Fig. 471. Diagram of structure of Palm or Yucca. 472. Structure of a Corn stalk, in transverse and longitudinal section. 473. Same of a small Palm stem. The dots on the cross sections represent cut ends of the woody bundles or threads.
426. An Asparagus-shoot and a Corn-stalk for herbs, and a rattan for a woody kind, represent the first kind. To it belong all plants with monocotyledonous embryo ([40]). A Bean-stalk and the stem of any common shrub or tree represent the second; and to it belong all plants with dicotyledonous or polycotyledonous embryo. The first has been called, not very properly, Endogenous, which means inside-growing; the second, properly enough, Exogenous, or outside-growing.
[427.] Endogenous Stems, those of Monocotyls ([40]), attain their greatest size and most characteristic development in Palms and Dragon-trees, therefore chiefly in warm climates, although the Palmetto and some Yuccas become trees along the southern borders of the United States. In 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, concentrically arranged, 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 the woody bundles (Fig. [471]) which below taper out and are lost at the rind.
Fig. 474. Short piece of stem of Flax, magnified, showing the bark, wood, and pith in a cross section.
[428.] Exogenous Stems, 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 Flax-stem, Fig. [474]) as in a Maple-stem of the first year's growth, except that the woody layer is commonly 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 central cellular part, dividing the cellular core within, the pith, from a cellular bark without. As the wood-bundles increase in number and in size, they press upon each other and become wedge-shaped in the cross section; 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 kind is exhibited in the figures, which are essentially diagrams.