1898] THE CHEMISTRY OF THE FOREST LEAF 179 



the well-known and characteristic decomposition products, normal or 

 putrefactive, one of which, to take an example, indoxyl-sulphate of 

 potassium (present in human urine) is easily oxidised to the powerful 

 pigment indigo. In 1873 Bommer showed that some very small 

 colourless granules scattered on the protoplasm of the Uowers of 

 Phajus tnaoulatus readily produce indigo when the oxygen of the air 

 comes in contact with the cell-contents ; and according to him the 

 experiments of Fremy fully suffice to prove that the origin of blue 

 chlorophyll (phyllocyanin) is clue to the presence of indican trans- 

 formed into indigo, which exists in chlorophyll though in excessively 

 minute proportion. We may observe that it is just this extreme 

 minuteness which forbids probably for ever the precise determina- 

 tion of the chlorophyll ian chromogen. We do not know what its 

 chemical constitution is, or whether it is a real dye-stuff with a 

 function either basic or acidic (I believe it is the latter). It has 

 been maintained that chlorophyll is not a substance derived from an 

 open chain of carbon atoms, and that very probably it is a derivative 

 of a benzenoid hydrocarbon ; but I think it is more probably a 

 derivative of polyuric or acrylic acid. The difficulties attending the 

 discovery of its real origin and chemical relationships seem to spring 

 from the fact that its outcome and presence in the leaf do not depend 

 on the aerial oxidisation incident to an expanded cellular surface, still 

 less on transpiration, but are solely and absolutely the direct and 

 immediate result of the life energy whose existence is indissolnbly 

 interwoven with the protoplasmic stroma of the chlorophyll granule. 

 A. Baeyer suggested that in the dissociation of carbonic acid, or 

 of water, protoplasm is at the bottom of the whole business, and 

 that chlorophyll plays only some subsidiary and indirect part, such 

 as that of temporarily fixing carbon dioxide as does haemoglobin, and 

 so facilitating the dissociation. According to Engelmann, while the 

 chlorophyll granules give off oxygen in the light, colourless proto- 

 plasm, cell membrane, and nucleus do not ; the green pigment itself 

 is not capable of so acting, it must be present in connection with the 

 living stroma of the chlorophyll granule. Timiriazeff held that the 

 function of chlorophyll is to absorb the rays which possess the 

 greatest energy, being transparent for those rays, and to transmit 

 this energy to the molecules of the carbon dioxide, which would not of 

 themselves undergo decomposition ; while Mr Berthelot has shown 

 that the presence of inert gases, such as nitrogen in the air, deter- 

 mines the phenomenon of the dissociation, they having the effect of 

 separating the atoms of the carbon dioxide, acting thus on the active 

 gas as a diminution of pressure would do. Nevertheless, and although 

 all these physical and mechanical agencies may aid and abet, the 

 question is, are they really and truly the practically determinant or 

 dominant causes of the effect ? It seems to be tolerably certain 



