ECONOMIC BOTANY. 127 



which it develops is removed periodically, and constitutes the 

 cork of commerce. A corky layer or periderm begins to be 

 developed when the tree is two or three years old. The first 

 cork that is produced is very inferior, but after its removal 

 good cork is yielded. When the tree is five j'ears old, the 

 worthless layer can be removed without injury to the phellogen, 

 or cork -producing cambium. This is done b)' making vertical 

 and horizontal incisions, reaching almost to the generating 

 layer of cells, or phellogen, within ; the bark is then carefully 

 pried off. This process cannot be performed with safety dur- 

 ing the active renewal of growth in the spring ; it can be 

 done, however, between May and October without in the least 

 harming the phellogen, provided the latter is not exposed to 

 rain. The layers become two-thirds to one inch (or more) 

 thick, and are removed every eight or ten years. When the 

 trees are fifteen years old, a valuable layer may be removed. 

 Cork continues to be produced till the trees are one hundred 

 or one hundred and fifty years old ; that is best, however, 

 which is removed from trees between fifty to one hundred 

 years old. The cork that grows in warm regions is better 

 than that produced in cold climates ; and that from trees on 

 the mountains is better than that fi'om trees in swamps. 



15. 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. pedunculata and 

 Q. sessUiflora. These have for centuries supplied the tanners 

 with bark, by the use of which leather of the best kind is 

 made. The inner bark of the Cork Oak (Q. suber) contains 

 much tannin and is used in Spain for tanning. The Oran or 

 African Oak (Q. coccifera) yields considerable tannin and is ex- 

 tensively exported for tanners' use. In Russia one of the 

 most important tanning materials is furnished by the Wil- 

 lows. In this country most of the bark used in tannery 

 comes from the Oaks— especially the White, Red, Scarlet, 

 Black, Burr and Chestnut Oaks ; the Hemlock (Pine family), 

 and ^^"attles, or species of Acacia (Bean family) growing in 

 Australia, are now extensively used. 



