VIII] AROMATIC COMPOUNDS 105 



Tannins. 



This is a large group of substances, many of which are of complex 

 composition. They arise in the plant from simpler compounds, such as 

 protocatechuic, gallic and ellagic acids. Their formation takes place in 

 various ways, either by condensation, accompanied by elimination of 

 water, or by oxidation, or both ; there may also be condensation with other 

 aromatic complexes. 



The tannins are widely distributed in the higher plants and, although 

 no very systematic investigation has been made, it is obvious that some 

 plants are rich in these substances, others poor, and others, again, 

 apparently entirely without them. The tannins generally occur in 

 solution in the cells of tissues of the root, stem, leaf, fruit, seed and 

 flowers: sometimes they are confined to special cells, tannin-sacs, but 

 after the death of the cell, the cell-walls of the dead tisssue become 

 impregnated with the tannin. In tannin-producing plants, the tannin 

 is generally found throughout the plant, and it probably tends to 

 accumulate in permanent or dead tissues, such as the bark (dead cortex 

 and cork), woody tissue, underground stems, etc. 



Tannins appear to be more frequent in woody than in herbaceous 

 plants, though in the latter they naturally only accumulate in the 

 persistent underground stems and root-stocks. In annuals, also, tannins 

 seem to be more rare: this may be due to the fact that in a short-lived 

 plant, comparatively little tannin is formed and is not so readily detected 

 as in the tissues of a perennial. 



In certain plants which are highly tannin-producing and are also 

 woody perennials, the bark becomes very rich in tannins. These barks 

 are consequently of considerable commercial importance for tanning of 

 leather. As examples may be taken species of Caesalpinia, Spanish 

 Chestnut (Castanea), Eucalyptus, Oak (Quercus), Mangrove (Rhizophora), 

 Sumac (Rhus). Tannins also occur in quantity in galls, especially on 

 species of Quercus. 



As a class, the tannins are non-crystalline and exist in the colloidal 

 state in solution. They have a bitter astringent taste. They have 

 certain properties and reactions in common, i.e. they precipitate gelatine 

 from solution, are themselves precipitated from solution by potassium 

 bichromate, and give either blue or green colorations with solutions of 

 iron salts. Many tannins occur as glucosides but this is by no means 

 always the case. 



