THE METABOLISM OF PLANTS. 235 



It is still a debated question whether or not the plant is capable of 

 making any use of the glucosides. The researches which have been 

 made on the subject refer only to tannin. It must be remembered that 

 micromechanical methods do not enable us to distinguish between the 

 glucoside tannin, and tannic (digallic) acid : it is quite possible that the 

 glucoside is of use ill that it yields glucose, and that the acid is not. 

 We will recur to this point below. 



We will now treat of the aromatic substances other than 

 the glucosides. The following are those which commonly 

 occur in plants : 



Pyrocatechin, Phloroglucin, Saligenin, 



Benzoic acid, cinnamic acid, tannic acid, gallic acid, salicylic acid, 



Resins, Balsams, and Ethereal Oils. 



Pyrocatechin, C 6 H 4 (OH) 2 and phloroglucin, C 6 H 3 (OH) 3 ,are derivatives 

 of benzene, the former being orthodioxybenzene, the latter probably 

 paratrioxybenzene. Pyrocatechin has been found in various parts of 

 plants, more especially in fading leaves : the mode of its origin has been 

 suggested above. 



Phloroglucin (paratrioxybenzene), C 6 H 8 (OH) 3 is a substance of common 

 occurrence in the bark of trees : it is a product of the decomposition of 

 substances like phloretin and quercitrin which are derived from glucosides. 



Saligenin C 7 H 8 O 2 = C 6 H 4 (OH), CH 2 (OH), is a derivative of toluene ; 

 it is orthomethoxyphenol ; it is formed as shewn above, by the decom- 

 position of the salicin and of the populin which are present in the bark of 

 Willows and Poplars respectively. 



The aromatic acids are of common occurrence in plants. Benzoic 

 acid (C 7 H 6 O 2 ) is prepared from gum-benzoin, the produce of Styrax 

 Benzoin, but it is present in small quantity in many other plants, and 

 together with cinnamic acid (C 9 H 8 O 2 ) in various balsams, such as storax 

 and balsam of Peru. These acids are probably formed by the oxidative 

 decomposition of glucosides which, like amygdalin, yield benzoic alde- 

 hyde (oil of bitter almonds) under the action of ferments. 



Salicylous acid (C 7 H 6 O 2 ) occurs in various flowers, especially in those 

 of the genus Spiraea : it is probably formed by the oxidation of its cor- 

 responding alcohol saligenin. 



Tannic and gallic acids are very generally present in plants. Tannic 

 acid (Ci 4 H 10 O 9 ) occurs in two somewhat different chemical forms, in the 

 tissues of almost all plants ; it is found, according to the observations of 

 Sachs and of Petzold, in the immediate neighbourhood of those parts of the 

 plant, growing-points for instance, in which metabolism is most active. 

 It is doubtless derived from tannin as indicated above. Gallic acid 

 (C 7 H C O 5 ), though not so universally present as tannic acid, has been 



