. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 590 FILICALES The form of the parent cells of the sporangia is not constant; commonly the cell has a square base, and the first segment-wall passes obliquely to the basal wall (Fig. 329 a and c), the next segment-wall being inserted obliquely on the first: the result is from the outset a sporangium with a short massive stalk, as shown in Fig. 329 d. In other cases the parent cell is more wedge-shaped, and the first segment-wall cuts an oblique lateral wall (Fig. 329 b) : thus the segmentation in Thyrs
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 590 FILICALES The form of the parent cells of the sporangia is not constant; commonly the cell has a square base, and the first segment-wall passes obliquely to the basal wall (Fig. 329 a and c), the next segment-wall being inserted obliquely on the first: the result is from the outset a sporangium with a short massive stalk, as shown in Fig. 329 d. In other cases the parent cell is more wedge-shaped, and the first segment-wall cuts an oblique lateral wall (Fig. 329 b) : thus the segmentation in Thyrsopteris wavers between two types—the one characteristic of larger, the other of smaller spore-output. The further segmentation of the sporangial head follows the type usual for Leptosporangiate Ferns. Very soon, however, the sporangium takes an. unsymmetrical form, the more strongly growing side being that directed towards the apex of the receptacle (Fig. 329 d): the oblique annulus, which in this genus presents peculiar characters, makes its appearance early, and occupies a position comparable to that in Gleictienia : the orientation of the sporangia is thus on the Gleicheniaceous type, which, when repeated and associated with lateral dehiscence, offers the advantages for spore-dissemi- nation noted in the Hymenophyllaceae, and effective in other Gradatae. The central cell undergoes the usual segmentation to form a tapetum of the usual type, and 12 to 16 spore-mother-cells. From countings of the spores it has been concluded that the typical number for each sporan- gium is from 48 to 64. The sporangia when mature are of large size and rather peculiar form: seen from without they present some rather unusual features. Fig. 329 f shows one presenting its central face: the annulus, starting from the base,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations ma
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