DIV. 



* MORPHOLOGY 163 



Bottle cork (Fig. 58) is formed of thick layers of soft wide cork-cells, interrupted 

 by thin layers of flat cork-cells marking the limits of the year's growth ; this can 

 be recognised in an ordinary cork. The pores filled with a loose powder which 

 penetrate the whole thickness of the cork in a radial direction are the lenticels 

 (cf. p. 59). The first layer of cork of the Cork Oak is artificially stripped off down 

 to the cork-cambium after fifteen years. A new cambium then forms a few cells 

 deeper which provides the cork of economic value ; this is removed every 6 to 8 

 years. Since such dead coatings of cork cannot keep pace with the dilatation of 

 the stem they gradually become fissured. 



3. Formation of Bark. All tissues external to the cork-cambium 

 are cut off from supplies of water and food materials and consequently 

 die. The dead tissue, including the layer of periderm, is termed BARK. 

 According to the depth at which the periderm is formed this may 

 include only the epidermis or a larger or smaller proportion of the 

 cortex. The first layer of cork-cambium in stems and roots usually 

 soon ceases to be active ; this does not happen in the Beech. A 

 new layer of cork forms deeper in the stem, and its activity in turn 

 comes to an end ; another layer forms still more deeply as shown in 

 Fig. 186. Ultimately the layers of cork are forming in secondary 

 tissues, in the living parenchyma of the zone of bast ; thus in old 

 stems all the living tissue external to the cambium is of secondary 

 origin and the bark includes dead secondary tissues. These are 

 emptied of their food material and contain only by-products of the 

 metabolism. The bark cannot follow the further increase in thickness 

 of the stem or root, but is cast off in scales or torn by longitudinal 

 fissures. It forms an even more complete protection than the cork 

 against both loss of water and overheating. 



Since in the formation of bark the more external and oldest parts 

 of the bast are thus shed, the zone of bast remains relatively thin. 

 Mechanical tissues can only be permanent constituents of the stem 

 when formed internal to the cambial ring, i.e. in the wood. 



If the layers of the secondary periderm constitute only limited areas of the cir- 

 cumference of the stem the bark will be thrown off in scales, as in the SCALY BARK of 

 the Pine, Oak. and Plane tree ; if, on the contrary, the periderm layers form com- 

 plete concentric rings, hollow cylinders of the cortical tissues are transformed into 

 the so-called KINGED BARK, such as is found in the Grape-vine, Cherry, Clematis, 

 and Honeysuckle. 



When the bark peels off from the stem in layers this is not a purely mechanical 

 result ; it depends on an ABSCISS LAYER consisting of thin-walled cork-cells or 

 phelloid cells (cf. p. 59) which are formed between the other layers of cork with 

 thickened walls. These absciss layers are ruptured by the hygroscopic tensions set 

 up in the bark. Bark which is not easily detached becomes cracked by the con- 

 tinued growth in thickness of the stem, and has then the furrowed appearance so 

 characteristic of the majority of old tree-trunks. 



The usual brown or red colour of bark, as in similarly coloured heart-wood, 

 is occasioned by the presence of tannins, to the preservative qualities of which 

 is due the great resistance of bark to the action of destructive agencies. The 



