1903.] The Corniced Function of the Green Plant. 453 



the maximum economic coefficient of assimilation in our case was 3 '3 

 per cent. But besides this chemical work there is the physical work of 

 chloro vaporisation going on. In 1894 I made a series of evaluations 

 of the quantities of water evaporated under the same conditions in the 

 same tubes over mercury. Given the conditions of the experiment- 

 evaporation in closed vessels in a saturated atmosphere, one may be 

 sure that the greater part of the process must be attributed to chloro- 

 vaporisation.* 



This line here (fig. 10) represents the expenditure of total energy due 

 to chlorovaporisation. It amounts to 8 per cent. This third line repre- 

 sents the sum of work both chemical and physical done in the leaf. It 

 will cross at a certain point the line of available energy and, beginning 

 from this point, the whole available energy would be spent trans- 

 formed into work and consequently the two processes, the chemical 

 and the physical, must interfere as in Deherain's experiment. Past this 

 point, and of course somewhat earlier, since in no case may the 

 economic coefficient be expected to rise to 100 per cent., the leaf will 

 consequently be short of energy for maintaining both processes at their 

 maximum, and the curve of the chemical process must therefore sink 

 rapidly with the decreasing intensity of the light. At this other point 

 of intersection the whole available energy will not suffice to keep at 

 its maximum the process of assimilation alone. These considerations, it 

 seems to me, give us a sufficient explanation for the first part of the 

 curve. 



I may add that these results are in full accordance with the beautiful 

 experiments of Mr. Horace Brown and Mr. Escombe. In direct sun- 

 shine they found the economic coefficient was but 27 per cent. When 

 the intensity was reduced to about one-tenth, the economic coefficient 

 rose to 95 per cent., nearly the whole being transformed into work. 



The next point to be considered, the reason for this form of the 

 upper part of the curve, is somewhat more perplexing. The simplest 

 supposition explaining this course of the process would be to admit 

 that past a certain point the radiant energy incident on the leaf will be 

 in excess. But can it be really admitted that the solar energy may be 

 in excess, remembering always the very modest figure representing the 

 economic coefficient of chemical work 1 



The chemical process going on in the green leaf is very often con- 

 sidered as somewhat paradoxical. Indeed it takes place at ordinary 

 temperatures, whilst the principal chemical phenomenon which underlies 

 it the dissociation of carbon dioxide can be realised in our labora- 

 tories only at very high temperatures, amounting to thousands and 

 thousands of degrees. Here is a diagram of Le Chatelier's representing 

 the dissociation of carbon dioxide in its relation to temperature and 

 pressure (fig. 11). But this paradox is of course only apparent, and if 



* I have since made some experiments with variegated leaves of Aspidistra. 



