10 EMBRYOGENESIS IN PLANTS 



primary formative activity is also located in the distal region, this 

 region having become organised as an apical meristem. In fact, the 

 subsequent development of the organism is primarily due to the 

 activity of this meristem. It remains in the embryonic state throughout 

 the vegetative development, acts as a self-determining, morphogenetic 

 region, giving rise to the organs and tissues of the enlarging axis (or 

 thallus); and it exercises a physiological dominance over the other 

 parts of the embryo, thus contributing to its regulated, harmonious 

 development. That these observations have a general application is 

 seen in the close parallelism of development (or homology of organisa- 

 tion) exemplified by the apical meristems, and the somata to which they 

 give rise, in AscophyUum nodosum (a brown algae), Dryopteris aristata 

 (a fern), and any representative dicotyledon, e.g. Lupinus albus or 

 Primula polyantha (Wardlaw, 1953). 



Each of the successive embryonic phases of any selected species 

 raises its own problems. Our aim is to discover the underlying causes, 

 i.e. the forces determining the sequence of orderly developments 

 characteristic of the species. In particular, we shall have to inquire 

 into the phenomenon of polarity, the predominance of the distal cell 

 group, the mode of nutrition of the embryo, the organisation of the 

 distal apical meristem, the inception and elaboration of the histological 

 pattern, the formation of leaf and root primordia, and the regulated 

 development of the embryo as a whole. The importance of these 

 investigations needs no emphasis : they are of the essence of biological 

 inquiry; and the more we know about factors in the embryogeny, the 

 more adequate will be our understanding of the subsequent develop- 

 ments as the organism grows to the adult state. 



GENIC CONTROL OF DEVELOPMENT 



Growth from the zygote or spore to the adult state is characterised 

 by an orderly sequence of developments, during which the organism 

 increases in size, becomes structurally complex, and develops the 

 specific characters by which it can be recognised. It may be accepted 

 as a working hypothesis that, in every aspect and phase of development, 

 factors in the hereditary constitution are involved. In that these 

 factors, or genes, determine and control metabolic processes, they are 

 in some way involved in all growth and cellular activity, and therefore 

 in morphogenesis. Such a view is generally accepted by biologists. 

 But how the genes act, individually and collectively, is a problem that 

 is as difficult as it is important. A comprehension of genie action in 

 embryogenesis is clearly a matter of first importance to the biologist. 



The time of action of particular genes is important in all develop- 

 ment, but it has a special interest and importance in embryogenesis. 



