EXOGENOUS STEMS. 



75 



cellular basis through which the bundles of wood pass, and each is inclosed by a cuticle 

 or bark (endogens are said to have no bark). The cellular system is horizontal, and 

 constitutes the woof of the structure ; whilst the vascular and woody system is longi- 

 tudinal, and corresponds to the warp. 



Exogenous Stems. On examining a section of the stem of an oak, or any other 

 of our forest trees (Fig. 127), we observe the following parts first, the pith, , or its 

 remains, in the centre ; secondly, the B 



bark, rf, on the outside ; thirdly, a mass 

 of wood, b, between the two, broken 

 up into portions by the concentric 

 deposition of its layers, and by a 

 series of lines or rays, c, which pass 

 from the centre to the circumference. 

 Thus there are always pith, bark, 

 wood, and medullary rays (Fig. 127). 



It has already been mentioned that 



Fig. 127 



A, transverse, and B perpendicular section of an exo- 

 genous stein, showing parts of which it is composed. 



a, the central pith ; b, four layers of woody fibre ; c, the 

 cambium in the spring ; d, the bark ; e, the medullary 

 rays. 



each stem has two systems, the cellu- 

 lar and the vascular ; and the parts 



just mentioned must belong to one 



or other of those systems. Thus the 



pith, medullary rays, and bark belong 



to the horizontal or cellular system, and the wood, with its associated ducts, constitutes 



the longitudinal or vascular system. 



This division of stems comprehends nearly every wooded plant of our climate. 

 The Pith occupies the centre of the stem (Fig. 128, a), and remains throughout the 

 period of growth of some trees, as of the elder 

 (Sambucus nigra), or is absorbed after a few years, 

 as in the oak and almost all large trees. In the 

 latter class of plants there are some remains of 

 the pith for many years after the process of ab- 

 sorption has commenced ; but at length no vestige 

 can be detected, and its position is known only 

 by the central spot around 

 which the wood is placed 

 in circles. It is, however, 

 at this period found in 

 young shoots just as it 

 was at the earliest mo- 

 ment of the formation of 

 the plant (Fig. 129). 

 When it exists, it passes 



Fig. 128.- -A scheme of the parts of an 

 exogenous stem. 



a, the pith ; ft, the bark ; c, medullary 

 rays uniting the pith and the bark 

 (greatly exaggerated) ; d, woody 

 fi 



bre. 



uninterruptedly from the ri ?- 129.-*ection of young 



*_ shoot of the MAPLE TREK 



root to tne end ot each 



branch and leaf-bud ; but is sometimes thickened, and rendered 

 more dense, as in the ash, at the nodes the place, indeed, 

 where all the structures are somewhat compressed. 



Its structure is at all times cellular ; and, for the most 

 part, the cells are hexagonal in form, as shown in Fig. 11. The cells are commonly 



(Acer campestre), show- 

 ing the large size of the 

 pith, a; the bundles of 

 wood of one year's growth, 

 and the bark with its 

 hairs. 



