EFFECT OF NATURAL VARIATIONS OF CHLOROPHYLL CONTENT 1265 



set in. On the other hand, leaves may remain green in the autumn and 

 nevertheless show declining v^ values; sometime these green but inactive 

 leaves recover their efficiency after having been returned to higher temper- 

 ature for several hours. To sum up, Willstatter and Stoll's determinations 

 of va. fmd Ia suggest that the concentration of the limiting catalyst has a 

 tendency to change proportionately with [Chi], but that this rule admits of 

 many, and sometimes^ striking, exceptions. 



o 



00 



o 

 o 



E 



CO 

 CO 



z 

 >- 



CO 



o 



H 



o 



X 



a. 



120 



100 



Green leaves 



0.25 0.50 075 1.00 



LIGHT INTENSITY, fractions of full sunlight 



Fig. 32.2. Light curves of green and aurea leaves of 

 Sambucus (after Willstatter and Stoll 1918). 



In addition to numerous experiments on the assimilation numbers of 

 various leaves in strong light, Willstatter and Stoll also determined the 

 complete light curves of the green and aurea leaves of Sambucus. The 

 results are represented in figure 32.2. The yellow leaves contained only 

 5-10% of the normal chlorophyll content, and probably absorbed not more 

 than one third of the amount of white light absorbed by the green forms 

 (c/. page 685). This difference may explain the slower rise in the rate 

 of photosynthesis of yellow leaves in weak light, and their failure to reach 

 complete saturation even in the strongest light used. The extrapolated 

 saturation value of the yellow variety appears, however, to be equal to, or 

 only slightly below, that of the green one. In other words, figure 32.2 

 agrees approximately with the prototype of the two solid curves in figure 

 32.1B (and differs from that of fig. 28.20(1)). 



