16 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



layers forming beneath it, as in Cork, which is the epiphlceum 

 of Quercus Suber. 



124. The Endophloeum or Liber consists of cellular tissue 

 resting on the alburnum, of laticiferous tissue (36), and of 

 pleurenchyma (23). The tubes of the latter are often thick- 

 ened rapidly by a deposit of sedimentary matter ; in which 

 case, sections of the tubes present the appearance of concentric 

 circles ^ 65 66 . Hence arises the toughness of the tubes of 

 pleurenchyma which occur in the liber, and are manufactured 

 into cordage, as in the Lace-bark tree, the Lime-tree, &c. 



125. Occasionally the liber is only formed during the first 

 year's growth; after which it is enclosed in wood, and is 

 eventually found near the pith. This has as yet been observed 

 only in the Menispermaceous order. 



126. The power of renewing themselves by the production 

 of new matter upon their inner surface, is apparently given to 

 the layers of bark in order to compensate for the gradual and 

 incessant distension of the wood beneath them. 



127. As the older parts die, from becoming too small to 

 bear the strain upon them, new parts form, each in its allotted 

 place, and take the station of that which went before it. 



128. The secretions of a plant are often deposited in the 

 bark in preference to any other part. 



129. Hence chemical or medicinal principles are often to be 

 sought in the bark rather than in the wood. 



130. The immediate functions of the bark are to protect the 

 young wood from injury, and to serve as a filter through which 

 the descending elaborated juices of a plant may pass horizon- 

 tally into the stem, or downwards into the root. 



131. It also contains the laticiferous vessels (36), by which 

 the latex is conveyed to all parts of the surface of a plant. 



132. The MEDULLARY RAYS or PLATES consist of compressed 

 parallelograms of cellular tissue (muriform cellular tissue), be- 

 longing to the medullary system. 



133. They connect together the tissue of the trunk, main- 

 taining a communication between the centre and the circum- 

 ference. 



134. They act as braces to the woody and vasiform tissue 

 of the wood. They convey secreted matter horizontally from 

 the bark to the heart-wood, and they generate adventitious 

 leaf-buds. 



