594 CUPULIFERiE. 



History — The astringent properties of all parts of the oak^ were 

 well known to Discorides, who recommends a decoction of the inner 

 bark in colic, dysentery and spitting of blood. Yet oak bark seems at 

 no time to have been held in great esteem as a medicine, probably on 

 account of its commonness; and it is now almost superseded by other 

 astringents. For tanning leather it has always been largely employed. 



Description — For medicinal use the bark of the younger stems or 

 branches is collected in the early spring. It varies somewhat in appear- 

 ance according to the age of the wood from which it has been taken: 

 that usually supplied to English druggists is in channelled pieces of 

 variable length and a tenth of an inch or less in thickness, smooth, of a 

 shining silvery grey, variegated with brown, dotted over with little scars. 

 The inner surface is light rusty-brown, longitudinally striated. The 

 fracture is tough and fibrous. A transverse section shows a thin, greenish 

 cork-layer, within which is the brown parenchyme, marked with nume- 

 rous rows of translucent colourless spots. The smell of dry oak bark is 

 very faint; but when the bark is moistened the odour of tan becomes 

 evident. The taste is astringent and in old barks slightly bitter. 



Microscopic Structure — The outer layer of young oak bark con- 

 sists of small flat cork-cells; the middle layer of larger thick- walled 

 cells slightly extended in a tangential direction, and containing brown 

 grains and chlorophyll. This tissue passes gradually into the softer 

 narrower parenchyme of the inner bark, which is irregularly traversed 

 by narrow medullary rays. It exhibits moreover a ring, but slightly 

 interrupted, of thick-walled cells (sclerenchyme) and isolated shining 

 bundles of liber fibres. 



Groups of crystals of calcium oxalate are frequent in the middle and 

 inner bark, but the chief constituents of the cells are brown granules of 

 colouring matter and tannin. As the thickness of the bark increases 

 the liber is pushed more to the outside, the middle cortical layer being 

 partly thrown ofi* by secondary cork-formation (rhytidoma, see pp. 354 

 and 588). Hence the younger barks, which alone are medicinal, are 

 widely different from the older in structure and appearance. 



Chemical Composition — The most interesting constituent is a 

 peculiar kind of tannin. Stenhouse pointed out in 1843 that the 

 tannic acid of oak bark is not identical with that of nutgalls; and such 

 many years afterwards was proved to be the case. 



The first-named substance, now called Querci-tannic Acid, yields 

 by destructive distillation pyrocatechin, and according to Johanson 

 (1875) very little pyrogallol. By boiling it with dilute sulphuric acid 

 querci-tannic acid is split up into a red derivative and sugar. A 

 solution of gelatine is precipitated by querci-tannic acid as well as by 

 gallo-tannic acid; yet the compound formed with the latter is very 

 liable to putrefaction, whereas the tannin of oak bark, which is accom- 

 panied by a large amount of extractive matter, furnishes a stable com- 

 pound, and is capable of fonning good leather. 



As querci-tannic acid has not yet been isolated in a pure state, the 

 exact estimation of the strength of the tanning principle in oak bark 

 has not been accomplished, although it is important from an economic 

 as well as from a scientific point of view. The method of Neubauer 



1 Probably not Q. Rohur L. 



