90 EMBRYOGENESIS IN PLANTS 



thus established during the initial development of the gametophyte. 

 The larger, distal cell, which contains most of the protoplasmic contents 

 of the spore, now divides either by a transverse or a longitudinal wall. 

 Fig. 21c; other divisions of a somewhat irregular character follow 

 and an apical cell is sometimes distinguishable. The mature gameto- 

 phyte is a lobed thalloid structure. The venter of the archegonium is 

 surrounded by gametophyte tissue. 



Although a considerable amount of variation has been observed in 

 the position of the first dividing wall of the zygote in the same and in 

 different species, the embryo is invariably exoscopic. The first wall may 

 be typically transverse, as in E. arvense and other species, Fig. 21a; 

 but in E. debile it may in some instances be vertical, i.e. aligned in the 

 axis of the archegonium. Fig. 2 Id (Campbell, 1928). The next two 

 walls are at right-angles to each other and to the first or basal wall, 

 the embryo thus attaining to the octant stage. These octant cells are 

 not invariably of equal size. In E. maximum the largest quadrant of the 

 epibasal hemisphere at once becomes the apical cell of the shoot. 

 According to Sadebeck (1878, 1900), the epibasal cell in E. arvense and 

 E. maximum is so divided by three intersecting walls that a tetrahedral 

 apical cell is formed. Fig. 21a, together with three peripheral cells 

 which give rise to (or correspond to) the three fused leaves of the 

 primary leaf sheath. In the hypobasal segment, comparable divisions 

 give rise to the foot and the first root, the latter with a conspicuous 

 apical cell and an incipient root cap. In other words, Sadebeck relates 

 the primary organogenic developments to an exact system of cellular 

 segmentation. Bower (1935), however, has emphasised that the 

 hypothesis of an exact correspondence between segmentation pattern 

 and organogenesis should not be too closely pressed and suggests that 

 Sadebeck rather tended to interpret embryonic development 'in 

 accordance with the custom of his time,' i.e. his interpretation was 

 based on the assumption of a regular relation between organ formation 

 and the embryonic cleavages; and (to quote Bower) 'as a consequence 

 he drove a direct comparison between the results of his own observa- 

 tions on E. arvense and E.paJustre and the embryology of the leptospor- 

 angiate ferns which was already known.' In Bower's view it should 

 not be assumed, either in Equisetum or in other pteridophytes, that the 

 embryology 'will accord consistently with any set segmental scheme.' 

 This, indeed, emerges from Campbell's study of E. debile; for although 

 a highly regular octant stage has been observed in this species, Fig. 21e, 

 there is considerable variability in the position of the first wall. The 

 root, moreover, is formed from the epibasal segment, the whole of the 

 hypobasal region becoming an enlarged foot, Fig. 21f-j. At this stage 

 the embryo has a somewhat flattened form. In E. hiemale, also, the 



