. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. S48 FILICALES are finely scalariform, and are scattered round the periphery of the xylem- core, which is itself composed of tracheides, intermixed with parenchyma (Fig. 306). The petiole is traversed by a single strand, which comes off from the protostele with only superficial disturbance of it: in fact the mature plant maintains the simple relation of the protostele and trace which is seen in its seedling (Fig. 307). The foliar strand in the climbing petiole is an almost cylindrical body, with bays o
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. S48 FILICALES are finely scalariform, and are scattered round the periphery of the xylem- core, which is itself composed of tracheides, intermixed with parenchyma (Fig. 306). The petiole is traversed by a single strand, which comes off from the protostele with only superficial disturbance of it: in fact the mature plant maintains the simple relation of the protostele and trace which is seen in its seedling (Fig. 307). The foliar strand in the climbing petiole is an almost cylindrical body, with bays of phloem protruding into the xylem : it is probably a derivative form of the more usual flattened type, contracted in accordance with the climbing habit. In Schizaea the obliquely ascending or erect stock shows a medullated stele: endoderrrfal. Fig. 306. Transverse section of the rhizome of Lygodiuyn dichotomum. X 50. ^A = phloem; x — xylem. (After Boodle.) = endodermis; pockets are often present at the nodes, or an isolated internal endodermis is occasionally seen, but no internal phloem. Internal tracheides occur in the medullary region, sometimes isolated, sometimes almost bridging across the central pith (S. molliucand). It has been clearly demonstrated that as the stele in the seedling expands, no internal phloem appears: these facts favour a theory of amplification of the stele in Schizaea rather than one of reduction. In Aneimia Phyllitidis and most other species, as also in Mohria, the mature stem is dialystelic, having a hollow reticulate vascular cylinder similar, except for the leaf-traces being only a single strand, to that of Nephrodium filix mas: each mesh is a leaf-gap and the leaf-trace is inserted at its base. But in A. mexicana and other creeping species the dialystelic state is replaced by closed vascular ring or solenostele. The simpler type of Lygodium is probably the more primitive, and as. Please note that these images are extracted from scanned page im
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