430 
PHYSIOLOGY. 
book II. 
SO excessively feeble, as to be wholly insufficient to overcome 
the blueness* of the carbon, if it were blue. The fact is, 
that the cause of carbon in the system of vegetation being 
green, belongs to that numerous class of facts of which no 
explanation can be given, in the existing state of human 
knowledge. 
“ Although we are justified by the mass of evidence, in as- 
serting that the green colour of plants is owing to the fixation 
of carbon in their tissue, in consequence of the power that 
light possesses of decomposing their carbonic acid, yet there 
are some exceptions that deserve attention. Humboldt found 
Poa annua and compressa, Plantago lanceolata. Trifolium 
arvense. Wallflower, and the Rhizomorpha verticillata, green 
in the subterranean galleries of the mines of Freyburg, al- 
though born in total darkness, but in atmosphere highly 
charged either with hydrogen or nitrogen. Ferns and 
Mosses, again, will be green where other plants are blanched ; 
and Humboldt found near the Canaries a Fucus which was 
bright grass-green, although it had grown at the depth of from 
25 to 32 fathoms (190 feet). Now, as light, according to the 
experiments of Bouguer, after traversing 180 feet, is weakened 
in the proportion of 1 to 1477*8, this Fucus must have been 
illuminated where growing by a power 203 times less than 
that of a candle at a foot’s distance. Are we to suppose that 
this feeble degree of illumination was sufficient to decompose 
the carbonic acid of such a plant, or was not the decomposi- 
tion rather owing to the operation of some unknown cause ? 
‘‘ Leaves, which, as we very well know, are usually green, 
may assume different colours in special cases. It is common 
to see in the autumn this green change to yellow, as in the 
Lombardy Poplar, &c.; or to red, as in the Berberry, the 
Sumach, the Virginian Creeper, and many kinds of Oaks. It 
is remarked that red colours are most common in leaves 
which contain some kind of acid, as the Vine, the Pear, the 
Viburnum, the Sorrel, &c. The red colouring matter obtained 
from leaves forms infusions which, like those from flowers, 
become more intense when acted upon by acids. Yellow 
leaves act in this manner like yellow flowers. It is supposed 
by some, that, while red is owing to the developement of acid. 
