Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain.

  • Coulter, John Merle, 1851-1928.
Date:
[1910]
Catalogue details

Licence: In copyright

Credit: Morphology of gymnosperms / by John M. Coulter and Charles J. Chamberlain. Source: Wellcome Collection.

    27/494 (page 13)
    solid and concentric. Near the periphery of the included xylem there occur small strands of protoxylem, which determine the mesarch character of the bundle. Thus far, the structure is distinctly fernlike; but in addition there is a narrow zone of secondary wood about the primary xylem cylinder. The elements of this secondary wood are ra- dially arranged, with numerous (multiseriate) bordered pits, and the zone is traversed by pith rays. The large leaf traces are collateral at their junction with the vascular cylinder, but they become concentric before entering the petiole. Although the leaf trace is single in H. Grievii, in other species of the genus it becomes double. Lyginodendron.1—One of the most common stems preserved in the British Coal-measures is Lyginodendron Oldhamium, fully described by Williamson and Scott (18), and possibly including a group of species. The stems are long, slender (3 mm. to 4 cm. in diameter), and branching. The vascular axis is a siphonostele, without internal phloem (fig. 11), as in Osmunda (fig. 4), the cylinder inclosing a large pith. A variable number (5-9) of isolated primary xylem strands occur next to the pith, being distinctly mesarch, with the larger part of the metaxylem centripetal (fig. 12). While most of the tracheids of the metaxylem have bordered pits, those next to the spiral elements are'usually scalariform (87). The main part of the vascular cylinder, however, consists of a broad zone of secondary wood, whose tracheids, with multiseriate bordered pits, occur in regular radial arrangement, and which is traversed by pith rays. Very evident gaps in the vascular cylinder are associated with the leaf traces, so that the siphonostele is phyllosiphonic and is thus definitely related to the siphonostele of Filicales. The leaf traces near the cen- tral cylinder are single and collateral, but soon they fork and gradually become concentric, so that in the petiole (once called Rachiopteris aspera) a pair of concentric bundles occurs. The association of col- lateral bundles in the stem with concentric bundles in the leaves is a distinct Osmunda character, and is met also among the cycads. Occasionally in L. Oldhamium there is a development of vascular bundles with reversed orientation (xylem next to that of the primary bundle) in the pith. Outside the cambium there is also evident a zone of phloem, broken up by the pith rays (fig. 13). 1 The generic name sometimes used is Lyginop/eris Potonie.