148 EMBRYOGENESIS IN PLANTS 



leaf to root; and (iii) that the shoot apex may be rather inconspicuous 

 at first and may have little or no vascular tissue dififerentiated below it. 

 The theory, which in some respects is evidently aptly descriptive for 

 some species, is supported by Vladesco and others ; but because of its 

 serious defects and limitations it has been rejected by Bower (1923) 

 and Bugnon (1921, 1924), and by the writer (Wardlaw, 1943, 1952) on 

 the basis of experimental investigations. The facts seem to favour an 

 axial theory of construction, the shoot apex being the primary morpho- 

 genetic region of the plant. 



Fig. 35b, c, shows that the dispositions of the epibasal and hypo- 

 basal segments relative to the surrounding prothallial cells, from which 

 nutrients will be absorbed, are closely comparable. As the epibasal 

 region is directed towards, and the hypobasal region away from, the 

 apex of the prothallus, the two regions may be differentially affected 

 by metabolic gradients within the prothallus. In this connection 

 Albaum (1938) has shown that a basipetal auxin gradient can be demon- 

 strated in the prothallus : the epibasal segment of the embryo would 

 thus be at a higher point on this gradient than the hypobasal segment. 



In general, the embryonic segmentation pattern is more regular and 

 exact in leptosporangiate than in eusporangiate ferns. Campbell (1940) 

 has noted that each of the octants, which are like tetrahedra with one 

 curved surface, shows what appears to be apical growth for a brief 

 period, i.e. the divisions of each octant are such that a 'three-sided' 

 apical cell is formed. But this, in fact, is exactly what would happen in 

 the division of a tetrahedron by walls of minimal area. Fig. 35b. 

 Vladesco (1935) has indicated that none of these three-sided cells 

 functions as an apical cell, whether of the leaf or the shoot. In the 

 root quadrant, in which the octant wall is somewhat oblique, the larger 

 cell begins to function as the apical cell of the first root. 



During its early development the embryo remains within the 

 prothallus and obtains all its nutrition from it, probably by way of the 

 foot. It is during this phase that the organisation of recognisable 

 meristems, i.e. of shoot, leaf and root, each with a single apical cell, 

 takes place in three of the quadrants. In the fourth quadrant the 

 original meristematic character is soon lost and it becomes a region of 

 distended parenchyma-like cells and is recognised as the foot. There is 

 a characteristic gradient of cell size from the foot into the shoot region. 

 Figs. 33f, m; 35c. As soon as organ formation becomes sufficiently 

 advanced, it can be ascertained that whereas the shoot and root are of 

 radial symmetry, the first leaf, and all subsequently-formed leaves, are 

 of dorsiventral symmetry and arise in a definite relationship to the shoot 

 apex. It is difficult to account for the inception of this orderly develop- 

 ment, or organisation, in the very young embryo, but such concepts as 



