252 



THE ALUMNI JOURNAL, 



the respective structures which develop 

 from them, which we are to find in the 

 older parts- So far as commercial barks 

 are concerned, we may ignore the struc- 

 ture {epidermis) resulting from the der- 

 matogen, as it is but temporary and is 

 torn asunder and destroyed by the pres- 

 sure of the rapidly growing parts which 

 it at first encloses. The same fate, by a 

 somewhat different process, sometimes 

 befalls the structure {cortex) developed 

 from the periblem, while at other times 

 it does not. If it does, say the advocates 

 of the morphological definition, then the 

 tree loses its bark and goes through 

 the remainder of its existence without 

 one and that which is peeled off from it 

 and is used under the name of bark is not 

 bark, because their definition limits the 

 bark to that portion of the integument 

 which is derived from the periblem. This 

 we regard as a reductio ad absurdum, and 

 therefore reject a definition or a construc- 

 tion which involves it. 



To understand the sense in which the 

 term bark is at present understood, and 

 which agrees with our practical defini- 

 tion, we must consider the development 

 and ultimate disposition of the cortex and 

 its resulting structures and also of those 

 of the pleron. It is clear that the origi- 

 nal or primary cortex cannot continue 

 indefinitely to enclose the central struc- 

 tures as they grow larger. If it do so, 

 it must also grow, and for this purpose a 

 cylinder, or a series of plates, of manu- 

 facturing tissues {phellogen) makes its 

 appearance, usually within the limits of 

 the cortex. The phellogen manufactures 

 tissue from both its inner and its outer 

 faces. That upon the inner is cortex, 

 very similar to the primary cortex. This 

 can receive nourishment the same as the 

 other central tissues. That upon the 

 outer face of the phellogen is cork, and 

 being impervious to the nutritive juices 

 from within, must cause by its presence 



the death of all tissue exterior to it. It 

 is this form of construction which we see 

 in the scaling of the barks of the ordin- 

 ary trees and shrubs. If, as occurs in 

 many cases, these scales falls off, it is 

 clear that all that ultimately remains of 

 the "bark" is the portion interior to the 

 phellogen. If now this phellogen does 

 not penetrate more deeply than the inner 

 limit of the cortex, there will always be 

 left as a covering for the tree a certain 

 por1ionofbark.no matter which of the 

 two definitions we accept. But, if, as in 

 the case of an old Calisaya, the phellogen 

 shall penetrate more deeply than this, 

 then the whole of the primary cortex and 

 its resulting structures may be lost to 

 the tree, which shall thenceforward be 

 covered by structures derived from the 

 plerom. Such a tree therefore will pos- 

 sess a bark in accordance with our practi- 

 cal definition, but not in accordance with 

 the morphological one. The confusion 

 and misunderstanding which would arise 

 from such a change in terminology can 

 be only in small part appreciated from 

 the above consideration. Let us see what 

 other tissue besides that of corticle 

 development is included in the bark as 

 we use the term. Just as the cortex re- 

 quires the action of a manufacturing 

 tissue for its growth, so does the struc- 

 ture {stele or central cylinder) developing 

 from the plerom. Such manufacturing 

 tissue, in plants yielding commercial 

 barks, is in the form of an extremely thin 

 cylinder {cambium) originating not far 

 from the outer boundry of the plerom. 

 Like the phellogen, it manufactures 

 tissue from both its inner and outer faces. 

 That upon the inner face becomes wood, 

 that upon the outer bast. If now we 

 attempt to remove the bark from a tree, 

 separation naturally occurs at the cam- 

 bium, and we take away, besides the 

 tissues derived from the periblem, if any 

 remain, that portion derived from the 



