144 EMBRYOGENESIS IN PLANTS 



area (see D'Arcy Thompson, 1917, 1942; Thomson and Hall, 1933). 

 The positions of the walls are not determined by external factors such 

 as light or gravity but by factors within the zygote or adjacent prothallial 

 tissue. Vladesco (1935) maintains that, with occasional exceptions, the 

 sequence in wall formation described above is general in leptosporangi- 

 ate ferns, this being contrary to the classical view that the transverse 

 wall is laid down before the median wall. Hofmeister, Hanstein and 

 Pringsheim held that the segmentation into quadrants by the basal and 

 transverse walls was highly significant in determining the inception of 

 the several organs of the embryo. Vladesco considers that this view 

 should now be abandoned. 



Leptosporangiate fern embryos have no suspensor. At the first 

 division of the zygote, however, as in Pten's serratula. Fig. 35a, the 

 epibasal segment may be slightly smaller than the other (Atkinson, 

 1894). In Marsilea, Fig. 36b, the hypobasal segment is the smaller 

 (Campbell, 1918). Vladesco (1935) states that the epibasal segment is 

 usually the smaller. If we regard the approximately spherical zygote as 

 a reaction system, this unequal division would be indicative of cyto- 

 plasmic or rnetabohc differences at the anterior and posterior poles. 

 Longitudinal sections of the embryo of Gymnogramme at the octant 

 stage, in the plane of the transverse walls, show that it is ovoid and 

 elongated along its polar axis. Fig. 34 (Vladesco, 1935). 



Now, in the small sphere of meristem.atic cells illustrated in Fig. 

 35b, it is evident that a considerable amount of biochemical or physio- 

 logical differentiation must already have taken place; for different 

 regions — which can be indicated as the anterior-superior, the anterior- 

 inferior, the posterior-superior, and the posterior-inferior quadrants — 

 soon develop into (1) the shoot apex, (2) the first leaf, (3) the root and 

 (4) the foot, respectively. In short, the main organographic regions are 

 determined at an early stage. 



The earlier investigators tried to relate the formation of the several 

 organs to the individual quadrants in a more or less precise manner. 

 A more critical scrutiny of the facts of the embryonic development, 

 however, shows such views to be untenable. That individual quadrants 

 do give rise to particular organs is not in question, but there is no close 

 and obligatory relationship between the segmentation pattern and organ 

 formation. Although the orientation of the walls during the division 

 of the octants is such as to define what appear to be 'three-sided' apical 

 cells, these do not, in fact, function as the apical cells of the leaf and 

 shoot. Indeed, the delineation of the so-called epibasal and hypobasal 

 discs (Vouk, 1877) is by no means constant : it is not found in Scolopen- 

 drium vulgare. The first leaf, as in Gymnogramme sidphwea (Vladesco, 

 1935), and many other ferns, originates equally from both of the 



