FACTORS IN EMBRYOGENESIS 37 



EXPERIMENTAL INVESTIGATION OF EMBRYOS 



Thus far, the facts of plant embryology have been mainly those 

 obtained by the methods of the anatomist and histologist. Much 

 remains to be done in the fields of experimental morphogenesis, 

 genetics, biochemistry and biophysics. Indeed, the new phase of 

 embryology should be characterised by an experimental outlook. The 

 results of experimental investigations of embryos in different systematic 

 groups will be given in the appropriate chapters. 



At this point it may be advantageous to consider briefly some of the 

 general ideas and conclusions that have emerged from recent experi- 

 mental studies of other embryonic regions, e.g. the shoot apex, which 

 exemplifies what has been described as a 'continued embryogeny.' We 

 should not, of course, assume out of hand that these conclusions have 

 a direct application to the small developing embryo, but there may be 

 phenomena common to both which can be more readily studied in the 

 large tissue mass afforded by the adult apex. 



Polarity is important in the shoot apex in vascular plants and is 

 closely associated with many of its morphogenetic activities. It is 

 difficult to modify by experimental treatment and has important 

 effects on the movement of metabolites. Thus auxins move rapidly in a 

 basipetal direction but only very slowly in the opposite direction. 



The symmetry of the apical region, which is largely due to intrinsic 

 factors, is important in morphogenesis. In typically dorsiventral plants, 

 such as species of Selaginella, the dorsiventrality is not readily modified 

 by experimental treatment. But in some species the symmetry shows a 

 certain lability, plants growing in the shade being dorsiventral, whereas 

 those exposed to full light are radially symmetrical. Many species, 

 however, are completely and persistently dorsiventral. 



With certain qualifications, the embryonic cells at the shoot apex 

 of a particular species are of a characteristic size. Thus, whereas the 

 apical cell in some ferns may develop to a very large size during the 

 ontogeny, small cells are characteristic of the adult shoot apex in most 

 gymnosperms and flowering plants. A survey of seed plant apices, 

 however, shows how very diverse are the sizes and arrangements of their 

 meristematic cells (Wardlaw, 1953). In diploid and polyploid members 

 of the same species, the latter have consistently larger meristematic 

 ceHs than the former. It may therefore be concluded that there is a 

 relationship between the genetical constitution of a species and the size 

 of its embryonic cells at the shoot apex. This relationship may be of 

 interest in the study of embryos in the same and in different taxonomic 



groups. 



The shoot apex is recognised as being the primary morphogenetic 



