82 ' OEGANOGBAPHY. 



{fig. 155, e, e). They are commonly so thin when separated that 

 they appear like the leaves of a book, and hence the supposed" 

 origin of the term liber applied to the inner bark. The name 

 liber is, however, sometimes considered to be derived from the 

 inner bark of trees having been formerly used for writing upon. 

 This distinction of the liber into layers is generally soon lost, 

 in consequence of the pressure to which it is subjected from the 

 growth of the wood beneath, which increases, as we have seen, by 

 additions to its outer surface. 



The outer layers of the bark, after a certain period in their 

 life, which varies somewhat in different plants, generally become 

 cracked in various directions in consequence of the pressure 

 which is exerted upon them by the growth of the wood beneath, 

 and thus assume a rugged appearance, as in the Elm and Cork- 

 oak. In some trees, as the Beech {Fagus sylvatica), the bark, 

 however, always retains its smoothness, which circumstance 

 arises, partly from the small development of the parenchymatous 

 layers, and partly from their great distensibility. Other smooth- 

 barked stems, such as those of the Holly and Ivy, owe their 

 peculiarities in this respect to similar causes. 



"When the bark has thus become rugged, it is commonly 

 thrown off in large pieces, or in plates or layers of various sizes 

 and appearances. The epidermis in all cases separates early 

 from the epiphlceum, by which it is replaced. By this exfolia- 

 tion and peeling off of portions of the bark, its thickness is 

 continually diminished. This decaying and falling away of 

 the old bark does not in any way injure the tree : hence, it is 

 evident that the old layers of the bark, like the inner layers 

 of the wood, have nothing to do with its life and growth after 

 a certain period. The new layers of wood, the cambium-layer, 

 and the recently formed liber, are the parts of an exogenous 

 stem which are alone concerned in its active development and 

 life. 



Having now described the different parts which enter into 

 the structure of an exogenous or dicotyledonous stem, we will, 

 in conclusion, recapitvdate them, and place them in a tabular 

 form : — 



1. JPith or Medulla, belonging to the parenchymatous system. 



2. Medullary Sheath, composed chiefly of 



spiral vessels. 

 S. Wood, composed of interrupted zones, 



one of which is formed annually on 



the outsiHe of the previous zones, and 



consists ordinarily in perennial plants 



of Wood-cells and Pitted Vessels. 

 4. Medullary Bays, consisting of muriform parenchyma, con 



necting the pith and the bark. 



These belong to the 

 fibro - vascular sys- 

 ■ tem, and together 

 form the wood pro- 

 perly so called. 



