BARK 143 



derm, each of the successive peridermal layers abuts with its margins 

 against its immediate predecessor, so as to cut oft' a portion of the bark 

 in the form of a scale. It is in this way that the so-called scale-bark 

 originates. But where the first peridermal layer is already deep- 

 seated, the subsequent periderms are more or less concentric therewith: 

 thus a ring-bark composed of a succession of annular zones is produced. 

 The anatomical structure of bark, of course, depends not only on 

 the character of the peridermal layers themselves, but also upon the 

 nature and variety of the tissues cut off by the periderm. Sometimes 

 these tissues consist entirely of cortical parenchyma, but in other cases 

 they may in addition comprise masses of collenchyma, fibrous strands, 

 crystal-sacs and resin-ducts. 



Special mention must further be made of those anatomical arrange- 

 ments which serve to increase the mechanical strength of bark. It is 

 self-evident that any strands of mechanical tissue which originally 

 helped to give strength to a stern or root, must likewise increase the 

 tenacity of the bark in which they become incorporated. A similar 

 result, accompanied by an increase in the average hardness and solidity 

 of the tissues, is produced by the conversion of primarily thin-walled 

 parenchymatous elements into sclerenchyma. This transformation may 

 be confined to individual elements, or may extend to whole groups of 

 cells, and the elements affected may either retain their original form 

 or else undergo a considerable change as regards both shape and size ; 

 an instance of the latter condition is provided by the multi-radiate 

 stone-cells which are largely responsible for the very firm texture of 

 certain kinds of bark. 



It is characteristic of bark that it is forever undergoing exfolia- 

 tion, the outermost scales continually peeling off from the underlying 

 mass. That this process does not merely represent a passive desquama- 

 tion, or, in other words, is not a necessary mechanical consequence of 

 the increased girth of the organ, may be inferred from the fact that the 

 plant produces special absciss-layers which facilitate the separation of 

 the scales. The arrangements resemble those which occur in leaf-bases 

 in connection with the autumnal leaf-fall. The peridermal absciss- 

 tissues consist of special zones, which are interpolated between the 

 layers of ordinary cork-cells ; they agree with the latter as regards the 

 shape of the constituent cells, but are distinguished by the entire 

 absence of suberisation. For this reason they have been termed 

 " absciss-phelloids " by Von Hohnel, who has devoted special attention 

 to the anatomical features and, in some degree also, to the physiological 

 behaviour of these layers. Von Hohnel discriminates between active 

 and passive types of absciss-phelloid. It is characteristic of a passive 

 absciss-phelloid that the cells of the cork are thick-walled and hard, 



