ENDOGENOUS STIIUCTURE. bl 



209 That portion of the stem Avhich serves this important 

 purpose; is the alburnum (203). Tlu-ough its ducts and filn'es 

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

 communicate./ Plaving 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 medullaiy 

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

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

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

 purpose of maintaining the conditions requisite for endosmosej 

 (159, a). 



§ 3. OF THE ENDOGENOUS STRUCTURE. 



210. In W\Q endogenous stem there is no distinction of pith, 

 wood, and bark, nor does a cross-section exliibit 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 fnst 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 their outer side, as in the exogen.J 



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, wlierc the cel- 

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

 tiiey tiu-n outwards, and interlace themSehcs with those which were pre\iously 

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

 become exceedingly dense and hard, even so as to resist the stroke of the a>:e._^ 



b. The age of most eAjiogcnous trees, as the palmSiAvould seem to be limited 

 by this peculiarity of gi'Swth. Tiie 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, arc often hollow, with solid 

 joints ; as in the grasses and bamboo. 



V 



