. Fossil plants : for students of botany and geology . Paleobotany. XX] RECENT FERNS 311 to a type of stele consisting of a central core of xylem surrounded by phloem, pericycle, and endodermis. While admitting that steles of this type may sometimes be the result of the modifica- tion of less simple forms, we may confidently regard the protostele as representing the most primitive form of vascular system. The genus Lygodium affords an example of a proto- stelic fern; a solid column of xylem tracheae and parenchyma is completely encircled by a cylinder of phloem succeeded by a multi-layered per
. Fossil plants : for students of botany and geology . Paleobotany. XX] RECENT FERNS 311 to a type of stele consisting of a central core of xylem surrounded by phloem, pericycle, and endodermis. While admitting that steles of this type may sometimes be the result of the modifica- tion of less simple forms, we may confidently regard the protostele as representing the most primitive form of vascular system. The genus Lygodium affords an example of a proto- stelic fern; a solid column of xylem tracheae and parenchyma is completely encircled by a cylinder of phloem succeeded by a multi-layered pericycle and an endodermis of a single layer of cells. In this genus the stele is characterised by marginal groups of protoxylem ; it is exarch. An almost identical type is. Fig. 238. Stele of Trichomanes scandens : px, protoxylem ; s, endodermis. From Tansley, after Boodle. represented by species of Gleichenia, but here the stele is mesarch, the protoxylem being slightly internal (fig. 237, C). Trichomanes scandens (fig. 238) has an exarch protostele like that of Lygodium; but, as Boodle' has suggested, the protostelic form in this case is probably the result of modification of a collateral form of stele such as occurs in Trichomanes reniforme (fig. 237, E). A second type of stele has been described in species oi Lindsaya'' in which the xylem includes a small group of phloem near the dorsal surface. This Lindsay a type is often passed through in the development of " seedling " ferns and may 1 Boodle (00). - Tansley and Lulham (02).. 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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