GENERAL CONCLUSIONS 335 



afford a means of explaining, or bridging, some of the gaps in the fossil 

 record. De Beer (1951, p. 140) also points out that a character which 

 appears in the early embryogeny is not necessarily primitive or early in 

 the phylogenetic sequence. In short, a knowledge of phylogeny does 

 not enable us to explain the events in the embryonic development. 

 'Even if we had a complete phylogenetic series of adults ancestral to 

 any given descendant, it would not help us to understand the processes 

 of fertihsation, cleavage, induction, differentiation, organogeny, etc., 

 which take place in the ontogeny of that descendant. The historical 

 descriptive study of evolution has no bearing on the causal analytic 

 study of embryology' (de Beer, p. 142). And conversely, even the most 

 adequate knowledge of ontogeny would not of itself explain the 

 essentially historic facts of phylogeny. 'But since phylogeny is but the 

 result of modified ontogeny, there is the possibility of a causal analytic 

 study of present evolution in an experimental study of the variability 

 and genetics of ontogenetic processes.' 



THE PLACE OF EMBRYOLOGY IN BOTANICAL SCIENCE 



One of the more general aims in contemporary botanical science is 

 to explain how specific organisation is brought about. The relevant 

 studies properly begin with investigations of zygotic constitution and 

 development and the assumption of form and structure in the growing 

 individual plant under the influence of genetical, physiological, physical 

 and environmental factors. The knowledge so gained will make for a 

 more adequate understanding of the conformation of the adult plant 

 and it may well lead to a new or modified outlook on the taxonomic and 

 phylogenetic significance of various morphological and structural 

 features. 



In recent years a number of concepts have gained ground as 

 affording at least partial explanations of formative processes in plants, 

 e.g. that the shoot apex is a self-determining primary morphogenetic 

 region, that leaf formation and phyllotaxis may be due to the inception 

 of growth centres or morphogenetic fields at the apex, that the inception 

 or initial differentiation of vascular tissue is due to basipetal effects 

 proceeding from the apices of shoot and leaf primordia, that physio- 

 logical gradients and the nutritional status of the apex are important 

 in morphogenesis, and so on. These conceptions, largely based on 

 morphological and experimental studies of the apex, do not appear to 

 be in any way invalidated by the present survey of embryonic develop- 

 ment. On the contrary, they are supported; and, indeed, it may be 

 claimed that our conception of factors which may determine the incep- 

 tion of pattern and the progressive organisation during the individual 

 development, has been enhanced and deepened. 



