. Fossil plants : for students of botany and geology . Paleobotany. xv] LEPIDODENDRON 117 The structure of the cortex of a shoot in which secondary growth, both in the stele and in the outer cortex, has pro- gressed further than in the specimen shown in fig. 148 is represented in fig. 151. The section (fig. 151, A) measures 7 x 3-8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood (fig. 151, E, x and x% The almost smooth surface of the primary wood (fig. 151, E, x) is succeeded by the secondary xylem, a?, characterised at its inner edge by the tapere
. Fossil plants : for students of botany and geology . Paleobotany. xv] LEPIDODENDRON 117 The structure of the cortex of a shoot in which secondary growth, both in the stele and in the outer cortex, has pro- gressed further than in the specimen shown in fig. 148 is represented in fig. 151. The section (fig. 151, A) measures 7 x 3-8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood (fig. 151, E, x and x% The almost smooth surface of the primary wood (fig. 151, E, x) is succeeded by the secondary xylem, a?, characterised at its inner edge by the tapered ends of the radial rows of scalariform tracheids between. Fio. 151. Lepidodendron vasculare. An older stem than that shown in fig. 148. (From a section in the Manchester Museum. No. 351.) which occur several delicate parenchymatous cells (fig. 151, E, a). The occurrence of such isodiametric elements, often exhibiting a delicate spiral thickening band, is a characteristic feature of the boundary .between primary and secondary wood in lepido- dendroid stems. The secondary wood is penetrated by numerous medullary rays and in some of them are seen strands of narrow spirally thickened tracheae—the leaf-traces—which are in organic continuity with the exarch protoxylem of the priraarj^ wood. The leaf-traces are oval and mesarch. The space, c^, (fig. 151, A) originally occupied by the delicate middle cortex, is succeeded by a shell of outer cortex composed chiefly of. 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 Seward, A. C. (Albert Charles), 1863-1941. Cambridge : University Press
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