GENERAL CONCLUSIONS 319 



growth pattern throughout the ontogenetic development. But environ- 

 mental and other factors are also active in determining the nature of the 

 embryonic development and hence, to arrive at any comprehensive 

 understanding, all the factors which may be involved must be duly 

 considered. 



FACTORS AND RELATIONSHIPS IN EMBRYOGENY 



It is assumed that all the factors in the embryonic development 

 work in accordance with the laws of physics and chemistry. The factors 

 and relationships involved may be indicated as follows. 



Genetical Factors. If, as is generally accepted, these determine both 

 the general and specific metabolism of plant cells and tissues, then 

 substances such as growth-regulating substances, enzymes, etc., are 

 ultimately gene-determined. Since a gene is essentially a large organic 

 molecule, the reactions which it stimulates and controls must proceed 

 in conformity with the laws of physical chemistry as applied to organic 

 reaction systems. As a working hypothesis, a fertilised ovum may be 

 regarded as a complex, gene-determined, reaction system. According 

 to the components of the system and the sustaining environmental 

 conditions, characteristic chains of reactions will be set in motion, and 

 the resulting biochemical pattern, or patternised distribution of meta- 

 bolites, will constitute the basis for the visible morphological and 

 histological developments. 



Physiological and Physical Factors. Important aspects of the 

 embryonic development are referable to physical factors and to physio- 

 logical processes and relationships that are common to all green plants. 

 Diffusion gradients, the action of gravity, light, surface tension and 

 other forces, the tendency towards a state of equilibrium in a dynamic 

 system, various factors in the environment, and organismal phenomena 

 such as correlation, the reciprocal relationships of parts, the inception 

 of growth centres, and functional activities, may all be involved in the 

 embryonic process. Here it is important to note that differently 

 constituted reaction systems may yield closely comparable organismal 

 patterns (Turing, 1952; Wardlaw, 1953). The characteristic develop- 

 ment of an embryo, then, is due to a whole nexus of factors and 

 relationships ; and while genetical factors are primary, other, essentially 

 extrinsic, factors are among the proximate causes of the observed 

 developments. 



Protoplasmic Organisation. The direct action of genes and of 

 various physical and environmental factors does not, however, afford 

 the basis for an adequate account of embryogenesis. The zygote of a 

 particular species is not simply a complex diffusion reaction system: 

 it is an organismal system, with a specific organisation, perhaps with a 



