54 



VEGETABLE PHYSIOLOGY. 



[LESSON 19. 



nous plant, after three years' growth ; this is shown in Fig. 89. The 

 yearly growth of the woody fibre is marked by the figures 1, 2, 3, 

 and the same letters apply to like tissues in both figures, A being the 

 transverse and B the longitudinal section. The pith, a, a, consisting 

 of hexagonal cellular tissue, 5, b, >, pitted or dotted vessels, and c, c, 

 c, woody fibres of successive bark ; d, d, spiral vessels of the medullary 

 sheath ; e, e, layer of cambium cells, between wood and bark ; /, /, 

 inner fibrous layer of bark , g, g, cellular envelope, forming middle 

 layer of bark ; A, A, outer corky layer of bark ; i, i, medullary ray 

 which, in the transverse section (A) is seen running without interrup- 

 tion from the pith to the bark ; but in the logitudmal section (B) it 



FIG. 



Exogenous, or Dicotyledonous wood. 



is mutilated, owing to the slight flexure which usually occurs, and 

 which generally prevents us from tracing the ray in an undivided 

 straight line, when the stem is cut longitudinally. 



317. Thus it will be found that the tissues have been produced in 

 the following order : first year the pith, surrounded by spiral vessels, 

 or medullary sheath, outside of which are the pitted vessels and 

 fibrous tissue of the first year's growth. 



318. In the second year, the pitted vessels succeed to the woody 

 fibrous tissue of the first year, and these vessels are followed by the 

 deposition of woody fibre, forming the second annual layer. 



319. The third year commences with the formation of pitted ves- 



