Coal; its history and uses . les which go to the leaves. Aboutthe same time the outer cellular investment becomesdifferentiated into the inner and outer layers of thebark, to which a third and intermediate layer is soonadded. When this stage is completed the stem may beheld to correspond with that of Lepidodendron Harcourtii. An exogenous growth of vessels now takes place in acambium-layer which surrounds the vascular meduUarycylinder. The new vessels are added in radiating laminse,and separated from each other by medullary rays. Wethus reach the stage corresponding to Lepidodendronselaginoide
Coal; its history and uses . les which go to the leaves. Aboutthe same time the outer cellular investment becomesdifferentiated into the inner and outer layers of thebark, to which a third and intermediate layer is soonadded. When this stage is completed the stem may beheld to correspond with that of Lepidodendron Harcourtii. An exogenous growth of vessels now takes place in acambium-layer which surrounds the vascular meduUarycylinder. The new vessels are added in radiating laminse,and separated from each other by medullary rays. Wethus reach the stage corresponding to Lepidodendronselaginoides, and by continued development the yet morecomplex stems are produced. It is the exogenous COAL PLANTS. 95 cylinder which especially distinguishes the adult states ofthe more highly organised species—first, from youngtwigs, secondly, from such forms as Lepidodendron Har-courtii, and lastly from the recent Lycopods. It will be observed that the exogenous zone is not indirect communication with the leaves, though continuous. Fie. 8.—Diagrammatic vertical section of Lepidodendron. a, pith; J,medullary vascular cylinder ; c, exogenous woody zone; A, i, k, layersof bark; I, leaf-stalks and leaves; m, vascular bundles of leaves; a,vascular bundles of roots ; o, rootlet. From Williamson ( Phil. Trans,vol. clxii. pi. ii. 1872). with the vascular bundles of the roots through the exo-genous cylinder shortly to be described in Stigmaria. Onthe other hand, the vascular medullary cylinder of the 96 COAL. CHAP. III. stem is in close relation with the leaves by means of thevascular bundles of which it is made up, but it does notextend into the roots, nor has it any continuity with theexogenous zone. Pood-material taken up by the rootswould therefore ascend freely into the exogenous zone ofthe stem, and there find itself cut off from the leaves;similarly food-material formed in the leaves would descendinto the vascular medullary cylinder, but be unable to passinto the exogenous zone or th
Size: 1295px × 1929px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlo, booksubjectcoal
