STEM. 15 



83. The epidermis, the stratum suberosum, the stratum 

 parenchymatosum, and, lastly, the stratum librosum. 



84. The epidermis is the external layer of the bark, 

 and is formed of a simple layer of small cellules having 

 pretty thick walls, and covered with stellate hairs. It 

 is continuous with the cuticle of the leaves, and is not 

 found on the old branches or trunk. 



85. The stratum suberosum, or corky layer, lies be- 

 neath the epidermis, and is composed of from three to 

 five layers of colourless cellules with thin walls, and de- 

 void of granules. In some plants this layer scarcely ever 

 exists perfectly ; in others, it becomes highly developed, 

 as in the Cork Oak. In our own country, the Plane 

 tree approaches nearest the Cork in an evident develop- 

 ment of the stratum suberosum. This layer of the bark 

 does not often attain a very great thickness, and is 

 easily destroyed. 



86. The stratum parenchymatosum forms the third 

 layer from the circumference, and is composed of cel- 

 lular tissue, containing a good deal of green colouring 

 matter, though sometimes having colourless cells filled 

 with crystals. 



87. The stratum librosum, or liber, is the fourth and 

 last layer of the bark. This layer is not always to be 

 separated from the last mentioned one, except in old 

 branches or trunks ; it consists internally of fibrous 

 tissue, and externally of cellular. In consequence of 

 the twining course of the bundles of the fibrous tissue 

 of this layer, there are formed regular interfibrillar 

 spaces, which give to the whole a pretty net-like appear- 

 ance, and the meshes of this net- work are filled with 

 parenchymatous cells which may be looked upon as the 

 medullary rays of the bark. (Meyen, Neues system, SfC.J 



88. Considering an Exogenous stem as regards ele- 

 mentary structure, we may say that its cellular system 

 is seen chiefly in the pith and part of the bark, its vas- 

 cular in the medullary sheath and internal portions of 

 the woody layers, and its fibrous in the external parts 

 of the woody zones and inner portion of the bark, 



c*2 



