ORGANS OF XUTEITION. 



69 



or die yearly, except that the wood in such plants is generally 

 firmer and in larger proportion. As growth procee(fi in the 



Fig. 161. 



Firi. 161. Horizontal section from the centre to the circumference of the 

 stem of the Maple, three years old. m. Pith. t. Spiral vessels, v. Pitted 

 vessels. /. Fibres or wood-cells, c. Cambium layer, s. Epiphloeum ; 

 ■within which may be ot)served three cortical layers, marked lpl,pl, 

 p I, corresponding to the tliree years' growth, m c. Jsewly forming 

 bark. The figures 1, 2, 3, refer to the three successive years' growth 

 of the wood. 



second year, a new zone of wood is formed on the outside of the 

 one of the previous year (fg. 161, 2), while at the same time 

 a new fibrous layer is added to the inside of the bark, I. These 

 layers are developed out of the vitally active cells of the cam- 

 bium layer c, already alluded to as being situated on the out- 

 side of the indefinite vascular bundles which form the wood of 

 Exogenous plants. The medullary rays {Jig. 163, A, i), at the 

 same time increase by addition to their outside, and thus con- 

 tinue to keep up the connexion between the pith and the bark. 

 In succeeding years we have in like manner new layers of wood 

 and fibrous bark, one of each for every year's growth (fig. 161), 

 while the medullary rays also continue to grow from within 

 outwards. Each succeeding year's growth is therefore essentially 

 a repetition of that of the first year, except as regards the pith, 

 which does not increase in size after the first year, so that in old 

 stems we have no more distinct regions than in those of the 

 first year. We have therefore in all exogenous stems but four 

 separate parts, namely, pith, wood, medullary rays, and bark ; 

 which we shall now describe in the order in which they are placed. 

 1. Pith or Medulla. {Figs, 161, m, and 163, a,a.) — This consists 

 essentially of ordinary parenchyma, and it forms a more or less 

 cylindrical or angular column which is situated commonly at, or 

 towards the centre of the stem. Under ordinary circumstances 

 the pith is not continued into the root, but it is always in con- 

 nexion directly with the terminal bud of the stem, and also at 



