BARK. 167 



by the cambium, always represents only a relatively thin layer of the tissues produced 

 from the cambium. After the , primary cortex has been removed by the processes 

 mentioned, the older layers of the secondary cortex gradually take part in the 

 formation of bark ; and as the inner older wood is transformed into heart wood, and 

 •the original pith and the primary vascular bundles surrounding it have likewise long 

 ceased to take part in the vital processes, the organ concerned (an older shoot-axis 

 or root) now consists only of such living masses of tissue {secondary cortex and 

 alburnum) as owe their existence entirely to the activity of the cambium. The 

 varying hardness and other material peculiarities of the bark (and especially the 

 way in which it gradually becomes split up by means of continually deeper longitu- 

 dinal cracks, often connected in a net-like manner) depend on the properties of 

 those masses of cortical tissue which provide the material for the development of 

 bark. If large quantities of elastic fibres are formed in it, the bark also exhibits a 

 fibrous fracture ; if, as very often happens, numerous stone-cells {sclerohlasts) are 

 present in the parenchymatous cortical tissue, these are found again in the bark- 

 scales. The latter will also be rich in calcium oxalate when this has been 

 accumulated previously in large masses in the secondary cortical tissue, as is 

 commonly the case. 



As in the epidermis the stomata exist as pa,ssages of communication between the 

 atmosphere and the air contained in the intercellular spaces ; so also in the periderm, 

 and later in the bark, peculiar organs occur by means of which, it is assumed, 

 a certain communication between the atmosphere and the interior of the cortical 

 tissue is established. These organs are the Lenticels. They arise even in the 

 first period of vegetation of lignifjang shoot-axes, and project from the smooth 

 periderm of annual and perennial shoots usually as pale coloured roundish warts. 

 With the increase in circumference of the organ they become widened, and at length 

 appear as transversely elongated masses of tissue, which swell up in moist weather 

 like cushions, their number increasing with the advancing age of the branch or 

 stem. The lenticels may be considered as peculiar localised growths of the peri- 

 derm. Where the periderm is developed in the epidermis or close beneath it, the 

 lenticels arise before it or simultaneously with it beneath the not very numerous 

 stomata of the shoot-axis. In the tissue lying under a stoma, a phellogen, convex 

 inwards, is formed; and from this, serially arranged cork-tissue is developed 

 externally, and phelloderm internally. This phellogen is immediately continuous 

 with the rest of the periderm-forming tissue of the shoot-axis ; only its activity in 

 developing cells is more vigorous, especially towards the outside. Moreover the cells 

 originating from the phellogen externally are distinguished from those of the ordinary 

 periderm, in that they possess intercellular spaces, and that the external mass of the 

 lenticel forms a very loose powdery tissue, broken up into its individual cells. This 

 maybe termed lenlicel-tissue, and is plainly shown in Fig. 172. The cells of this 

 lenticel-tissue remain thin-walled and for a long time living, and, on contact with water, 

 capable of swelling and even of growth. At the conckision of the period of vegetation 

 a dense layer of cork, not traversed by intercellular spaces, is formed in the lenticel; 



• What is here said concerning the Lenticels is taken from De Bary's 'Vergl. Anal, der 

 Vegetations-organe,' § 179. 



