. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 0 0- FILICALES flattened expansion : and developmentally it is found that this arises by transverse growth of the superficial tissues, especially at the wings them- selves. The result in Osmunda is a mere flattened leaf-base ; but in Todea saperba the development extends also as a transverse commissure across the adaxial face of the leaf-stalk. This development, so exceptional in Ferns, may be compared with the stipular development in the Marattiaceae, which is also extended as a commissure across the
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 0 0- FILICALES flattened expansion : and developmentally it is found that this arises by transverse growth of the superficial tissues, especially at the wings them- selves. The result in Osmunda is a mere flattened leaf-base ; but in Todea saperba the development extends also as a transverse commissure across the adaxial face of the leaf-stalk. This development, so exceptional in Ferns, may be compared with the stipular development in the Marattiaceae, which is also extended as a commissure across the leaf-stalk. The two cases, though differing in detail, appear alike in morphological nature. The sporangia of Todea are borne only on the under surface of the leaves, but in Osmmida they are inserted in- differently on both sides of the much- contracted part, so that at maturity it appears to be completely covered by them. There is no protective indusium. The sporangia themselves are relatively large, of pear-like form, and thick-stalked. They consist at maturity of a single layer of cells forming the wall, but with a few tabular cells within : a group of polygonal, thick-walled cells in a lateral position, but nearer the distal end, is recognised as the annulus : it is related to the slit of dehiscence so that the latter passes from the centre of the annulus, over the distal end, and approaches the stalk on the opposite side of the sporangium (Fig. 294). The line of dehiscence, de- fined structurally by narrow thin-walled cells, gapes widely at ripeness ; this arrangement requires elbow-room, which the lax arrangement of the sporangia readily allows. Those sporangia which are in near proximity to one another originate simultaneously : there is no interpolation, nor any marked sequence of their origin. Nor is there any regularity in their orientation : in Osmunda the sporangia face in the most various directions : and even in Todea, where their arrangement has some reference t
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