CHAPTER VIL 



BARK. 



Bark in Eiionymus and Beech — Superficial periderm — Bark of the 

 Birch — Ring- bark — Scaly bark of Plane and Sycamore — Scaly 

 bark of Scots Pine — Fissured bark — Tow-like peeling of bark — 

 Hardness of bark — Soft bark — Stone-bai'k — Protective functions 

 of bark — Thick bark of light-demanding trees — Thin bark — 

 Injury from sunlight and frost. 



We have seen, from our study of the shoots and twigs 

 (Vohime I), that the tegumentary system gradually under- 

 goes changes which result, in most cases, in the replace- 

 ment of the epidermis by cork, or periderm-layers, and 

 that when these layers, which are impervious to water, 

 develope deep down in the cortical tissues of a branch or 

 stem, all the tissues outside the water-tight layers of cork, 

 die off and constitute Bark. 



There are, moreover, other characters of bark — using 

 the Avord in this wider and looser sense — which are due to 

 the place of origin of the periderm and the mode of its 

 action in cutting out the true bark, as described above. 



In Euonyinus, for instance, and even in the Beech in 

 certain cases of very old stems, the deeper cork-layers are 

 formed irregularly, in spots as it were, in the cortex, 

 resulting in the developtnent of isolated scattered patches 



