166 PHOTOSYNTHESIS 



to this conception, also an enzymatic agent. This latter protoplasmic 

 factor is contained in hoth the chlorophyll-rich and light-green or yellow 

 varieties in about the same amount. Temperature affects primarily this 

 factor. Now the temperature coefficient of photosynthesis is considerably 

 higher in leaves rich in chlorophyll than in leaves poor in chlorophyll. 

 This Willstatter and Stoll explain on the basis that an increase in tem- 

 perature and consequent augmented activity of the protoplasmic factor, 

 can result in a higher rate of photosynthesis only under conditions in which 

 the other steps in the photosynthetic process are also raised, or at least 

 are not lower than the ones determined by the enzymatic factor. That is, 

 it must be possible for the plant to absorb enough light to supply the 

 greater energy necessary to carry on these other steps at a rate which is 

 equal to that of the enzymatic factor, now augmented by a rise in tem- 

 perature. Therefore, higher temperature can result in a higher rate of 

 photosynthesis only if there is sufficient chlorophyll present which can 

 absorb this increased amount of energy required. These latter conditions 

 are met only in leaves rich in chlorophyll. In leaves poor in chlorophyll 

 the activity of the enzymatic factor may also be raised by higher tempera- 

 tures but it will not result in a total increase in photosynthesis, because 

 the leaf cannot absorb sufficient energy to increase the rate of the purely 

 photochemical steps in the series of reactions comprising photosynthesis. 



Willstatter and Stoll found that etiolated leaves after they had de- 

 veloped a fraction of the chlorophyll-content of normal leaves surpassed 

 the latter in photosynthetic activity. They assume that this higher activity 

 of etiolated leaves is due to a greater development of the internal enzymatic 

 factor. It is possible, on this assumption, that light is not so favorable 

 for the development of this enzymatic factor as darkness or at least in 

 high intensity exerts an inhibiting effect. Although there is hardly suffi- 

 cient evidence to permit of speculation, is it not possible that if light does 

 exert an inhibiting effect on this internal factor, that the "time factor" of 

 photosynthetic activity may be explained on this basis? Also it may be 

 possible that the fatigue effects, described by Pantanelli and discussed 

 above, are to be explained on the ground that light, while essential for the 

 photochemical reactions, is injurious to the protoplasm in high illumina- 

 tion intensities. The deleterious effects of light, particularly the blue- 

 violet rays, on enzymes and proteins have been repeatedly demonstrated.^^" 



In fact Ewart^^^ has shown that "it is possible to produce a condition 

 of permanent light rigor, i.e., death, in the chlorophyll grains over an 

 exposed region of a cell of Chara without affecting the vitality of the 

 cell, i.e. the plasma of the chlorophyll grain appears to be more sensitive 

 than the general protoplasm of the cell." 



^Reynolds-Green, Trans. Rov. Soc. London, 188, 167 (1897). Emmerling, Ber. 

 chem Ges., 34, 3811 (1901). Chauchard, Conipt. rend., 158, 1575 (1914). Pin- 

 cussen, Biochem. Zeit., 134, 457 (1922). Schanz, Arch. Ges. Physiol, 164, 445 

 (1916). 



^^Ewart, /. c, 443. 



