Z The Philippine Journal of Science : 1917 
of chemical processes. It is found that in general the latter follow the 
vant Hof law, * °* *. 
Livingston and Livingston ‘ in discussing these problems make 
the following statement: 
In much of the work that has been published on vital temperature 
coefficients, relatively simple physiological processes have been considered, 
and it seems allowable to conclude, at least tentatively, that most of the 
elementary chemical processes of living things go on according to the 
principle of van’t Hoff and Arrhenius, and that such processes possess 
temperature coefficients, within the ordinary limits of environmental tem- 
peratures, of an order of magnitude of from about 2.0 to about 2.5. This 
may be regarded as a fundamental principle in physiology. 
The consensus of opinion on this subject is that the principle 
applies only within certain limits and not at minimum and max- | 
imum temperatures for the processes concerned. 
Photosynthesis is one of the plant processes that is usually 
cited as following the van’t Hoff principle. However, since 
carbon dioxide assimilation is dependent on light, it is to be ex- 
pected that the temperature coefficients will be similar to those 
of photochemical reactions, rather than to those of ordinary chem- 
ical processes. Photochemical reactions, almost without excep- 
tion, have much smaller coefficients than those required by the 
van’t Hoff principle. The ratios for the velocities of photochem- 
ical reactions for 10° intervals, given in a table by Plotnikow,’ 
are between 1.00 and 1.42. Sheppard * gives a less complete list 
with values between 1.00 and 1.34. It is interesting to note that 
Weigert,°® in his comprehensive discussion of photochemical proc- 
esses, mentions photosynthesis as one of two photochemical re- 
actions that show unusually high coefficients. 
This discrepancy appeared rather surprising and induced us 
to undertake a series of experiments for the further investiga- 
tion of the question of the temperature coefficients of photo- 
synthesis. The available literature, however, seemed to show 
so convincingly that photochemical temperature coefficients do 
hold for photosynthesis, that it seemed advisable to oer a 
discussion of the literature at the present time. 
* Livingston, B. E., and Livingston, G. J., Temperature coefficients in 
plant geography and climatology, Bot. Gazette 56 (1918) 349-375. 
‘Plotnikow, J., Photochemische Studien. IV ther den photochemischen 
Temperaturkoeffizienten von Brom, Zeitschr. Phys. Chem. 78 (1911) 573. 
* Sheppard, S. W., Photo-chemistry (1914) 304. 
* Weigert, F., Die chemische Wirkung des Lichts, Sammlung Chem. u. 
Chem.-Tech. Vortrége 17 (1911-12) 183-296. 
