STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 77 



component of the protoplasm. This component they assume to be a 

 specific enzyme contained in the stroma. 



So far as these conclusions go they are apparently in undeniable 

 agreement with observational facts, for it is well known that even 

 mild disturbances of the structure of the chlorophyllous cell result 

 in inhibition of photosynthesis. Of great interest also is their 

 observation that the internal factor is affected by temperature in 

 much the same manner as most enzyme reactions. 



In Willstaetter and Stoll's experiments on the effects of variations 

 in temperature and light intensity with leaves rich and poor in 

 chlorophyll, the disproportionahty between chlorophyll-content 

 and photosynthetic activity becomes clearly evident. Moreover, 

 [leaves poor in chlorophyll are more dependent upon variations in 

 light intensity, while those rich in chlorophyll show greater variations 

 with temperature. The two components of the photosynthetic 

 apparatus thus operate in such a manner that the pigment reacts to 

 variations in light intensity more in the case of the leaf with low 

 chlorophyll-content than the one with high chlorophyll-content, 

 while the internal factor reacts more directly to temperature and 

 manifests itself to a larger degree in the leaves of high chlorophyll- 

 content. This fact can be expressed in terms of limiting factors in 

 such a way that in leaves poor in chlorophyll this component is the 

 limiting factor, while in leaves rich in chlorophyll the internal factor 

 is the limiting one, and complete utilization can not be made of the 

 higher chlorophyll-content on account of the relatively low activity 

 of the internal factor. 



The most valuable experimental data on this subject are those 

 pertaining to the temperature coefficients. Unfortunately, however, 

 while there have been made a number of careful determinations of 

 this nature, these represent a variety of rather isolated observations 

 on different plant material, with varying methods, and the nutritional 

 conditions of the plant have not been taken into consideration suffi- 

 ciently. We are at present engaged in a more exhaustive study of 

 the subject with mature leaves. 



Warburg^ found that the temperature coefficient with high light 

 intensity and high carbon-dioxid concentration decreases with in- 

 creasing temperature, so that at 5° it is 4.3 and at 32° it is 1.6. With 

 low light intensity the temperature coefficient is about unity. These 

 observations can be explained on the basis of Blackman's theory of 

 limiting factors; but on careful consideration the question is also 

 raised whether this theory is not due for a revision or extension, not 

 as to its observational basis, but rather regarding the interpretation 

 of the dynamics of the factors involved. Temperature relations 



1 Wahburg, O. Biochem. Zeitschr., 100, 230-370 (1919). 



