318 PHYSIOLOGY. BOOK II. 



province of Chemistry rather than of Botany, need not be 

 recapitulated here. It will be more useful to make some 

 o-eneral 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 dearee 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 owmg 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 acqviire 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 degi'ee to the quality 

 of timber, — a point of no small importance to all countries. 



It is in a gi'eat degree to the carbon incorporated with the 

 tissue, either in its own proper form, or as resinous or astrin- 

 gent matter, that the diflPerent 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 



