284 BARKS 



The bark has a scarcely perceptible odour, but a strongly 

 astringent taste. 



The student should observe 



(a) The glossy silvery cork, 



(b) The line of sclerenchymatous cells, 



(c) The striated fibrous inner surface ; 



and should compare the bark with 



(i) Willow bark, which usually possesses a dull greenish brown 

 cork, paler inner surface, and no line of sclerenchymatous 

 cells ; 



(ii) Witch-hazel bark, which has a dull grey cork and reddish 

 pink colour. 



Constituents. The principal constituent of oak bark is the tannin, 

 quercitannic acid, C 17 H 16 9 (Etti), of which it contains from 5 to 

 20 per cent, according to the age of the bark. Oak bark, as described 

 above, should contain 15 to 20 per cent. ; trunk bark contains 

 only 5 to 8 per cent., or if the dead outer bark has been removed, 

 8 to 10 per cent. 



The bark also contains gallic acid, ellagic acid, quercite, phloro- 

 glucinol, &c. 



Quercitannic acid has been obtained as an amorphous, yellowish brown or 

 reddish white powder readily soluble in water and alcohol. When boiled with 

 dilute sulphuric acid it is converted into oak-red, C 38 H 26 17 , a reddish brown 

 substance insoluble in water, alcohol and ether. Whether dextrose is 

 simultaneously produced is doubtful, probably it is not, and oak -red may be 

 regarded as the anhydride of quercitannic acid. 



Gallic acid, C 7 H 6 5 , (trihydroxybenzoic acid), is colourless and crystalline ; it 

 is produced when gallotanmc acid is boiled with dilute sulphuric acid and occurs 

 in a number of plants. 



Ellagic acid, C 14 H 6 O 8 , is pale yellow and crystalline ; it forms a deep yellow 

 solution with caustic alkalies and dark blue with ferric salts. 



Quercite C 6 H 12 O 5 , is a sweet, crystalline pentahydric alcohol also found in 

 acorns. 



Phloroglucinol (phloroglucin), C 6 H 3 (OH) 3 , occurs in several plants and may 

 be obtained from many resins, tannins, &c., by fusing with caustic potash. 



Use. Oak bark has been used medicinally as an astringent, but 

 is not much prescribed at present. 



Note. Quercus JSuber, Linne, the cork oak (Mediterranean, Algeria) yields 

 commercial cork. The cork first formed is unsuitable for technical purposes 

 and is sold as ' virgin ' cork. That which is subsequently produced from the 

 phellogen is stripped from the tree every 8 or 10 years, steamed, scraped and 

 pressed into flat strips. 



