EXOGENOUS STEM. 379 



/era or Fir tribe with its allies (Figs. 166, 170-172), in which the 

 woody fibres are of unusually large diameter, and have the pe- 

 culiar "glandular" markings already described (§ 231). In any 

 stem or branch of more than one year's growth, the woody struc- 



Transverse section of young stem of Clematis:— a, pith; &, &, 6, woody bundles; c, c, c, medullary 



rays. 



ture presents a more or less distinct appearance of division into 

 concentric rings, the number of which varies with the age of the 

 tree (Fig. 167). The composition of the several rings, which are 

 the sections of so many cylindrical layers, is uniformly the same, 

 however different their thickness ; the arrangement of the two 

 principal elements, however, — namely the woody fibre and the 

 ducts, — ^varies in different species; the ducts being sometimes 



Fig. 166. 



i Iff 



Portion of transverse section of stem of Cedar :~a, pith ; b, b, wood ; c, bark. 



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 in-egularity (if 

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

 figured appearance to the transverse section (Figs. 167, 168). 

 The general fact, however, is, that the ducts predominate towards 

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



