STRUCT UEE OF STEMS 703 



fluid ; but it gradually dries up and loses its colour ; and not un- 

 frequently its component cells are torn apart by the rapid growth 

 of their envelope, so that irregular cavities are found in it ; or if 

 the stem should increase with extreme rapidity it becomes hollow, 

 the pith being reduced to fragments, which are found adhering to 

 its interior wall. The pith is immediately surrounded by a delicate 

 membrane, consisting almost entirely of spiral vessels, which is 

 termed the medullary sheath. 



The woody portion of the sten* (fig. 541, b, b) is made up of woody 

 fibres, usually witli the addition of ducts of various kinds ; these, 

 however, are absent in one large group, the Coniferce or fir-tribe 

 with its allies (figs. 545-548), in which the prosenchymatous cells 

 or trache'ids are of unusually large diameter, and are marked by 

 the bordered pits already described. In any stem or branch of more 

 than one year's growth the woody structure presents a more or less 

 distinct appearance of division into concentric rings, the number of 



FIG. 542. Transverse section of stem FIG. 548. Portion of the 



of RJiamnus (buckthorn), showing same more highly 



concentric layers of wood. magnified. 



which varies with the age of the tree (fig. 542). The composition of 

 the several rings, which are the sections of so many cylindrical 

 layers, is uniformly the same, however different their thickness ; but 

 the arrangement of the two principal elements namely, the cellular 

 and the vascular tissue varies in different species, the vessels being 

 sometimes almost uniformly diffused through the whole layer, but in 

 other instances being confined to its inner part ; while in other 

 cases, again, they are dispersed with a certain regular irregularity 

 (if such an expression may be allowed), so as to give a curiously 

 figured appearance to the transverse section (figs. 542, 543). The 

 general fact, however, is that the vessels predominate towards the 

 inner side of the ring (which is the part of it first formed), and that 

 the outer portion of each layer is almost exclusively composed of 

 cellular tissue. Such an arrangement is shown in fig. 541. This 

 alternation of vascular and cellular tissue frequently serves to mark 

 the succession of layers when, as is not uncommon, there is no very 

 distinct line of separation between them. 



