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 flowers of 
Phajus maculatus 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 due 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 chlorophyllian 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 aérial 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 indissolubly 
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. Tuimiriazeff 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 
