ELEMENTS OT STKUCTURAI. BOTANY. 105 



however, long />«>/' colls are formed iu the iiuu^r part, 

 next tlio wood, which psiit i-< tluToaftor specially dcsig- 

 Datcd tlie lihir. Tho outer ring of all, cuclosing the 

 whole stem, is tho epiilermis or s/.-jh. 



170. It is now to be observed that, year after year, the 

 rings of wood are increased in thickness /'// llic viiiltipli- 

 viition of their outer celh. There is, consequently, 

 rt/ii«i/.s- a layer of soft cells between the wood and the 

 baik. This is known as tho cmitliium luifir, and it 

 is liere that tho whole growth of an exogenous stem 

 takes place. The soit cells on its inner side arc gradu- 

 ally transformed into woody tissue and vessels, whilst 

 those on its outer side become tho bast cells of the 

 liber, and others form the extension of tho medullary 

 rays. 



Bear in mind, then, that tho exogenous stem is char- 

 acterized (1) by tho formation of its wood in rings, (2) 

 by tho presence of the continuous cambium-layer, and 

 (3) by the presence of a true bark. 



171. Let us now consider the structure of an endo- 

 genous stem. Fig. HiO represents a section of one. 



Here, again, tho white portion is 



cellular tissue, whilst tho dark 

 parts are tho fibro-vascular bundles. 

 This stem is at once distinguished 

 from the other by tho isolation of 

 these bundles^. They never co- 

 alesce to form a ring. That por- 

 Fig. 169. tion of each bundle, whicli is nearest 



the centre of tho stem, corresponds to tho wood of the 

 exo^en, whilst the outer portion of each consists of 

 cells which resemble the exogenous bast-cells, but ifin-e 



