356 
PHYSIOLOGY. 
BOOK II. 
the blotches 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 were in reality an unhealthy state of a 
plant, — a supposition, however, which there are no sufficient 
grounds otherwise 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 ensues ; when it does 
not take place, the organ which developes in darkness, pre- 
serves the primitive white colour of the tissue ; and when it 
lias taken effect incompletely, the results are intermediate 
between the two preceding eases. 
" The deposition of carbon thus induced does not act upon 
the vegetable membrane ; that is always of its original pearly 
lustre. But it forms a peculiar matter called green chromule ; 
the abundance or scantiness of which is what causes the differ- 
ent tints of leafy surfaces. The action of the membrane pro- 
duces 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 pro- 
ducing a green appearance in vegetation ? The old physio- 
logists 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 in- 
spection, for the colouring matter may be separated from the 
tissue with the greatest facility, and it still preserves its 
colour; and, besides, the yellow of tissue, if any, is so exces- 
