114 CELLULAR TISSUE. 



walled layers, characterise the corky masses of old tuberous stocks of Tamus 

 elephantipes^- 



As has often been already indicated in what has been said above, the 

 successive layers of one corky mass are either formed throughout of almost similar 

 cells, or concentric zones of unlike properties alternate with one another. 



The first case includes most cork-layers consisting as a rule of more or less 

 flattened cells, which as thin skins cover wound-surfaces, roots, rhizomes, and the 

 cortex of stems. The second case includes in part equally thin layers, e. g. those 

 on the branches of Philadelphus, where one layer of cells greatly elongated radially 

 always alternates with one to two layers of flat cells ; on the other hand more 

 especially thicker masses of cork, such as those of Tamus elephantipes, and the 

 stems of dicotyledonous trees. The great flaps of cork on the cortex of Boswellia 

 papyrifera consist, as already stated, of multiseriate layers of flat cells, which 

 alternate with uniseriate thin-walled cells, with a fine fibrous thickening and 

 silicified walls. A similar alternation of multiseriate layers of flat strongly-thickened 

 cells and wide thin-walled ones occurs in the white corky covering of young 

 birch stems, in the corky masses on the stems of Quercus Suber, Acer campestre, 

 Liquidambar, &c. According to Hartig^ and Sanio' each narrow-celled layer in 

 the Birch corresponds to the inner limit of one annual increment of growth, as 

 in the wood, only inverted (comp. Chap. XV)*. Also in Quercus Suber' the number 

 of the concentric zones corresponds with the specified number of years through 

 which the production of cork on the tree had lasted. Whether such relations 

 between stratification and annual increment are more generally distributed remains 

 still to be investigated. 



It is manifest that the firmness or toughness of a layer of cork must vary according to 

 the form and thickening of the walls of the cells, even if one assume that the physical 

 properties of the corky substance are universally similar. As a fact, one finds the flat- 

 celled, thick-walled layers firm and tough, resisting energetically both the increase of the 

 enclosed parts and the causes of injury acting from without : the wide-celled thick- 

 walled forms are soft, more easily burst by the increase of the enclosed parts, and more 

 easily injured from without. Alternating wide- and flat-celled masses, as especially those 

 of the Birch and of Boswellia papyrifera, peel -off, when old, by splitting of the delicate 

 wide-celled layers from one another. 



According to Mohl's system ", it is usual to distinguish the wide-celled softer form as 

 cork, in the narrower sense, from the tough-walled masses of cork which are termed 

 Periderm. Since this distinction can never be sharply drawn, it may here be entirely 

 given up, and the above-described sort of tissue be termed Cork (Suber), while the term 

 Periderm may be applied to all phellogenetic cortical products (to be more exactly treated 

 in Chap. XV) of which the cork is a part. 



^ Compare Mohl, Verm. Schr. p. 190. In the old specimens investigated the hard layers are 

 much stronger according to Mohl's statements than in the yoimger ones. 



' Forstl. Cultnrpfl. p. 306. '^ I.e. p. 83. 



• Von Merklin, Mel. Biolog. de I'Acad. S. Petersburg, opposes Hartig's statements. 

 ° C. de CandoUe, in Memoires de la Soc. de Physique de Geneve, XVI. p. i (1861). 

 ° Verm, Schr. p. 212. 



