FUNCTION.] LIGHT INFLUENCES SECRETIONS. 301 



and the less its flavour. Hence it is that the most odor- 

 iferous 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 extra-tropical Asia. The Peach, the 

 Vine, and the Melon, also, nowhere 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 

 astringent 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 brittle 

 Wainscot Oak of the Black Forest is produced by the very 

 same species as produces the tough and solid naval timber of 

 Great Britain. Starch, again, in which carbon forms so large 

 a proportion, and which, in the Potato, the Cassava, Corn, 

 and other plants, ministers so largely to the nutriment of man, 

 depends for its abundance essentially upon the presence of 

 light. For this reason, Potatoes grown in darkness are, as 

 we say, watery, in consequence of no starch being developed 

 in them ; and the quantity of nutritious or amylaceous matter 

 they contain is in direct proportion to the quantity of light 

 to which they are exposed. For this reason, when orchard 

 ground is under-cropped with Potatoes, the quality of their 

 tubers is never good ; because the quantity of light intercepted 

 by the leaves and branches of the orchard trees, prevents the 

 formation of carbon by the action of the sun's rays upon the 

 carbonic acid of the Potato plant. Mr. Knight has turned 

 his knowledge of this fact to great account, in his application 

 of the principles of vegetable physiology to horticultural 

 purposes." 



