Coal; its history and uses . from the medullary axisto the leaves. They consist of small collections of vesselssurroiinded by cellular tissue derived from the innerbark. Apertures (the lenticular spaces ) are left in theexogenous or radiating woody cylinder and the bast-layerfor their passage. The lenticular spaces, like the leaves,are disposed quincuncially, and the quincuncial bosseswhich mark their position are characteristic features of the CHAP. Til. COAL PLANTS. 97 internal casts of the bast-layer (formerly named Knorria,on the supposition that they were independent stems). The differenc
Coal; its history and uses . from the medullary axisto the leaves. They consist of small collections of vesselssurroiinded by cellular tissue derived from the innerbark. Apertures (the lenticular spaces ) are left in theexogenous or radiating woody cylinder and the bast-layerfor their passage. The lenticular spaces, like the leaves,are disposed quincuncially, and the quincuncial bosseswhich mark their position are characteristic features of the CHAP. Til. COAL PLANTS. 97 internal casts of the bast-layer (formerly named Knorria,on the supposition that they were independent stems). The differences of internal structure between the stemof a Lepidodendroid tree and the root (Stigmaria) requirea. few words of explanation. Stigmaria has the centralpith, the radiating and exogenous laminae of the woodyzone, the medullary rays, and the thick bark of the Lepi-dodendron stem. But the inner vascular cylinder, form-ing part of the medullary axis and sending off vascularbundles to the leaves, is wanting in the root. It is re-. FlG. 9. Restoration of part of Stigmaria, with attached rootlets. FromWilliamson ( Phil. Trani. vol. 162, pt. 1, 1872). placed by vascular bundles derived from the vascularlaminte of the woody zone, which pass outwards in tongue-like processes through meshes in the wood and bark. Thevessels in each bundle gradually diminish in number to-wards the outside of the stem, and but few actually passout into the rootlet. The cellular sheath which investsthe vascular bundle of the rootlet unites by its flask-shaped base with the thick epidermis or hypoderm of themain root. Comparing the Lepidodendron stem just describedwith one of our recent forest-trees we may notice, as H 98 COAL. CHAP. III. points common to both, the central pith ; the woody zone,exogenous, or increasing by external addition, and partedinto wedges by radiating vertical plates of cellular tissue(the medullary rays); the bark divided into layers ; andthe vascular bundles given off to the leaves. P
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Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlo, booksubjectcoal
