. Morphology of gymnosperms. Gymnosperms; Plant morphology. i6 MORPHOLOGY OF GYMNOSPERMS two genera Lyginodendron and Heterangium approach most nearly to the Osmundaceae and Gleicheniaceae respectively; in Megalo- xylon, on the other hand, the structure of the primary xylem affords evidence that the Lygodium type of stem was also represented in the cycad-fern alliance which played so prominent a part in paleozoic ; Another characteristic feature of Megaloxylon is the structure of the metaxylem, which is usually made up of very short tracheids, often broader than long, intersper
. Morphology of gymnosperms. Gymnosperms; Plant morphology. i6 MORPHOLOGY OF GYMNOSPERMS two genera Lyginodendron and Heterangium approach most nearly to the Osmundaceae and Gleicheniaceae respectively; in Megalo- xylon, on the other hand, the structure of the primary xylem affords evidence that the Lygodium type of stem was also represented in the cycad-fern alliance which played so prominent a part in paleozoic ; Another characteristic feature of Megaloxylon is the structure of the metaxylem, which is usually made up of very short tracheids, often broader than long, interspersed with parenchyma, a structure suggesting a storage rather than a conducting region. This feature is shared by the Permian Za- lesskya, which has been shown to be a primi- tive member of the Osmundaceae (87). Cycadoxylon.— The stems of this genus have been re- ported from the Lower Coal-measures and from the Permian, and it is evident that the tjrpe was an important one during the Paleo- zoic. Some of the preserved vascular cyl- inders are large enough (14 cm. in diameter) to suggest stems as large as those of small trees. The anatomy resembles that of Lyginodendron, in fact one of the species was described as L. robustum. There is the same siphonostele, with abundant second- ary wood traversed by broad pith rays and containing elements of the same character. Two differences, however, are notable. One is the endarch character of the primary vascular strands. This character is not clear in all the forms, and in some a small amount of centripetal xylem is claimed; but in any event, the. Fig. 13.—Lyginodendron Oldhamium: part of transverse section of outer portion of stele; iC, second- ary wood; r, medullary rays; cb, cambium; ph, primary phloem; ph^, secondary phloem; ss, secretory sac; pd, periderm; X52.—After Williamson and Scott (18).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration
Size: 1617px × 1544px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublisherchicagouniversityo
