. L'origine d'une flore terrestre, une théorie fondée sur les faits de l'alternance. La morphologie des plantes. MARATTIACEAE 52S Il est inutile de décrire en détail ici : notre objet sera plutôt à mettre en relation avec les systèmes moins complexes d'autres fougères, et avec l'autre des fossiles. C'est la plus facile à faire par référence à la jeune plante, et de ces genres qui sont moins compliqués dans leur état de maturité ; il y a beaucoup de variété de complexité dans les différents genres de vie de la famille. Il est constaté que Kaulfussia et Archangiopteris sont relativement simples, tandis que

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. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. MARATTIACEAE 52S It will be unnecessary to describe it in detail here: our object will be rather to bring it into relation with the less complex systems of other Ferns, and with the cognate fossils. This is most readily done by reference to the seedling-structure, and to those genera which are less complicated in their mature state ; for there is considerable variety of complexity in the different living genera of the family. It is found that Kaulfussia and Archangiopteris are relatively simple, while Angiopteris is the most complicated of all, Marattia and Danaca taking a middle position. In the seedlings of them all the axis is traversed by a monostele : in Danaea simplicifolia it has a solid xylem-core, which is maintained till several leaf-traces have been given off from it, naturally without any leaf-gap :Y it then becomes crescentic, and expands into a dictyostele with leaf-gaps, while a central strand or commissure arises from the concavity of the dictyostele, and pursues an upward course with occasional fusions at the successive leaf-gaps.- The same type of structure is closely followed in the mature stem of A/r/iangiapteris, 3 though on a simpler scale ; in fact, this stem still retains at maturity a stage rapidly passed through by the young plants of other more complex genera. A similar vascular system, consisting of a cylindrical dictyostele, with normally a single central strand, is found also in the mature axis of Kaulfussia ; but it is dorsi- ventral, and rather more elongated between the leaf-gaps, in accordance with its creeping habit.4 In Angiopteris and Marattia the final structure is more complex, though the initial steps are similar. There is in their seedlings also a solid protostele : in the central part of its xylem-core certain cell-rows cease to differentiate as tracheides, but give rise to a parenchymatous pith : the siphonostele thus formed becomes

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