EXOGENOUS STEM — BAEK. 



57 



gradually becomes altered by the formation of secondary deposits. 

 The bark consists of a cellular and vascular system. In this respect 

 it resembles the wood, but the position and relative proportion of these 

 two systems is reversed. In the bark the cellular system is external, 

 and is much developed ; while the vascular is internal, and occupies 

 comparatively a small space. The cellular portion of the bark con- 

 sists of an external layer, or Epiphloium (Ivl, upon, on the outside, and 

 pXo/Js, bark), and the cellular envelope, or Mesophlaum {//I'ssog, middle) ; 

 while the vasular system forms the internal portion called Liber, or 

 Endophlceum (ivSov, within). 



The inner bark, or endophlceum (fig. 116 / c), is composed of 

 elongated pleurenchyma niixed with laticiferous vessels and some 

 cellular tissue. It is separated from the wood by the cambium layer. 

 The pleurenchymatous tubes are thickened by concentric deposits in 

 their interior, and thus they acquire a great degree of tenacity. The 

 liber of the Lime tree and of Antiaris saccidora (the sack tree of 

 Ooorg) are used to form mats, cordage, and bags ; 

 and the toughness of the fibres of the inner 

 bark of flax, hemp, and of many of the nettle 

 and mallow tribe, render them fit for various 

 manufacturing purposes. The liber is sometimes, 

 from its uses, called the bast-layer. Occasionally 

 it is continuous and uninterrupted, as in the 

 Vine and Horse-chestnut ; at other times, as in 

 the Oak, Ash, and Lime, the fibres are separated 

 during the progress of growth, and form a sort 

 of network, in the interstices of which the 

 medullary rays are seen. The fibres of the 

 lace-bark tree {Lagetta lintearia) are similar. 

 In figure 119 is represented the bark of Daphne 

 Laureola ; / indicating the woody fibres of 

 liber, and r the medullary rays. The en- 

 dophlceum increases by layers on its inside, 

 which are thin, and may be separated like the 

 leaves of a book, and hence the application of the name liber. The 

 term Viher may be derived from the fact of the inner bark being used 

 for writing upon. 



The cellular envelope, or mesophloeam, lies immediately on the 

 outside' of the liber. It consists of polyhedral, often prismatical cells 

 (fig. 116 e c), usually having chlorophyll, or green colouring matter, 

 in their interior, but sometimes being colourless, and containing 

 raphides. They are distinguished from those of the epiphloeum by 

 their form and direction, by their thicker walls, their green colour, 



Fig. 119. Network formed by liber of Daphne Laureola. //, Fibrous bundles, r r. 

 Medullary rays. 



Pig. 119. 



