METABOLISM AND TRANSPORT OF 

 MATERIALS. 



1.— THE ORGANIC COMPOUNDS IN PLANTS. 



Carbon Compounds. — Metabolism in Living Plants. 



CAEBON COMPOUNDS. 



It is naturally to be expected, from analogous relations in the inorganic world, 

 that the variety to be observed in the vegetable kingdom as to colour, taste, and 

 smell, should depend upon the diversity of the materials manufactured in the in- 

 dividual species. Numerous characteristic materials have been shown by the re- 

 searches of chemists to belong to certain species, and in the names given to many of 

 these (as in the terms oxalic acid, benzoic acid, salicin, amygdalin, asparagin, nicotin, 

 strychnin, atropin, cocain, &c.) we recognize the names of well-known plants. But 

 it would be erroneous to suppose that the series of substances belonging to the 

 vegetable kingdom would be exhausted by the sugars, acids, salts, alkaloids, oils, 

 ethers, and pigments which are already known to us by their varied effects on our 

 nerves of taste, smell, and sight. What is accurately known, indeed, in this respect 

 is apparently only a fraction of what actually exists. In the meantime we cannot 

 venture on even an approximate estimate of all the substances produced by plants. 

 Only this much can be affirmed with certainty, that their number is far greater 

 than that of inorganic or mineral bodies. This is the more remarkable, since the 

 elements of which the inorganic compounds are built up are comparatively so many, 

 whilst the elements which serve as building materials for organic compounds are 

 so few. The fact is thus explained, that carbon, an element whose chemical nature 

 admits of its union with other elements in inexhaustible combinations, appears 

 as the centre of all organic compounds in plants. 



For the purpose of the following discussion it is fitting, first of all, to give here 

 a brief sketch of this important property of carbon. Chemists call carbon a tetrad 

 element, by which is meant that each atom of carbon enters into combination with 

 four atoms of another element, and can form a mechanically inseparable group, i.e. 

 a molecule. It can be shown that each atom of a tetrad element possesses four 

 centres of attraction, i.e. four connection points, to which the atoms of other 

 elements become attached, and where they are held fast. These points are called 

 bonds of union, and are said to be saturated when other atoms have become 



