. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. APICAL MERISTEMS 651 irregular and intermediate conditions have, been found between the Marattiaceous type with four prismatic initials and that of the Lepto- sporangiate Ferns, where there is only one (Fig. 351). The axis has usually a single conical initial, but it is doubtful whether this is always so, and irregularities certainly occur (Fig. 352). The leaf of the Osmundaceae is alone among Ferns in showing the more complicated three-sided initial in place of the two-sided common for the Lepto- spo
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. APICAL MERISTEMS 651 irregular and intermediate conditions have, been found between the Marattiaceous type with four prismatic initials and that of the Lepto- sporangiate Ferns, where there is only one (Fig. 351). The axis has usually a single conical initial, but it is doubtful whether this is always so, and irregularities certainly occur (Fig. 352). The leaf of the Osmundaceae is alone among Ferns in showing the more complicated three-sided initial in place of the two-sided common for the Lepto- sporangiates, while the wings of the leaf have a segmentation of the Marattiaceous rather than of the Leptosporangiate type (Fig. 353). In. Fig. 353. Sections transversely through young wings of the leaves of various Ferns. A=Angi- opteris evecta, showing that there is no single marginal cell, but a small-celled meristem at margin of the wing. B=a similar section of Todea barbara, with like structure. C=similar section of Scolopendrium vuteare, showing regular segmentation of a marginal cell (fn) by alternating cleavages. Z>=sunilar section of Trichomanes radicttns, showing marginal cell (wz) with transverse segmentation. X 2t6. point of fact, all the meristems of the Osmundaceae show nearer relation to those of the Marattiaceae than do any other Leptosporangiate Ferns that have been examined. These facts are in obvious accord with the segmentation of the sporangia {see above, p. 637): it appears, therefore, that in stem, leaf, root, and sporangium those types of Ferns which are held as more primitive commonly show a more bulky initial construction, while the true Lepto- sporangiates, which are held as derivatives, show in all their parts a less bulky type of segmentation. It has. been seen above that the spore- output per sporangium follows a similar sequence, and indeed the numbers might be taken as a rough indication of the complexity of organisation â for the plant at larg
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