. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. EMBRYOGENY OF THE PTERIDOPHYTES 669 it as possible, it could not be done more exactly than is shown in Sade- beck's drawings (Fig. 214, p. 393): one octant enlarges and thrusts the less active octants aside; and'its central angle immediately becomes one of the angles â of the pyramidal initial, which then continues to segment in a sequence of which the original octant walls were the first terms. The succession of the cleavages is shown diagrammatically in Fig. 357 a and b. The necessary consequence is
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. EMBRYOGENY OF THE PTERIDOPHYTES 669 it as possible, it could not be done more exactly than is shown in Sade- beck's drawings (Fig. 214, p. 393): one octant enlarges and thrusts the less active octants aside; and'its central angle immediately becomes one of the angles â of the pyramidal initial, which then continues to segment in a sequence of which the original octant walls were the first terms. The succession of the cleavages is shown diagrammatically in Fig. 357 a and b. The necessary consequence is an appearance in section accurately shown in Fig. 358 a, in which it â will be seen that the apical segmentations conform with great exactitude to those shown in the Fig. 358. Drawings of embryos. A, of Equisetum (after Sadebeck). B, of Marsilia. (after Hanstein). C, of Adiantum (after Atkinson). They all illustrate with accuracy the origin of the apical cell of the axis, according to the scheme shown in Fig. 357. Even in Leptosporangiate Ferns, notwithstanding the influence of a large and precocious cotyledon, the same relation of the apical cell of the axis to the octant segmentations may be observed. It is accurately shown in Hanstein's drawing of the embryo of Marsilia salvatrix (Fig. 358 b), where the apical cell with its first segment directly adjoins the octant-wall. It is equally clear in Campbell's Fig. 178 f1 for Onoclea sensibilis, while Fig. 358 c, after Atkinson, showing the embryo of Adiantum, indicates the same cleavages there also. Thus, even in embryos where there is a single initial cell, that cell is carved out so as to be in the point nearest the centre of the epibasal hemisphere that is consistent with their mode of segmen- tation. In the Marattiaceous Ferns, where there is no constant single initial at the apex of the stem, the matter is not so clear; but Fig. 292 leaves no room for doubt that the position of the apex of the stem is substantia
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