" ENDOGENOUS STRUCTURE. 81 



209. That portion of the stem which senses this important 

 purpose is the alburnum (203). Through its ducts and fibres 

 the sap is elevated to the leaves, with the vessels of which they 

 communicate. Having been there elaborated by exhalation and 

 decomposition into a certain nutritious fluid called latex, it de- 

 scends by the laticiferous tissues of the fiber. Of tlfis descend- 

 ing sap a part is carried inward from the bark by the medullary- 

 rays, and thus diflused through the whole stem ; the remainder 

 descends to the roots, and is in the same manner difllised 

 through their substance, both for thehr nourishment, and for the 

 pui-pose of maintaining the conditions requisite for endosmose 

 (159, a). 



§ .5. OF THE ENDOGENOUS STRUCTURE. 



210. In the endogoious stem there is no distinction of pith, 

 wood, and bark, nor does a cross-section exhibit any concentric 

 arrangement of annual layers. (Fig. 26; 3, 5.) 



211. It is composed of the same tissues and vessels as that 

 of the exogen, that is, of ceUular tissue, woody fibre, spiral ves- 

 sels, and ducts; the fii-st existing equally in all parts of the 

 stem, and the rest imbedded in it in the form of bundles. 



212. Each bundle consists of one or more ducts, with spiral 

 vessels adjoining their inner side next the centre of the stem, 

 and woody fibres on then- outer side, as in the exogen. 



a. A new set of these bundles is formed annually, or oftener, proceeding from 

 the leaves and passing downwards in the central parts of the stem, where the cel- 

 lular tissue is most abundant and soft. After descending awhile in this manner, 

 they turn outwards, and interlace themselves with those which were pre-viously 

 formed. Hence the lower and outer portions of the pahns, and other endogens, 

 become exceedingly dense and hard, even so as to resist the stroke of the axe. 



