. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. MARATTIACEAE 527 The cortex which surrounds the central region in the fossil stems was traversed by large numbers of downward-growing roots, having characteristic Marattiaceous structure : the cortex with its contained roots was often distended to great bulk, while outside it a mass of densely woven roots is sometimes seen. It seems probable that these specimens represent the basal region of arborescent stems, which, though greater in stature than the living forms, were Marattiaceous in their characte
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. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. MARATTIACEAE 527 The cortex which surrounds the central region in the fossil stems was traversed by large numbers of downward-growing roots, having characteristic Marattiaceous structure : the cortex with its contained roots was often distended to great bulk, while outside it a mass of densely woven roots is sometimes seen. It seems probable that these specimens represent the basal region of arborescent stems, which, though greater in stature than the living forms, were Marattiaceous in their characters. Not only is this so as regards general structure, but also in the details of the tissues, into which, however, it is impossible to enter fully here;l the recognition of their affinity with the modern Marattiaceae is thus further confirmed. Embryology. The embryology of the Marattiaceae shows features distinct from that of other Ferns. The prothallus is of the normal flattened Fern-type, though of larger size and more massive construction. The sexual organs are borne on its under side, and are deeply sunk in its tissue, not projecting from its surface, as in the Leptosporangiates. The first segmentation of the zygote is by a basal wall transverse to the axis of the archegonium, whereas in most Ferns it is nearly coincident with it (b, b, Fig. 292 a). The basal wall is followed by segmentation into octants, and the relation of the parts of the embryo to these is essentially similar, to what is seen in other Ferns : the epibasal hemisphere, here turned away from the archegonial neck, forms the cotyledon and the apex of the stem : the hypobasal hemisphere contributes the soot and foot (Fig. 292 a). But here the segmen- tation Proceeds further before the Several basal wall; the arrow points to the neck of the ^ . archegonium. B= a similar section of an older parts are denned than IS the Case in the embryo, showing its position in the prothallus. r . 1 system; /r= prothallus; tzr