. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. EXTERNAL CHARACTERS 627 Osmundaceae and Matonineae, and occasionally in other genera, for instance in Cyatkea and in Pteris. On the other hand, axillary branches are found with a high degree of constancy in the Botryopterideae and Hymeno- phyllaceae. In other Ferns buds are found in varying relation to the leaf-bases, and at various other points upon the leaves: all these appear to be different in their nature and origin from the terminal, dichotomous branches above mentioned. The architecture of the
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. EXTERNAL CHARACTERS 627 Osmundaceae and Matonineae, and occasionally in other genera, for instance in Cyatkea and in Pteris. On the other hand, axillary branches are found with a high degree of constancy in the Botryopterideae and Hymeno- phyllaceae. In other Ferns buds are found in varying relation to the leaf-bases, and at various other points upon the leaves: all these appear to be different in their nature and origin from the terminal, dichotomous branches above mentioned. The architecture of the leaves of Ferns, with their complex and variable out- lines, presents features which are important for comparison. For long the developmental interest centred in the apical segmentation, as exhibited in the Leptosporangiate Ferns with their single initial cell. It was not till 1874 that Sadebeck extended that in- terest to the marginal growth of the ultimate pinnules, and showed in the case of Asplen- ium Shepherdi that the last branchings of the veins are true dichotomies (Fig. 347). The same was shown later by Prantl in the Hymenophyl- laceae : such dichotomy may be held to be wide-spread in Ferns, and its results are apparent in the external outline of many young leaves; for instance, it cannot be missed in the case of Allosorus crispus, quoted by Goebel (Fig. 348): here the successive pinnules are produced as branches of a dichotomy, and are successively relegated to a lateral position right and left: the whole pinna is thus a sympodial development of a dichotomous system, though when mature it presents an appearance of pinnation. Dichotomous branching is a very obvious feature even in the mature leaves in some of those Ferns which are believed to be relatively primitive. For instance, in the Schizaeaceae, and especially in Schizaea itself, while the same is seen with modifications in the pinnae of Again, in Matonia and Dipteris it is very obvious, though with sy
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