^8 PHOTOSYNTHESIS 



concentration of carbon dioxide and at a high Hght intensity, 

 although it was not shown in all cases that either of these 

 factors was so high that they were never limiting. With a 

 'saturating' light intensity the assimilation number measures 

 the activity of the dark processes per unit chlorophyll; if 

 measured with lower light intensities the ratio of the rate 

 of photosynthesis to the concentration of chlorophyll will 

 approach more and more to being simply a measure of the 



June July Aug. Sept. Oct. 

 Time of year 



FIG. 4.9. Variation throughout the growing season of chlorophyll 

 content and of assimilation number in Tilia cordata (•) and 

 Acer pseudoplatanus (O). (After Willstatter and Stoll.) 



activity of the chlorophyll. Under their conditions Will- 

 statter and Stoll found a considerable range in assimilation 

 number amongst different species of plants. In species with 

 a high assimilation number (or low chlorophyll content) it 

 was found that there was not in general a linear relation 

 between rate of photosynthesis and chlorophyll concentra- 

 tion; for species with high concentrations of chlorophyll the 

 assimilation number tended to be more nearly constant with 



