ORGANS OF NUTRITION. 87 



less in a radial direction, and form by their union a compact 

 tissue, commonly 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 sometimes 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 {Quercus Suber) (Jig. 173, s),this layer 

 becomes excessively developed and forms the substance called 

 cork, and hence the name corky or 

 suberous layer which is frequently ap- 

 plied to it. Large developments of cork 

 also occur on some other trees, as 

 various species of Elm ( Ulmus alata, 

 racemosa, &c.). It commonly hap- 

 pens that the cells of which the epi- 

 phloeum 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 alter- 

 nate with them, so that the whole 

 layer appears to be subdivided into 

 several secondary layers. In the Birch, 

 again, this distinction into layers is 



remarkably evident (^$r. 177)." Here /'tv?. 177. Transverse section 

 a number of layers of dark- coloured of a portion of Birch- 

 firmly compacted tabular cells, a, compa^^Yab^'iar' ceils: 

 maybe seen alternating with others 6.&. Layers of loose thin - 



r ^ . ■ J i' i.'i waUed cells alternating 



of a loose nature and ot a white co- with the former, 

 lour, 6. 



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

 direction to that of the wood, for while the latter increases by 

 additions to its outer surface, the former increases by additions 

 to its inner. The bark is therefore endogenous in its growth. 

 Each layer also grows separately ; thus the liber by the ad- 

 dition of new matter from the cambium-layer on its inside ; 

 the mesophloeum by the addition of cells next the liber ; and 

 the epiphlcBum by cells next to the mesophloeum. The two 

 outer layers, constituting the cellular system of the bark, rarely 

 continue 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 from the cambium-layer. 

 In some trees these layers may be readily observed, at least up 

 to a certain period, as in the oak (Jig. 163). They are com- 

 monly so thin when separated that they appear like the leaves 

 of a book, and hence the supposed origin of the term liber 

 G 4 



