CORK. 



"3 



The suberisation of the walls begins, in the cases investigated, directly after 

 the separation of the cork-cells by the divisions of the meristematic layer, and before 

 the cork-cell has attained its definitive size and thickness of wall. In the above- 

 mentioned Melastomaceae, according to Vochting, the wall even of meristematic cells 

 still capable of division is suberised, provided the first divisions have once taken 

 place. Where thickening masses slightly or not at all suberised appear within a 

 totally suberised outer lamella, they appear, according to Sanio, later than the 

 suberisation in the latter. 



In accordance with its continued active growth, the young cork-cell is filled 

 with protoplasm (nucleus) and cell-sap even after it has become differentiated 

 by the occurrence of suberisation. Thus a young cork-layer, even when very 

 completely developed, may remain transparent, and a branch covered by it (e. g. 

 Tilia) may long appear green to the naked eye, by reason of the chlorophyll of 

 the cortical parenchyma showing through it. Many cork-cells may remain for a 

 long time in this condition of independent growth, e. g., those of Sambucus 

 nigra, which last through the winter, and even form chlorophyll. Finally, and 

 at most after a year, there appear important internal changes. In one case — ■ 

 especially in the thin-walled forms with large cavities — the contents dry up to in- 

 significant residues, which often (Betula) attach themselves as granules to the wall: 

 the space enclosed by the membranes is filled with air. In the other case the cavity 

 is occupied by a dense, almost homogeneous, more or less darkly brown-coloured 

 mass : I leave it as undecided whether this completely fills the space, as Sanio states, 

 or whether air bubbles occur also. This is usually the case in flat, plate-like cork- 

 cells, like those of the bark of Fagus, Castanea, Tilia, Pirus, &c. 



When the air makes its appearance, the death of the cell has begun. The thin- 

 walled cells and masses of cork, which are filled with air, are incapable of other 

 than purely passive changes, and sink into gradual decay, e.g. Quercus suber, 

 Ulmus, Betula, &c. Also in the case of the flat cells filled with brown contents 

 it is probable that this mass consists of dead protoplasm and contents, and that the 

 death of the cell is indicated by its appearance. It is at all events beyond doubt 

 that in the stems, roots, &c. which grow in thickness, the cork-cells in question are 

 finally torn and decay. On the other hand however these flat cork-cells, as in the 

 above-cited trees, may be seen to increase for a long period in size in the direction 

 of the periphery; there is a decrease it is true of the radial diameter, but not any 

 great change of structure, or of the thickness of the walls. Destruction begins only 

 at a later period, and with it the disappearance of the brown contents. Accordingly 

 the question may be raised, and recommended for further investigation, whether the 

 increase in size of these cells depends upon a purely passive extension, or is 

 connected with a real growth, an increase in mass, at least of the walls. 



In the larger masses of cork of many plants single cells, genetically equivalent 

 to the cork-cells, assume the properties of short sclerenchymatous elements 

 (compare Sect. 29), resembling in all important points the so-called stone-cells. 

 The cork oak especially supplies examples of this, where such elements occur 

 singly in all possible places, but especially near the lenticels (Chap. XV), and m the 

 worse sorts of cork are known only too well by the dealers. Massive irregularly 

 concentric zones of such stony, brown elements, alternating with numerous thin- 



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