. Fossil plants : for students of botany and geology . Paleobotany. 286 PITYEAB [ Fio. 486. Pitys antiqua. Central region of stem, showing primary xylem strands ^s black dots. (Kidston Col- lection, 598 A.) and on the examination of the sections. Fig. 486 represents a diagrammatic sketch of a section (2-7 x 2-7 cm.) of the central region of a stem which had lost most of its secondary wood. In some cases the pith reaches a diameter of 3 cm.; it consists of large parenchymatous cells broader than deep, several of which contain a black substance and are probably secretory elements: the interc
. Fossil plants : for students of botany and geology . Paleobotany. 286 PITYEAB [ Fio. 486. Pitys antiqua. Central region of stem, showing primary xylem strands ^s black dots. (Kidston Col- lection, 598 A.) and on the examination of the sections. Fig. 486 represents a diagrammatic sketch of a section (2-7 x 2-7 cm.) of the central region of a stem which had lost most of its secondary wood. In some cases the pith reaches a diameter of 3 cm.; it consists of large parenchymatous cells broader than deep, several of which contain a black substance and are probably secretory elements: the intercellular spaces are fairly large. Horizontally extended gaps, due to shrinkage and collapse, occur in the pith and give to it an appearance not unlike that of Cordaites. Numerous small elliptical strands of primary xylem,0-15—0-3mm. in diameter, are scattered in the peri- pheral region (fig. 486) at varying distances from the inner edge of the secondary wood and occasionally, when about to bend outwards as a leaf-trace, a primary xylem- strand abuts directly on the secondary xylem. The protoxylem occupies a more or less central position in each mesarch strand (fig. 487, B, fx): the centrifugal tracheids have the Araucarian pitting while the centripetal elements are spiral. A few parenchy- matous cells are associated with the more central tracheids. A leaf-trace about to bend outwards into the zone of secondary wood is double, but at a slightly higher level in its course it assumes the form of a single strand. The foliar bundles are thus single and not true double traces. Scott recognised, at the extreme edge of the pith, an association with outgoing leaf-traces of a single reparatory strand deeper in the medullary tissue. In this species there is no evidence that an outgoing trace is accompanied by an arc of secondary xylem, but that such was the case is rendered highly probable by the discovery of an arc of wood added by the cambium to a leaf-trace in Pitys Withami^. The radi
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishercambr, bookyear1898
