94 



Carbon Assimilation. 



here variations in light intensity are without influence on the 

 assimilation, while temperature variations influence the rate of 

 assimilation considerably. However, no experimental results are 

 given which justify these conclusions, and however interesting they 

 may be they can at present only be accepted as postulates. 



Etiolated Leaves. 



The complexity of the processes involved in carbon assimilation 

 is also made clear by examination of the behaviour of etiolated leaves. 



On this subject observations were made by Miss Irving (1910 

 at Blackman's suggestion. Biackman assumed that in the case 

 of etiolated leaves " the whole assimilatory apparatus might be 

 efficiently developed except the green pigment, and as this increased 

 by degrees, so the power of photosynthesis would increase. Thus 

 the amount of chlorophyll present would then be the limiting factor 

 for assimilation, and interesting data might be looked for relating 

 the amount of pigment present to the amount of photosynthesis 

 that could be effected." 



This expectation was not confirmed by the preliminary experi- 

 ments carried out by Miss Irving, who 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. 



In Miss Irving's experimental arrangement the plants were 

 supplied with their own respiratory carbon dioxide, and the light to 

 which they were exposed was the feeble light from a north window. 



Willstatter, using a much stronger light intensity (48,000 lux) 

 and 5% carbon dioxide, obtained results which justified Blackman's 

 expectations that chlorophyll is really a limiting factor. There is a 

 possibility that in Miss Irving's experiments light was a limiting factor. 



Table XXIII. 



AssintilatioH Numbers of Etiolated Leaves becoming Green. 



Phaseolus vulgaris. 



Temperature 25"C, 5% carbon dioxide. Light intensity, 48,000 lux. 



