 

 does not convey the implied meaning. Literally, this term means 

 'Hhe best temperature," while in reality at this point the factor 

 of inactivation, or the destruction of the chloroplasts, is already 

 considerable. A somewhat lower temperature should be recog- 

 nized here as the ''best" at which the above time factor of 

 inactivation is as yet almost imperceptible. 



Fig. 63. — Blackman's scheme, ex- 

 plaining the optimum in the tempera- 

 ture curve. 



