318 
PHYSIOLOGY. 
BOOK II. 
province of Chemistry rather than of Botany, need not be 
recapitulated here. It will be more useful to make some 
general observations upon the practical application of the 
physical laws we have been examining. 
As light is the great agent by which the decomposition, 
recomposition, and assimilation of the juices of plants take 
place, and as it must be obvious that the intensity of the action 
of vegetable secretions, or their abundance, will depend upon 
the degree of their elaboration, it follows that these must be 
in direct proportion to the quantity of light they have been 
exposed to. As has been observed by the author of the 
article Botany, in the "Library of Useful Knowledge," " We see 
in practice that the more plants are exposed to light when 
growing naturally, the deeper is their green, the more robust 
their appearance, and the greater the abundance of their odours 
or resins ; and we know that all the products to which these ap- 
pearances are owing are highly carbonized. On the contrary, 
the less a plant is exposed to sunlight, the paler are its 
colours, the laxer its tissue, the fainter its smell, and the 
less its flavour. Hence it is that the most odoriferous herbs 
are found in greatest perfection in places or countries in which 
the sunlight is the strongest — as sweet herbs in Barbary 
and Palestine, Tobacco in Persia, and Hemp in the bright 
plains of extratropical Asia. The Peach, the Vine, and the 
Melon, also, no where acquire such a flavour as under the 
brilliant sun of Cashmere, Persia, Italy, and Spain. 
This is not, however, a mere question of luxury, as odour 
or flavour may be considered. The fixing of carbon by the 
action of light contributes in an eminent degree to the quality 
of timber, — a point of no small importance to all countries. 
It is in a great degree to the carbon incorporated with the 
tissue, either in its own proper form, or as resinous or astrin- 
gent matter, that the different quality in the timber of the 
same species of tree is principally owing. Isolated Oak 
trees, fully exposed to the influence of light, form a tougher 
and a more durable timber than the same species growing in 
dense forests ; in the former case its tissue is solidified by the 
greater quantity of carbon fixed in the system during its 
growth. Thus we have every reason to believe that the 
