SKCTION IG.] 



ANA'l'OMY OF STEMS. 



139 



Yuccas become trees along tJic southern borders of (lie United States. lu 

 such stems the woody bundles are more numerous and crowded toward the 

 circumference, and so the harder wood is outside ; while in an exogenous 

 stem the oldest and hardest wood is toward the centre. An endogenous 

 stem lias no clear distinction of pith, bark, and Avood, concentrically ar- 

 ranged, no silver grain, no annual layers, no bark that peels off clean from 

 the wood. Yet old stems of Yuccas and the like, that continue to increase 

 in diameter, do form a sort of layers and a kind of scaly bark when old. 

 Yuccas show well the curving of tiie woody bundles (Fig. 471) which 

 below taper out and are lost at the rind. 



428. Exogenous Stems, those of Dicotyls (37), or of 

 from dicotyledonous and also polycotyledonous embryos, have 

 a structure which is familiar in the wood of our ordinary 

 trees and shrubs. It is the same in an herbaceous shoot 

 (such as a Flax-stem, Fig. 474) as in a Maple-stem of the 

 first year's growth, except that the woody layer is com- 

 monly thinner or perhaps reduced to a circle of bundles. 

 It was so in ihe tree-stem at the beginning. The wood aU 

 forms in a cyliudei-, — in cross section a ring — around a cen- 

 (ral cellular part, dividing the ceUidar core within, the pith, from a cellu- 

 lar bark without. As the wood-bundles increase in number and in size. 



laiits comm? 



tliey press upon each other and become wedge-shaped in the cross sec- 

 tion ; and they continue to grow from the outside, next the bark, so that 

 they become very thin wedges or plates. Between the plates or wedges 

 are very thin plates (in cross section lines) of much compressed cellular 

 tissue, which connect the pith with the bnrk. The plan of a one-year-old 

 woody stem of this kind is exhibited in the figures, which are essentially 

 diagrams. 



429. "When such a stem grows on from year to year, it adds annually a 



Fig. 47t. Slmrt pierc of stem of Flax, ni.igiiifieil, showing tlie bark, wood, and 

 pitli in a cross section. 



Fig. 47.^. Diagram of a cross section of a very young exogenous .stem, showing 

 MX woody bundles or wedges. 47(3. Same later, witii wedges increased to twelve. 

 477. Still later, the wedges filling the sjiace, separated only by the thin lines, or 

 medullary rays, running from pith to bark. 



