88 ORGANOGUAPHY. 



applied to the inner bark. The name hber is, however, some- 

 times considered to be derived from the inner bark of trees 

 having been formerly used for writing upon. This distinc- 

 tion of the liber into layers is generally soon lost, in conse- 

 quence 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, from the distension to which 

 they are exposed from the growth of the wood beneath, generally 

 become cracked in various directions and assume a rugged ap- 

 pearance, as in the Elm and Cork-oak, In some trees, as the 

 Beech (Fagus sylvaticd), the bark, however, always retains 

 its smoothness, which circumstance arises, partly from the 

 small development of the cellular layers, and partly from their 

 great distcnsibility. Other smooth- barked stems, such as those 

 of the Holly, Ivy, &c., 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 thick plates of various sizes. In the Birch 

 and the Clierry, &c., the epiphloeum separates in thin transverse 

 plates, which have a silvery appearance. In some plants, such 

 as the Vine, Honeysuckle, &c., each layer of liber as it is de- 

 veloi)ed throws oif that of the preceding year, so that the 

 bark always presents a fibrous character, and a similar ap- 

 pearance is produced in some otiicr trees* when they have 

 arrived at a certain age. In other trees, as the Plane, a layer 

 of delicate cells, analogous to tlie epiphloeum, is formed be- 

 neath each layer of liber, so that the two layers alternate 

 with each other; these soon dry up in the outer layers, and 

 the liber thus separates in plates year by year. In many 

 Coniferous Plants, such as the Larch and common Pine, the 

 cellular envelope becomes extensively develoi)ed, by which a 

 kind of false cork is ])roduced, which soon comes ott" in scales. 

 The epidermis in all cases separates early from the epiphloeum, 

 by wliich it is replaced. 



By this exfoliation 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 ))criod. The new layers of wood, the 

 canihiuni-layer, and the recently formed lil)er, arc the ])arts of 

 an exogenous stcni which are alone concerned in its active de- 

 vclof)ment and life. 



J laving now described the different parts which enter into 

 the structure of an exogenous or dicotyledonous stem, we will 

 in conclusion recapitulate them, and place them in a tabular 

 form : — 



