ORGANS OF NUTRITION. 81 



botanists who adopt this nomenclature, apply the terra denn to 

 the inner living portion of the bark. 



The epiphlceum consists of one or more layers of tabular or 

 cubical cells, which are generally elongated more or less in a 

 horizontal direction, and form by their union a compact tissue, 

 or one without interspaces. It is this layer which gives to the 

 young bark of trees and shrubs their peculiar hues, which are 

 generally brownish or some colour approaching to this ; or some- 

 times it possesses more vivid tints. It is rarely coloured green, 

 which is the case in Negundo, according to Gray, from its inner 

 cells containing chlorophyll. In some plants, as in the Cork- 

 oak [Qiiercus Suber) {fig. 165, s), this layer becomes excessively 

 developed and forms the substance called cork, and hence the 

 name corky or suherous layer which is 

 frequently applied to it. Large deve- 

 lopments of cork also occur on some 

 other trees, as various species of Ehn 

 ( Ulmus alata, raceniosa, Szc). It com- 

 monly happens that the cells of which 

 the epiphlceum is composed have not 

 all the same appearance and colour. 

 Thus in the Cork-oak some are more 

 tabular or compressed and darker- 

 coloured than others which alternate 

 with them, so that the whole suberous 

 layer appears to be subdivided into 

 several secondary layers. In the Bircli, 

 again, this distinction into layers is 

 remarkably evident (A^. 169). Here j,,.^ ^^g Transverse section 

 a number of lavers of dark-coloured of a portion of Birch-bark, 

 firmly compacted tabular cells, a, a, may 4i^,TaJ'"^is."'C f Sers 

 be seen alternating with others of a of loose thin-waiied cells 

 loose nature and of a white colour, h,b. alternating witlUUe former. 



Growth of the Bark. — The bark developes in an opposite direc- 

 tion to that of the wood, for while the latter increases by ad- 

 ditions to its outer surface, the former increases by additions to 

 its inner. The bark is therefore endogenous in its growth ; while 

 the wood is exogenous. Each la3'er of the bark also grows sepa- 

 rately ; thus the liber by the addition of new matter from the 

 cambium-layer on its inside ; the mesophloeum by the deposition 

 of cells next to the liber ; and the epiphlceum by addition of cells 

 next to the mesophloeum. The two outer layers, which together 

 constitute the parenchymatous system of the bark, rarely con- 

 tinue to grow after a few years, but become dead structures on the 

 surface of the tree. The inner bark, however, continues to grow 

 throughout the life of the individual, by the addition of annual 

 layers on its inner surface. In some trees these layers may be 

 readily observed, at least up to a certain period, as in the oak 

 a 



