BARK 



141 



the original cork-cambium is deep-seated, the subsequent cambia 

 are continuous cylinders ; whilst when the first is superficial, the 

 later ones are often merely vertical strips whose margins are in 

 contact with one another. As the bark gets thicker and thicker 

 the outer portions are subjected to a growing tension, so that, 

 being dead tissue, fissures appear at the surface as irregular 

 longitudinal furrows well seen in the Oak and Elm. In many 

 cases, however, the oldest bark is shed, and this takes place 

 either as* rings (ring-bark, e.g. Birch, Cherry) or as scales (scale- 

 bark, e.g. Plane, Scotch Fir), according as the successive cork- 

 cambia are continuous cylinders or separate strips. 1 



After the shedding of the first-formed cork, lenticels may 

 develop from any part of an active phellogen. As a result of 

 subsequent stretching they assume various forms, which are often 

 characteristic of individual species. Thus on the bark of the 

 Birch and Cherry they appear as slightly raised horizontal lines, 

 whilst in the Poplar they are usually lozenge-shaped. The 

 cork-wings developed in many varieties of woody plants (e.g. Cork 

 Elm, Cork Maple, etc.) are due to the copious formation of cork 

 which, consisting as it does of dead cells, necessarily splits at 

 the surface ; the regularity of the fissures may, however, depend 

 in part on excessive local activity of the cork-cambium. 



It will have become apparent that cork serves to protect 

 surfaces which would otherwise be exposed, and indeed it is 

 even found covering wounds and the scars left by the shedding 

 of leaves, flowers, branches, etc. The fall of leaves is preceded 

 by the differentiation of a definite separating layer (abscission 

 layer) which is produced, with or without division, from the cells 

 at the base of the leaf -stalk (Fig. 69, S.). The layer in question 

 is generally recognisable by the smaller size of its cells, and its 

 position is often indicated externally by a slight constriction of 

 the petiole. Subsequently the middle lamellae between the cells 

 of this layer become mucilaginous and break down, so that for 

 a short time the leaf is connected with the stem solely by the 

 epidermis and vascular bundles. Beneath the abscission layer 

 a protective layer of cork, which subsequently becomes con- 

 tinuous with that of the stem, is developed, either before or 

 after the detachment of the leaf. The smooth layer of cork 

 1 Regarding commercial uses of bark, see pp. 48, 60, 62. 



