226 SCIENCE PROGRESS 



process. The reaction of the chlorophyll system being photo- 

 chemical, it may be assumed to have a temperature coefficient 

 not far removed from unity, while the enzymatic process has a 

 temperature coefficient of from 2 to 3. Temperature variations 

 do not influence the assimilation in yellow varieties, while 

 decrease in light decreases the assimilation. Willstatter there- 

 fore concludes that in this case the enzymatic system is more 

 developed than the chlorophyll system, which thus controls 

 the rate of reactions. In the green variety {i.e. the normal 

 leaf) variations in light intensity are without influence on the 

 assimilation, while temperature variations influence the rate 

 of assimilation considerably, this being explained by the 

 greater development of the chlorophyll system in this case, 

 while the enzymatic system limits the rate of the reaction. 



Another important fact is brought out by a study of the 

 assimilation numbers of etiolated leaves becoming green. 

 Miss Irving, in some experiments on etiolated leaves {Ann. of 

 Bot., 19 10) found not only that etiolated shoots possessed no 

 power of assimilation, but that shoots that had developed a 

 considerable green colour did not possess the power. Willstatter 

 contends that these results were due to Miss Irving's experi- 

 mental arrangement, in which the plants were only supplied 

 with their own respiratory carbon dioxide and were exposed 

 to the feeble light from a north window. Willstatter used a 

 much stronger light intensity (48,000 lux) and 5 per cent, carbon 

 dioxide. Under these circumstances he finds that not only is 

 there an appreciable assimilation, but that also the assimilation 

 numbers of etiolated leaves are very high, thus indicating that 

 the amount of chlorophyll is limiting assimilation. In Miss 

 Irving's experiments light and carbon dioxide were probably 

 limiting factors. 



Finally, Willstatter, like so many great chemists before 

 him, dogmatises as to the actual processes concerned in carbon 

 assimilation, but the considerations which lead him to assume 

 that carbon dioxide and chlorophyll form a dissociable com- 

 pound are not worth recording here, as the experimental 

 basis is very questionable, and he himself admits that only a 

 few experiments could be carried out owing to the exhaustion 

 of his stock of chlorophyll. 



Altogether the present trend of plant physiology seems to 

 aim less at getting the processes of the plant pictured by means 



