112 PHOTOSYNTHESIS 



latter serving as a measure of the rate of photosynthesis. For light filters 

 Wurmser used solutions of potassium dichromate, transparent up to 

 about 560 \i \i, cuprous chloride transmitting from 560 \i \i to 460 ^ \i and 

 ammoniacal copper sulphate, transmitting from about 540 fi [x to 450 ^ ^. 

 Wurmser's results, which are expressed in arbitrary values are summarized 

 as follov^s : 



Red Light Green Light Violet Light 



Photosynthesis = v 100 24 80 



Enegry absorbed = Pa 100 6 *^1 -,. 



Ratio = v:Pa 1.0 4.00 2.35 



This is indeed a surprising conclusion, while the highest value for photo- 

 synthesis was found to be in the red and next in the blue light, the highest 

 value in proportion to the energy absorbed was in the green. From these 

 observations Wurmser formulates a hypothesis of the mechanism of photo- 

 synthesis. This assumes that the photochemical reaction takes place on 

 the surface of the "pigment." A more exact statement would probably 

 be that the photochemical reaction takes place on the surface of the 

 chloroplasts, the chlorophyll acting as a photochemical sensitizer. ^ The 

 photochemical reaction is but the first step in a series of reactions in the 

 subsequent steps of which the activity of the protoplasm plays an impor- 

 tant role. The rate of photosynthesis thus depends not only upon the 

 energy absorbed but upon the activity of the protoplasm as well. Views 

 quite analogous to this have recently been expressed by a number of in- 

 ve.^tigators of the problem and will be discussed in greater detail in an- 

 other chapter. Wurmser's results do not permit, however, of an accurate 

 calculation of the photosynthetic efficiency in light of different wave- 

 lengths especially because his method of measuring light intensity has 

 no direct physical basis. 



Probably the most successful investigations of the efifect of different 

 wave-lengths on photosynthetic activity have been carried out by Warburg 

 and Negelein.^2 These are described in greater detail in the chapter on 

 Energy Relations of Photosynthesis. Warburg and Negelein conclude 

 that the efficiency of the photosynthetic process becomes less with decreas- 

 ing wave-length. There is apparently no relation between photosynthetic 

 efficiency and the absorption bands of chlorophyll. Thus the yield in the 

 red, a region of high absorption is greater than in the green, a region of 

 low absorption ; while the yield in the green is greater than in the blue, 

 the region of highest absorption. For the reduction of one molecule of 

 carbon dioxide with red (660 ^i i-i) or yellow (578 ^a h) light there are 

 required about four quanta, with blue light (436 n \i) about five quanta. 

 There are undoubtedly several factors which affect these results. Prob- 

 ably the most important of these is the concentration of the chlorophyll 

 and carotin in the plants. That these vary with the conditions of previous 

 culture of the plants is a matter of common experience. The influence 



"'Warburg and Negelein, Zcit. physik. Cheni., 106, 191 (1923). 



