CHAP. XVI. 
COLOUR. 
429 
are for the most part irregular ; in Endogens they are usually 
arranged in bands that follow the course of the principal veins. 
In these places it is clear that chromule^ or colouring matter, 
is either not found at all or in very small quantity ; but the 
cause of the deficiency is entirely unknown. It is, however, 
interesting to remark, that variegations of this kind are best 
preserved in sterile, and are soonest lost in fertile, soil, as if 
they w^ere in reality an unhealthy state of a plant ; a suppo- 
sition, however, which there are no sufficient grounds other- 
wise for entertaining. 
We have already seen that all parts which either are green 
or susceptible of becoming so, decompose the carbonic acid of 
the sap or of the atmosphere, when they are exposed to solar 
light ; that they part with the oxygen, and fix the carbon in 
their own tissue. Hence it was natural to conclude that this 
operation is connected with the formation of a green colour. 
In fact, when it takes place greenness does ensue; when it 
does not take place, the organ that developes in darkness pre- 
serves the primitive white colour of the tissue ; and when it 
has taken effect incompletely, the results are intermediate 
between the two preceding cases. 
“ The deposition of carbon thus induced does not act upon 
the vegetable membrane; which always retains its original 
pearly lustre. But it forms a peculiar matter called chlorophyll 
or green chromule ; the abundance or scantiness of which is what 
causes the different tints of leafy surfaces. The action of the 
membrane produces some effect, no doubt, either by reason 
of its own pallid hue, or its transparency or density ; or of the 
hairs with which it is often covered ; or of the air which it 
contains ; or, finally, of the waxy matter by which they are 
protected. But how does it happen that carbon, which is 
black, is capable of producing a green appearance in vege- 
tation ? The old physiologists supposed that it is in reality 
an intense blue, and not a black; and that, shining through 
the yellow sides of the cells, the combination of the two colours 
produced green. This notion, however, is disproved by the 
most casual inspection, for the colouring matter may be sepa- 
rated from the tissue with the greatest facility, and it still pre- 
serves its colour; and, besides, the yellow of tissue, if any, is 
