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 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 will be seen that the apical segmentations 

 conform with great exactitude to those shown in the diagram. 



ivh 



FIG. "358. 



Drawings of embryos. A, of Equisetum (after Sadebeck). B, of Marsiiia (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 Marsiiia 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 F 1 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 substantially 

 the same. In the Ophioglossaceae the segmentation in the embryo has 

 not been accurately made out, but sufficient is known to show that in 



1 Mosses and Ferns, p. 322. 



