Allgemeines. 531 



Chlorophyll, the one an oxidation product of the other. By the 

 action of nascent hydrogen on chlorophyll-solution a yellow 

 Solution is produced which on exposure to air at once becomes 

 green. The yellow substance — protochlorophyllin, or proto- 

 phyllin — is characterized by the total absence of bands in the 

 less refrangible part of the spectrum. That the production of 

 Chlorophyll from etiolin is an oxidation process was shown by 

 Dementieff in 1873, and Timiriazeff has obtained proto- 

 phyllin from etiolated seedlings of the sunflower, with no trace 

 of Chlorophyll spectrum at all. These facts prove that in the 

 Iiving plant there exists a colouring matter with the properties 

 of reduced Chlorophyll. It would be of still greater importance 

 to obtain a direct proof of the presence of a reduced constituent 

 in a green leaf — this fact could alone account for its playing 

 the part of a chemical sensitiser — but its detection on a green 

 leaf presents enormous difficulties. A partial bleeching has been 

 observed and attributed to the migration or change of volume 

 of the chloroplasts, but the author believes it to be due to the 

 reduction of Chlorophyll to protophyllin. 



It has been shown that contrary to the usual supposition the 

 maximum of energy is not in the dark infra-red rays but in the 

 rays of the visible spectrum absorbed by the green plant. A 

 further conclusion is deduced, namely that so far at least as 

 Chlorophyll is concerned the photochemical effect of a radiation 

 depends not only on the degree of its absorption, but at the 

 same time on its energy — or the amplitude of its vibrations. 

 Now that we know that only those rays absorbed by Chloro- 

 phyll effect the reduction of carbon dioxide it is evident that 

 the economic coefficient must principally depend on the degree 

 of absorption of light by the green matter of the leaf, and it 

 becomes important to know the fraction of the total radiation 

 available to the plant. 



Timiriazeff as the result of careful experiment finds the 

 maximum effect at about half direct sunlight. Up to a certain 

 point this effect is proportional to the intensity. Wiesner has 

 shown that a considerable part of the evaporation going on in 

 the green leaf is accomplished at the expense of radiant energy 

 absorbed by the Chlorophyll. This has been termed chloro- 

 vapourisation. Deherain pointed out that evaporation falls off 

 in the presence of carbon dioxide. This would seem natural 

 since the same supply of energy must be divided between the 

 two processes — diel we not know how great is the excess of 

 radiant energy in the leaf. The absorption of energy by the 

 Chlorophyll amounts to about 27 per cent. of the solar energy 

 ineident upon the leaf, and the maximum economic coefficient 

 of assimilation is 3,3 per cent. But in addition to this the 

 work of chlorovapourisation amounts to 8 per cent. The sim- 

 plest way would be to admit that beyond a certain point the 

 radiant energy ineident upon the leaf will be in excess. But 



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