THE NATURE OF PHOTOSYNTHESIS 139 



The fact that the temi^eratiire coefficient varies with temperature has 

 been employed to sul)Stantiate the view held by a number of investigators 

 that the photosynthetic process involves two reactions ( Willstatter and 

 Stoll, Warburg, Osterhout and Haas and others). These various hypothe- 

 ses differ somewhat in detail, but the facts upon which they are based 

 are about as follows. At high illumination intensity a rise of ten degrees, 

 from 15'' to 25", causes the velocity of photosynthesis to double. Here 

 a chemical reaction is evidently determining the rate of the process ; this 

 reaction Warburg has designated as the "Blackman reaction." At low 

 illumination intensity the rate of photosynthesis is' independent of the 

 temperature between 15' and 25^. This is what could be expected if 

 under these conditions a photochemical reaction, as, for instance, the 

 photolysis of a chlorophyll-carbonic acid complex were determining the 

 velocity. The assumption here is that all photochemical reactions have 

 low temi^erature coefficients. The photosynthetic process is made up 

 of two reactions, one an ordinary chemical reaction, which determines 

 the rate of the process at high illumination intensities and has a high 

 temperature coefficient, secondly a photochemical reaction which deter- 

 mines the rate of the process at low illumination intensities and has a 

 low temperature coefficient. These theories are taken up in greater de- 

 tail under the discussion of the chemistry of the photosynthetic process. 

 There is no satisfactory explanation of the high temperature coefficients 

 at low temperatures. It should l3e stated, however, that high values of 

 Qio at low temj^eratures are of very common occurrence in biological 

 processes. ^*^ 



A great deal more experimental work is required in order to elucidate 

 fully the relation between these two steps in the photosynthetic process, 

 the "dark" and the "light" reaction. In fact it cannot be accepted as 

 established that two such reactions actually exist or that their assumption 

 will prove adequate to explain what still api)ears to be an exceedingly 

 complex system. That the temperature coefficient varies with differ- 

 ent temperatures and with other external conditions there is little doubt. 

 Many of these facts have been brought to accord with the general prin- 

 ciples first outlined by Blackman's theory of limiting factors. This 

 theory and its modifications, however, tell us little of the kinetics of the 

 photosynthetic process ; they are the first attempts at an accurate expres- 

 sion of the relation of this process to the various factors which influence 

 it. In a sense this mode of attack is analogous to the thermodynamic 

 treatment of chemical reactions, though we are not in possession of the 

 exact data which such a method requires. But the thermodynamic treat- 

 ment is largely indej^endent of any molecular hypothesis we may have 

 formed regarding any process. In photosynthesis we are constantly en- 

 deavoring to determine the nature of the molecular changes involved 

 and to this end data are frequently applied which have but very indirect 



*"Kanitz, A., "Temperatur uiid Lebensvorgange," Berlin, 1915, p. 27. 



