142 DERMAL SYSTEM 



Pomaceae, in the genus Salix, in certain species of Viburnum, in 

 Nerium Oleander, in Staphylea pinnata and in certain other cases (Fig. 

 42 a, b). Usually, however, it is the outermost cortical layer or hypo- 

 dermis that gives rise to the superficial phellogen; this case is exemplified 

 bv most of our native trees and shrubs. 



In many woody plants the phellogen arises at a greater distance 

 from the surface, either in a deep-seated layer of cortical parenchyma 

 or as in most Dicotyledonous and Gymnospermous roots in a layer 

 belonging to the vascular cylinder. In the case of roots it is usually 

 the pericycle (pericambium) which provides the primary mother-cells of 

 the phellogen. It is evident that when a phellogen of this deep-seated 

 type begins to form cork, all the tissues outside it must soon be 

 entirely cut off from their water supply, and hence doomed to desicca- 

 tion, unless, indeed, they are served by special water-paths (as in the 

 case of Salicornia). The dried-up cortical tissues, together with the 

 corky layers, constitute the compound structure known as bark. 



The phellogen may sooner or later cease to divide, and itself become 

 converted into cork. This condition is characteristic of organs with a 

 limited growth in thickness, such as bud-scales and fruits (e.g. the 

 Apple). In certain other cases the primary phellogen becomes 

 inactive, but is replaced by a secondary phellogen of more profound 

 origin ; this process may be repeated again and again, so that a succes- 

 sion of periderms is formed, each more deep-seated than the last, and 

 each, as it were, condemning a fresh slice of the cortex to speedy 

 desiccation and death. 



IV. BARK. 



Bark may be regarded as a third and final stage in the develop- 

 ment of the dermal system ; it merits separate consideration not so 

 much on account of its physiological functions which do not differ 

 essentially from those of ordinary cork as because of its anatomical 

 complexity. A typical bark, namely, comprises, in addition to corky 

 layers, also a variety of tissues which originally belonged to other 

 systems. Superficial dead and dried-up tissues may indeed be utilised 

 by the plant for purposes which are not connected with protection. 

 That, on the other hand, any dried-up tissue even if it originally 

 formed part of a mass of succulent parenchyma can restrict transpira- 

 tion most effectively, may be demonstrated in the simplest way by 

 noting the rapid fall in the daily rate of transpiration exhibited by an 

 apple from which a slice has been removed. Masses of dried-up cells 

 also provide a fairly satisfactory means of protection against mechanical 

 injury. 



In those woody plants which at first produce a superficial peri- 



