52 VITAL ACTIONS. 



from the solid parts that were dissolved in it ; to the 

 depositioa thus effected of various earthy and other 

 substances, either introduced into plants, as silex and 

 metallic salts, or formed there, as the vegetable alka- 

 loids ; 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 ; tim- 

 ber, gum, sugar, acids, starch, oil, resins, odours, fla- 

 vours, aijd all 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. 

 76. It is, however, to be observed that, as has 

 already been stated (68), the capability of plants to 

 bear the action of direct light varies according to 

 their specific nature. One species is organised to suit 

 the atmosphere of a dense wood, into which diffuse 

 light only will penetrate ; another is planted by nature 

 on the exposed face of a sunburnt rock, upon which 

 the rays of a shadeless sun are daily striking ; in 

 these cases, the light which is necessary to the one 

 would be destructive of the other. The organic dif- 



