THE LIGHT FACTOR IN PHOTOSYNTHESIS 1035 



Ehrke (20) for different algae. These temperature optima are not fixed 

 but vary with changes in Hght intensity and carbon dioxide concentration. 

 They are shifted to lower temperatures with lower light intensities. 

 The meaning of these results is still obscure. 



Light and Internal Factors: Water. — Although water enters chemically 

 into the photosynthetic reaction, very little has been done of a quantita- 

 tive nature on the role of water as an internal factor. Dastur (16, 17) 

 has presented some evidence that there is some correlation between 

 water content and the rate of photosynthesis, though the relationship 

 of this internal factor to photosynthesis is still quite involved. 



Chlorophyll. — On the basis of their extensive quantitative investiga- 

 tions Willstatter and Stoll (143) concluded that the chlorophyll content 

 does not change during the course of photosynthesis and that there 

 is very little change in the ratio of chlorophyll a to h. Under experi- 

 mental conditions in which neither light intensity, temperature, nor 

 carbon dioxide concentration is determining the rate of photosynthesis, 

 the influence of the internal factors becomes most pronounced. In 

 various leaves of different chlorophyll content no direct proportionality 

 between chlorophyll content and photosynthetic rate is observable. 

 That is, there are other internal factors, besides chlorophyll, which 

 determine the rate. Under conditions of ample carbon dioxide supply, 

 a change in light intensity has little effect on leaves rich in chlorophyll, 

 while such a change produces decided effects in the photosynthetic rate 

 in leaves which are low in chlorophyll. On the other hand, leaves high 

 in chlorophyll are affected in their photosynthetic rates by changes in 

 temperature, while leaves low in chlorophyll show a relatively slight 

 change in their rates due to temperature. These facts have been inter- 

 preted thus: In the leaves rich in chlorophyll this factor is in excess of 

 an enzymatic factor which is strongly influenced by temperature. A 

 change in temperature, therefore, will exert an influence on the photo- 

 synthetic rate through the action on the enzymatic factor which in these 

 leaves is in relatively low concentration. Conversely, in leaves poor in 

 chlorophyll, temperature has little effect, because here the enzymatic 

 factor is in excess of the chlorophyll. Under these conditions, light 

 intensity controls the rate. Only under conditions in which the chloro- 

 phyll is completely utilized, i.e., with relatively high light intensity, 

 can the enzymatic factor exert its full influence. This interpretation 

 is largely based upon the Blackman theory that the rate of photosynthesis 

 is determined by the pace of the slowest factor. In all probability, 

 however, conditions are not quite so simple as would appear from such 

 an interpretation. 



Emerson (22) has found that increase in chlorophyll content in 

 Chlorella augments photosynthesis proportionately in any one series of 

 cultures. When, however, different series of cultures are compared the 



