. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 584 FILICALES attached to that of the leaf-trace. The stele varies considerably in its construction in different species: in Hymenophyllum there is less variation than in Trichomanes. In all cases the stele lies centrally, and is delimited by the endodermis, followed by a broad pericycle, while the phloem surrounds the centrally-lying xylem. In species of Hymeno- phyllum with large rhizomes, such as H scabrum or dilatatum, the metaxylem forms a ring enclosing the protoxylem together with some parenchy
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 584 FILICALES attached to that of the leaf-trace. The stele varies considerably in its construction in different species: in Hymenophyllum there is less variation than in Trichomanes. In all cases the stele lies centrally, and is delimited by the endodermis, followed by a broad pericycle, while the phloem surrounds the centrally-lying xylem. In species of Hymeno- phyllum with large rhizomes, such as H scabrum or dilatatum, the metaxylem forms a ring enclosing the protoxylem together with some parenchyma: the metaxylem often has the form of two bands, their definition having relation to the origin of the strands which pass to the roots (Fig. 328; Boodle, Fig. 10). In species with small rhizomes the metaxylem. Fig. 328. Transverse section of a node of Hy?nenophyllu7H dilatatum v. Forsterianum. Stele of rhizome to the right, leaf-trace to the left. //£=ph!oem; ^jr=protoxylem ; /=lower xylem-bands ; 7*=upper xylem-band. X200. (After Boodle.) forms a small band or mass, and the protoxylem is peripheral to it on the lower side: these two types are bridged by transitional forms. In Trichomanes the structure of the rhizome of T. reniforme corresponds in all essentials to that of H. scabrum, but with a considerable mass of parenchyma accompanying the central protoxylem. In others the parenchyma is scanty (T. radicals), while in T. trichoideum the stele is sub-collateral: these types find their analogues in Hymenophyllum. But other species of Trichomanes diverge along lines of their own : thus the stele is collateral in T. muscoides, with the xylem downwards : others (T. labiaturn and Motleyi) may have only a single tracheid, or none, and no phloem : these are plainly reduced types. Others again may show a solid mass of xylem, with scattered indefinite protoxylem (T. spicatum), or with the protoxylem peripheral. Please note that these images are extracted from scanned page images th
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