300 LIGHT INFLUENCES SECRETIONS. [BOOK n. 



CHAPTER XIV. 



OF SECRETIONS. 



THE result of the foregoing phenomena is, the formation 

 of numerous principles peculiar to the vegetable kingdom, 

 and the deposit of others which have heen introduced into 

 the system of plants in the current of the sap. Thus are 

 produced, on the one hand, the sugar of the Cane, and of 

 various fruits ; the starch of Corn, Potatoes, and other fari- 

 naceous plants ; the gum of the Cherry ; the tannin of the 

 Oak ; and all those multitudes of azotised, oily, resinous, and 

 other substances, of which the modern chemist has ascertained 

 the existence; and on the other hand, the various mineral 

 matters left as ashes after vegetable bodies have been burnt. 



As light* is one of the chief agents by which the decom- 

 position, 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 would seem 

 that these must be in direct proportion to the mere quantity 

 of light they have been exposed to. The author of the article 

 Botany, in the Library of Useful Knowledge, has remarked 

 that " 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 appearances are owing are highly carbonised. 

 On the contrary, the less a plant is exposed to sunlight, the 

 paler are its colours, the laxer its tissue, the fainter its smell, 



* For some highly interesting experiments upon the effect of light passing 

 through coloured media, in determining the appearance of the lower plants and 

 animals, see Morren's Essais sur I" ffeteroyenie dommante ; Liege, 1838. 



