TEMPERATURE, LIGHT, CARBON DIOXIDE 143 



initial values can, however, be obtained by extrapolation. 

 The values so found agree with the values obtained by 

 calculation from the coefficient for 10° C. 



Blackman's interpretation of this result is that the 

 primary relation between temperature and assimilation is 

 that between temperature and a chemical reaction ; that, if 

 we could measure the rate at the moment the high tempera- 

 ture takes effect, we should find it to have the value indicated 

 even at the critical temperature — probably between 50° C. 

 and 60° C. — where almost instantaneous death ensues. 

 Above 30° C. temperature has a second effect ; in some way 

 it has a progressively deleterious action on the protoplasm. 

 We do not know precisely the nature of this deleterious 

 action, and Blackman has called it simply the time factor, 

 a term which indicates its increasing effect with time, but 

 does not tie us to any theory as to its nature. 



It is clear that the old idea of an " optimum " tempera- 

 ture is purely fictitious ; for the temperature at which 

 assimilation appears to have a maximum value will depend 

 on the interval which elapses between the raising of the 

 temperature and the making of the determination ; the 

 shorter the interval the higher will He the apparent optimum. 

 This conclusion, as well as the whole mode of analysis of 

 the action of temperature, is fundamental not only for 

 photosynthesis, but for metabolic processes in general. 



Light and Carbon Dioxide. — The relations between 

 photosynthetic activity and light and carbon dioxide supply 

 are simpler ; here we are dealing with the necessary energy 

 and the necessary raw material. We might expect that if 

 we doubled the available energy or the available material, 

 then the rate of assimilation too would be doubled, and this 

 is what Blackman and Smith (191 1) found. The graph con- 

 necting assimilation with either of these factors is an inclined 

 straight line ; the relation is arithmetical. This only holds 

 within certain limits, for, with very high light intensities or 

 very high carbon dioxide concentrations, a " time factor " 

 is again introduced, and the rate falls off. For Elodea the 

 time factors set in only when the light intensity exceeds nine 



