. The elements of botany for beginners and for schools. Botany. SECTION 16.] ANATOMY OF STEMS. 139 Yuccas become trees along the southern borders of the 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 has no clear distinction of pith, bark, and wood, concentrically ar- ranged, no silver grain, no annual layers, no bark that peels off clean from tiie wood. Yet old stems of Y'uccas and the like, that continue to
. The elements of botany for beginners and for schools. Botany. SECTION 16.] ANATOMY OF STEMS. 139 Yuccas become trees along the southern borders of the 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 has no clear distinction of pith, bark, and wood, concentrically ar- ranged, no silver grain, no annual layers, no bark that peels off clean from tiie wood. Yet old stems of Y'uccas 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. 428. Exogenous Stems, those of Dicotyls (37), or of plants comuig from dicotyledonous and also polycotyledouous 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, Tig. 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 ill a cylinder, — in cross section a ring — around a cen- tral cellular part, dividing the cellular core within, the pith, from a cellu- lar bark without. As the wood-bundles increase in number and in size,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Gray, Asa, 1810-1888. New York American Book Co
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