APRIL. Ill 



question, and therefore proceed shortly to notice some of the secretions of 

 phmts. 



It must be obvious to the most common observer, that the pecuhar 

 products of plants vary, as much as do their flowers or outward 

 appearance. The exquisite fragrance of some is only equalled by tiie 

 most offensive odour of others. While some produce the most delicious 

 fruits, others cannot be tasted for their nauseating qualities. One class 

 furnishes us with food so indispensable to our very existence, that the 

 epithet " staff" of life" alone adequately conveys to us their importance; 

 while, again, we find others producing the most deadly poison ; while 

 the various other ingredients found in plants — as gum, sugar, starch, 

 tannin, resin, albumen — show how widely diff"erent are their secretions. 

 But whatever these may be, they are all dependent on the subtile 

 agency of light for their formation, and tlie qualities of their respective 

 products are increased, or otherwise, as the plants producing them have 

 been fully, or not, exposed to its influence. 



It follows, then, that by a vital function, common, in a greater or 

 less degree, to the entire vegetable kingdom, plants are enabled, under 

 the influence of solar light, to decarbonise the air in which they grow, 

 and by assimilating the carbon (found in the atmosphere combined with 

 oxygen in the form of carbonic acid), they furnish themselves with the 

 principal material for building up their own structure from the air, and 

 likewise for forming the base of such carbonact ous compounds as oily 

 matter, sugar, gum, and starch, found in all plants. In connection 

 with the decomposition of carbonic acid and water (which is effected by 

 the same agency), is the liberation of oxygen, which is restored to the 

 atmosphere, by which means this element is maintained in a state of 

 perpetual purity and freshness. By the decomposition of water, which 

 takes place in the cells of the plant, hydrogen is retained, and forms 

 one of the proximate elements of which resin, turpentine, and many 

 aromatic products consist. The formation of those constituents which 

 render plants valuable as producing food — as gluten, albumen, and 

 casein — is owing principally to the presence of nitrogen and the phos- 

 phates of lime and magnesia. These enter the plant in a soluble state 

 by absorption through the roots — the former in the shape of ammonia 

 — and, with the alkaloids, undergo the various chemical changes into 

 which they are afterwards formed, when exposed to the same potent 

 power whose influence we are considering. 



The exact way in which all the various products found in plants are 

 elaborated from the four simple elements of which they are composed, 

 is beyond the pale of scientific research ; we only know the great agent 

 is Light, acting in connection with heat and the vital powers of the 

 plant. 



We have before noticed, that we consider that the leaves of plants are 

 acted upon differently at certain stages of their growth. When young, 

 and while they are being developed, their principal action appears to be 

 the assimilation of carbon, which takes place very rapidly in bright 

 weather. At a later period of their growth, the sap undergoes a 

 change, which renders it more susceptible to chemical affinities, and 

 it is then that we find the peculiar secretion common to each plant in 

 greatest abundance. 



