44 



EXOGENOUS STEM. BA.RK. 



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 called the bast-layer, from its uses. 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 net-work, in the interstices 

 of which the medullary rays are seen. The fibres of the lace-bark 

 tree (Lagetta lintearia) are thus formed. In fig. 

 101, is represented the bark of Daphne Lau- 

 reola;/ indicating the woody fibres of liber, and 

 r the medullary rays. The endophlosum in- 

 creases 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 liber may be derived from the fact of the 

 inner bark being used for writing upon. 



84. The cellular envelope, or mesophlceum, lies 

 immediately on the outside of the liber. It con- 

 sists of polyhedral, often prismatical cells (fig. 

 fWOinjWUl 98 bis, e c), usually having chlorophylle, or green 

 r ttJmMxMi^S colouring matter, in their interior, but some- 

 times being colourless and containing raphides. 

 They are distinguished from those of the epi- 

 phloaum by their form and direction, by their 

 thicker walls, their green colour, and the inter- 

 cellular spaces which occur among them. This covering is usually less 

 developed than the outer suberous layer, but sometimes, as in the 

 Larch and common Fir, it becomes very thick, and separates like the 

 epiphloeum. In the cellular envelope laticiferous vessels occur. 



85. The Epiphloeum is the outer covering of the bark, consisting of 

 cells which usually assume a cubical or flattened tabular form (fig. 98 

 bis, p). The cells have no chlorophylle in their interior, are placed 

 close together, and are elongated in a horizontal direction ; and thus 

 they are distinguished from the cells of mesophlceum. In the progress 

 of growth they become often of a brown colour. This covering may be 

 composed of a single layer of tabular cells; but in some trees it consists 

 of numerous layers, forming the substance called cork, which is Avell seen 

 in Quercus suber, the Cork-oak (fig. 100 p); hence the name suberous, 

 or corky layer, which is given to it. The form of its cells varies in 

 some instances, being cubical at one part, and more compressed or tabu- 

 lar at another, thus giving rise to the appearance of separate layers. 

 After a certain period (sometimes eight or nine years), the corky portion 

 becomes dead, and is thrown off in the form of thickish plates, leaving 



Fig. 101. Network formed by liber of Daphne Laureola. //, Fibrous bundles, r r, Medullary 

 rays. 



