292 ECONOMIC BOTANY. 



placed in piles and weighted with stones and allowed to 

 dry, thus losing fifteen per cent, in weight. The outer- 

 most and innermost sides are then removed with a knife or 

 file, and the slabs placed in boiling water five to six 

 minutes. They are then dried, and found to have gained 

 one-fifth in volume. In Spain the cork layers or slabs 

 are sometimes drawri through a flaming fire, when they 

 take on externally a blackish and internally a deep gray- 

 ish-brown color. The cork slabs or plates are sometimes 

 nearly two inches thick ; they are plainly stratified, i.e., 

 the annual rings of growth are plainly seen. The cork- 

 cells are in section generally four-sided ; their walls are 

 very thin, and seldom lignified. The hard, brittle portion 

 of the cork consists of sclerenchyma. Cork contains more 

 carbon and less oxygen than cellulose; its important 

 chemical constituent is Suberin. The various uses of cork 

 are well known, and require no further mention. 



274. Of the various Tanning Barks used in different 

 countries, none compare with those from several species of 

 Oak ( Quercus). The most valuable species in Europe are 

 Q. pednmmlata and Q. aessiKJlora. These have for centu- 

 ries supplied the tanners with bark, by use of which 

 leather of the best kind, both as to color and wear, is made. 

 The quality of Oak bark is better when the trees grow on 

 good clay soil; poorer, when on calcareous soil. It has 

 beeii shown by analysis that Oaks of fifty years growth 

 yield thirteen per cent , and those one hundred years old 

 yield but eight per cent., of tannin. The inner bark of the 

 Cork Oak (§. Suber) contains much tannin, and is used 

 in Spain for tanning. The Oran, or African Oak (Q. cocd- 

 fera), yields fifteen per cent, of tannin, and is extensively 

 imported for tanner's use. In Russia, one of the most 



