SECTION 16.] 



ANATOMY OF STEMS. 



139 



Yuccas become trees along the southern borders of the United States. Ik 

 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 has no clear distinction of pith, bark, and wood, coucentrieally 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 aud a kind of scaly bark when old 

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

 below taper out and are lost at the rind. 



428. Exogenous Steins, those of Dicotyls (37), or of plants coming 

 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 Max-stem, Pig. 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 the tree-stem at the beginning. The wood all 

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

 tral cellular part, dividing the oeUular core within, the pith, from a cellu- 

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



they 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 bark. The plan of a one-year-old 

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

 diagrams. 

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



Fig. 474. Short piece of stem of Flax, magnified, showing the bark, wood, and 

 pith in a cross section. 



Fio. 475. Diagram of a cross section of a very young exogenous stem, showing 

 six woody trandles or wedges. 476. Same later, with wedges increased to twelve. 

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

 mediJlary rays, running from pith to bark. 



