80 THE STEM. 



204. The MEDULLARY RAYS are those fine lines which appear in 

 a cross section of the stern, radiating from the pith to the bark, 

 intersecting all the intervening layers. They consist of thin, 

 firm plates of cellular tissue ; being, like the pith, the remains of 

 that tissue, which at the first constituted the whole of the stem. 



a. These rays are quite conspicuous in vertical sections of the oak, or the 

 maple, where they are sometimes called the silver grain. 



205. The BARK is the external covering of the stem, consisting 

 of several integuments, of which the outer is the epidermis (35), 

 that next within the cellular integument, and the inner the liber. 



206. The structure of the two outer integuments is chiefly 

 cellular, and that of the inner, or liber, is both cellular and 

 woody. The cellular integument is very thick in Quercus 

 suber, and constitutes that useful substance cork. The liber 

 (Lat. the inner bark, hence a book, because it was manufactured 

 into parchment) is usually thin, delicate, and strong, and has 

 been often applied to useful purposes, as in those trees of Poly- 

 nesia from which cloth, mats, and sails are made. 



207. At the end of the spring a portion of the sap, now transformed into a 

 viscid, glutinous matter called cambium, is deposited between the liber and the 

 wood, becomes organized into cells, and forms a new layer upon each. Soon 

 afterwards, the new layers are pervaded by woody tubes and fibres, which com- 

 mence at the leaves and grow downwards. Thus the number of layers formed 

 in the bark and wood will always be equal. 



a. Since the growth of the bark takes place by internal accretions, it follows 

 that the older layers must be carried outwards and continually expanded. Thus, 

 although smooth and entire at first, they at length become shaggy and rough, 

 with longitudinal furrows and ridges, and finally they are cast off, as in the hem- 

 lock, spruce, walnut, &c. Not unfrcquently, however, the older layers are ex- 

 tended in horizontal grains, or fibres, encircling the stem, as in the white birch 

 (Betula papyracea). 



b. The peculiar virtues or qualities of the plant reside in the bark rather than 

 in the wood ; hence this is the part chiefly used for medicine, dyes, tannin, &c. 



c. That vascular system which is peculiar to the bark, serving for the circula- 

 tion of its fluids, is called the laticiferous tissue (34). It exists in the form of a 

 complete network of vessels, through which the sap moves in all directions. 



$ 2. FUNCTIONS OF THE STEM. 



208. We have already stated (156) that the stem serves to 

 convey the sap from the roots to the opposite extremities of the 

 plant. 



