134 THE BIOLOGY OF FLOWERING PLANTS 



TABLE XXVI 



Relation of Plastid Number to Assimilation 



The assimilation activity values are based on results of 

 an early investigation by C. A. Weber (1879). The chloro- 

 plast numbers are obtained by actual counts. The corre- 

 spondence is remarkable, but it is likely that it is partly due 

 to chance, for Weber's results were based on determinations 

 of dry weight increase after forty-eight days' growth under 

 greenhouse conditions, and can give no information as to 

 the maximum assimilating capacity of the plants. There 

 is no evidence that chlorophyll content is proportional to 

 plastid numbers. The relation is really one to plastid 

 material rather than to pigment. Lubimenko (1908) found 

 the chlorophyll content of broad-leaved trees higher than 

 that of conifers, and related a higher assimilating capacity 

 of the former to this. 



Willstatter and Stoll (1918) have studied this question 

 with more reHable methods. They have made estimations 

 of the actual amount of chlorophyll present, and have mea- 

 sured the rate of assimilation under conditions of high carbon 

 dioxide supply (5 and 10 per cent, carbon dioxide) and strong 

 illumination. They express the relation of assimilating 

 power to chlorophyll content as the ratio of carbon dioxide 

 in grams assimilated per hour, to i gram chlorophyll — that 

 is, the amount of carbon dioxide assimilated per hour by 

 the amount of leaf substance containing i gram of chloro- 

 phyll. This ratio is called the assbnilation number. 

 Table XXVII reproduces some of Willstatter's results for 

 leaves assimilating in light of the strength of sunlight, in an 

 atmosphere containing 5 per cent, carbon dioxide and at a 

 temperature of 25° C. 



