70 FORMATION OF SECRETIONS. 



from the air. Their stomates appear well adapted for that 

 purpose, by their position in most abundance on the under side 

 of leaves ; and the possibility of recovering drooping or sickly 

 plants, by syringing their epidermis copiously, seems to render 

 this fact almost certain. It is, however, imagined by some, 

 that leaves have no power of absorbing water, even in an elastic 

 state ; and that the renovation of plants by syringing is merely 

 owing to a diminution of perspiration, which is improbable. 



It is to the action of leaves, — ^to the decomposition of their 

 carbonic acid, and of their water ; to the separation of the 

 aqueous particles of the sap from the solid parts that were 

 dissolved in it ; to the deposition thus effected of various earthy 

 and other substances, either introduced into plants, as silex 

 and metallic salts, or formed there, as the vegetable alkaloids ; 

 to the extrication of nitrogen ; and, probably, to other causes 

 as yet unknown, — ^that the formation of the peculiar secretions 

 of plants, of whatever kind, is owing. And this is brought 

 about principally, if not exclusively, by the agency of light. 

 Their green colour becomes intense, in proportion to their 

 exposure to light within certain limits, and feeble, in proportion 

 to their removal from it ; till, in total and continued darkness, 

 they are entirely destitute of green secretion, and become 

 blanched or etiolated. The same result attends all their other 

 secretions; timber, gum, sugar, acids, starch, oU, resins, odours, 

 flavours, and aU the numberless narcotic, acrid, aromatic, 

 pungent, astringent, and other principles derived from the 

 vegetable kingdom, are equally influenced, as to quantity and 

 quality, by the amount of light to which the plants producing 

 them have been exposed. 



It is evident that the possibility of the downward distribution 

 described m the previous paragraph rests upon the certainty that 

 elaborated sap descends and is dispersed through the system. That this 

 occurs is so certain, that it would have been needless to maintain it by 

 further proof if some modern naturalists had not ventured to caE it in 

 question. To deny it is tantamount to questioning the existence of 

 wood, or its formation as it appears to our eyes when the bark is 

 stripped from a young growing shoot; as in a Lilac for example. In 

 such a case new wood can be demonstrated to descend from each leaf 

 downwards, till it is lost among the multitude of descending currents. 



