90 AROMATIC COMPOUNDS AND [CH. 



(6) Add a drop or two of iodine solution. A transient red colour appears. 



(c) Add a drop or two of lime water. A reddish or blue coloration will be given. 



(d) To a few c.c. of the nitrate in a porcelain dish add a little lead acetate solu- 

 tion. A precipitate is formed which turns red on addition of caustic potash solution, 

 and dissolves to a red solution with excess of potash. 



(e) To a few c.c. of the nitrate in a test-tube add a little potassium cyanide 

 solution. A pink colour appears, but disappears on standing. On shaking with air it 

 reappears. 



(f) To a few c.c. of the nitrate in a test-tube add a few drops of 10 % gelatine 

 solution. No precipitate is formed. 



(g) To a few c.c. of the filtrate in a test-tube add a little lead nitrate solution. 

 No precipitate is formed. 



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 tissue 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 



