SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 557 



the latter together with concentric zones of flat elements alternating with it. 

 Examples of this are afforded by Acer campestre, with broad, soft zones of cork ; 

 by the older, bark-forming cortex of Betula alba, also with broad zones of cork, 

 of similar structure to the white superficial periderm of younger stems, but firmer ; 

 but more especially by the Cork Oaks. The cortex of the stem of Quercus pseudo- 

 suber is covered, in the thick pieces of the stem investigated, which was at least 

 forty years old, by a crust of cork, attaining 2<=m in thickness, and cracked ex- 

 ternally; it looks like bad bottle cork, and has the same structure as the latter, with 

 reference to the cork-elements. It includes at various depths, numerous scattered 

 isolated scales of desiccated cortical tissue, which are scarcely more than 2™™- thick, 

 and icm to about 6<'m broad and long. The oaks which yield true cork, especially 

 Q. Suher, have, in the wild condition, the tendency to a similar formation of bark, but 

 with a far more abundant production of cork. The latter indeed appears chiefly as 

 superficial periderm (comp. p. 550); but it may also proceed, in the wholly intact 

 tree, from internal, repeated periderms, and thus cut off narrow portions of cortex 

 between broad zones of cork \ With" the object of obtaining technically valuable 

 cork, the tendency of the tree to the repeated internal formation of periderm is 

 made use of artificially in the Cork Oaks and the Birch. The numerous mis- 

 understandings regarding the modes of procedure and phenomena theii taking place, 

 some of which have re-appeared even in recent times, may justify a short account 

 of the facts in this place, leaving purely technical matters untouched. 



The intact Cork Oak'^ forms on stem and branches the superficial periderm above 

 described, producing the thick mass of cork, which becomes cracked externally. Even 

 on branches and stems many years old this form only is generally observed, covering the 

 cortex, the whole of which remains alive, while the internal periderms just mentioned 

 only occur exceptionally ; this however may depend on the fact that old intact stems 

 but seldom come under observation. In order to obtain cork for technical purposes, 

 the almost useless, superficially-formed layer (called the male) is removed from the stem 

 all round (demasclage) ; this is done carefully and smoothly, but not without everywhere 

 injuring and exposing the living cortical tissue — in the best case at least the layer of 

 phellogen and the phelloderm. While the cortical tissue begins to die off on the injured 

 surface, a new periderm appears, one or two millimeters below the latter, around the 

 entire stem ; and its phellogen produces a new layer of cork, which cuts oif the portion 

 of cortex lying outside it. This periderm grows quicker than the external male cork, 

 and is used technically as ' female cork.' The first peeling off of the male cork is under- 

 taken when the tree is about fifteen years old. A serviceable female layer of cork is 

 formed in about lo-iz years ; a layer now before me, twelve years old, has, for example, 

 an average thickness approaching i""^, without having undergone any further dressing. 

 The female layer of cork, when sufficiently thick, is now peeled off like the male one, in 

 order to be made use of, and is again replaced by the tree, in the way described above, 

 by an internal formation of periderm. The same process may be repeated periodically, 

 until the tree attains an age of about 150 years. If left to grow indefinitely, a female 

 layer of cork may attain an immense thickness ; I have a piece before me 17'=™ thick, 

 though of course of very bad quality. 



A quite similar method to that used for obtaining oak-cork is employed in the northern 



• Compare C. de CandoUe, Mem. Soc. Phys. de Geneve, XVI. p. 1 (186.1), Taf. I. Fig. 2. 

 ' C. de CandoUe, I.e. — Von MoM, Botan. Zeitg. 1848, p. 361.^— See also Fluckiger, Ph.irma-. 

 cognosie, p. 334. 



