1 lO CELLULAR TISSUE. 



Thicker masses of cork, attaining a thickness of many centimetres, are formed on 

 the cortex of Testudinaria elephantipes, and especially of the cork trees, which 

 derive their names from it. These will be treated of in connection with the bark 

 (Chap. XV). 



The cork-cells remain uninterruptedly connected with one another. Only in the 

 INIelastomeae (Chap. XV), which form the first layer of cork on the margin of the 

 bast, have intercellular spaces between the perpendicular edges of the cork-cells been 

 observed. The form of the single cell is approximately that of a parallelopiped, 

 usually with a five- to six-sided base, parallel to the surface covered by the layer. 

 The height of the parallelopiped is usually smaller than the diameter of the bases, 

 and the cells are thus more or less flattened, in extreme cases, to quite flat lameflae, 

 as on the stem of Fagus, Betula, Tilia, species of Prunus, Boswellia papyrifera, &c. 

 In other cases the radial and basal diameters are almost equal, or even the former 

 larger than the latter, as especially in the soft cork of Quercus suber, Acer campestre, 

 Ulmus, Aristolochia, &c., also in thin layers of cork, e. g. Philadelphus. 



The basal diameters are in most cases about equal : in many forms, e. g. old 

 stems of Betula, Prunus Cerasus, on the other hand, the cells are elongated consider- 

 ably in a transverse direction. 



The cork-cells of the above Melastomece form an exception to these rules, since 

 they have the shape of elongated four-sided prisms, the sides of which are parallel to 

 the longitudinal axis of the stem. 



The single wall-surfaces remain flat and straight, or show archings and undula- 

 tions. The latter holds especially for the lateral or radial faces of most of the less 

 flattened cork-cells : indeed these are usually undulated in the radial plane : rarely 

 (Pinus sylvestris, Larix ^) in the tangential plane, so that the cell appears in surface 

 vie\v to have a star-like outline. 



The structure and history of growth of the cork-cells are still but imperfectly 

 known. The following facts, founded mainly on Sanio's investigations, may at 

 present be stated on this point. 



As regards the structure of the walls, one can distinguish firstly thin-walled 

 cells with apparently almost homogeneous delicate walls, and others with thickened 

 walls. Examples of the first are supplied especially by the iso-diametric or radially 

 elongated cells of the soft masses of cork of the surface of the stem of Quercus 

 suber, Acer campestre, Aristolochia, and the wide-celled layers of the bark of the 

 Bin:h. The flat forms have usually, but not ahvays (Nerium), thickened walls : and 

 in that case the thickness of the walls is almost equal all round (e. g. Fagus, 

 BosweUia papyrifera), or the outer (e. g. Salix, Zanthoxylon fraxineum) or the inner 

 wall (e.g. INIespilus germanica, Viburnum opulus) is specially thickened : the thicken- 

 ing mass is either uniform or pitted. Fibrous thickenings are known in the uniseriate 

 layers of thin-walled cells, which alternate with strongly thickened multiseriate layers, 

 in the tough corky skin of Boswellia papyrifera ^. The delicate membrane here shows 

 narrow thickening bands, which protrude inwards, and branch here and there at an 

 acute angle. Further, Sanio found in the cork-cells of the branches of Melaleuca 



* Schacht, Lehrbuch, II. p. 572. 2 Mohl, Botan. Zeitg. 1S61, p. 229. 



