38 EMBRYOGENESIS IN PLANTS 



region of the plant. It may be characterised by a distinctive histological 

 and physiological organisation, as in leptosporangiate ferns such as 

 Dryopteris or Matteuccia. A comparable organisation is also thought 

 to be present in the apices of gymnosperms and seed plants, though 

 the histological pattern may be considerably less evident. Nevertheless, 

 in all groups of flowering plants and gymnosperms, the shoot apex is 

 characterised by a distinctive arrangement of its meristematic cells. 

 Thus the tunica may consist of one or several layers, it may constitute 

 a very definitely zoned tissue, or the zonation may be rather feebly 

 defined. Again, in some species, a central group of mother-cells may 

 give rise to the various regions of the apex ; and there are many other 

 variants of histological pattern. Since these several types of apical 

 organisation have their inception at some stage in the embryogeny, or 

 in the post-embryonic phase, it may well be that the embryologist can 

 both profit by the existing knowledge of apices and contribute to it. 

 Indeed, all too little is known of the inception of the characteristic 

 patterns in the apices of seed plants. 



The shoot apex gives rise to lateral members in an orderly fashion, 

 leaves being usually formed in a regular phyllotactic sequence while 

 buds normally occupy characteristic positions on the shoot. It will 

 evidently be a matter of interest to inquire at what stage in the embryo- 

 geny this regulated development first becomes apparent. 



Studies of the shoot apex have shown that the prevascular tissue 

 typically originates close to the most distal cell group. This can be seen 

 particularly well in the ferns, but also in some flowering plants. A 

 considerable body of experimental evidence supports the view that the 

 inception of the vascular tissue, whether in shoot, leaf or root, is due 

 to the basipetal diffusion of a substance (or substances) from the actively 

 growing apical meristem or distal cell group. 



In the contemporary view, the development of an axial structure 

 from a shoot apex is essentially epigenetic in character, i.e. each sub- 

 sequent development is determined by those that have gone before, and 

 all suggestions of preformation are ruled out. It is assumed here that 

 the development of the zygote to the adult state is also an epigenetic 

 process. Some factors are at work throughout the individual develop- 

 ment, others become incident at particular stages, and gradually, as the 

 result of the action of factors of many kinds, the characteristic form 

 and structure of the species become manifest {see table, p. 332). Studies 

 of the shoot apex have shown that both extrinsic and intrinsic factors, 

 size-structure and other spatial relationships, and the reciprocal 

 relationships of parts, must all be considered in attempts to explain the 

 observed developments. Similar considerations almost certainly apply 

 to investigations of embryogenesis. 



