ENDOGENOUS STRUCTURE. 81 



209. That portion of the stem which sen-es 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 liber. Of this descend- 

 ing sap a part is earned inward from the bark by the meduUaiy 

 rays, and thus diffused tln'ough the whole stem ; the remainder 

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

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

 purpose of maintaining the conditions requisite for endosmose 

 (159, a). 



§3. OF THE ENDOGENOUS STRUCTURE. 



210. In the endogenous 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 cellular 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 infier side next the centre of the stem, 

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



a. A new set of these bundles is foi-med annually, or oftener, proceeding from 

 the leaves and passing downwai'ds in the central parts of the stem, where the cel- 

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

 thcv turn outwards, and interlace themselves with those which were previously- 

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

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



b. The age of most endogenous trees, as the palms, would seem to be limited 

 by this peculiarity of growth. The stem at length becomes incapable of further 

 increase in diameter, and the lower portions of it so densely filled with the de- 

 scending fibres as to become impervious to all succeeding ones, and the tree 

 languishes and dies. 



c Endogenous stems, both herbaceous and woody, are often hollow, with solid 

 joints ; as in the grasses and bamboo. 



