330 EMBRYOGENESIS IN PLANTS 



of embryogenesis. The basic assumption is that the zygote of any 

 species is a highly complex and specific organic reaction system. 



If the initial distribution of metabolites in the ovum or zygote is 

 homogeneous, a drift towards heterogeneity soon begins, physiological 

 gradients in the surrounding tissue being probably causally involved. 

 These may determine the polarised distribution of particular meta- 

 bolites in the egg. In algae, the ova, if initially homogeneous, soon 

 become heterogeneous under the impact of factors in the environment. 

 This polarised distribution of metabolites, which underlies the fila- 

 mentous or axial development of the zygote, is of profound importance 

 in the ensuing embryonic development. Whether or not a zygote has a 

 specific cytoplasmic organisation of some kind must remain an open 

 question. The polarity of the young embryo, and all the morphological 

 and histological features by which its development is characterised, are 

 primarily referable to an antecedent patternised distribution of meta- 

 bolites. The inception of a biochemical pattern in the zygote, and the 

 successive changes in it during the embryonic development, are held to 

 be due to the reactions of the gene-determined components of the 

 system, to the impact of physical and environmental factors, and to 

 various organismal relationships which develop during growth. The 

 visible, specific organisation, in short, is due to the hereditary con- 

 stitution and to the inception and elaboration of a specific biochemical 

 pattern. The basic problem in embryogenesis and morphogenesis is 

 thus to discover how patternised distributions of metabolites, and, 

 in particular, of morphogenetic substances, are brought about. Turing 

 (1952) has indicated how an initially homogeneous diff'usion reaction 

 system, of the kind that may be present in a developing zygote, may 

 become heterogeneous and give rise to a patternised distribution 

 of metabolites, thus affording a basis for a morphological or histological 

 pattern. 



Since the main physiological processes are common to all classes of 

 autotrophic plants, we may assume that their ova contain (a) meta- 

 bolites, or reacting substances, which are common to all, e.g. inorganic 

 nutrients, carbohydrates, organic acids, amino-acids, enzymes, etc., and 

 (b) substances, or combinations of substances, which are characteristic 

 of the species. A zygotic reaction system may therefore be expected to 

 develop some morphological and structural features which are common 

 to all embryos and others which are particular to the species, genus or 

 family. In these reaction systems, also, particular genes, or gene- 

 determined substances, will enter actively into the reactions at par- 

 ticular times, this being determined by a number of circumstances 

 (Mather, 1948; Darlington and Mather, 1949) and they will therefore 

 have a greater or less importance at different phases of development. 



