2 EMBRYOGENESIS IN PLANTS 



growth, development, assumption of form, differentiation, and func- 

 tional activities of organisms, from their small and relatively simple 

 unicellular state to their large and complex adult state. Studies of 

 embryos thus provide essential information on the inception of the 

 varied form and structure of plants. 



If our aim is to seek unity in the great and wonderful diversity 

 which we find in the Plant Kingdom, and to understand how, and at 

 what point, the divergences between species and higher taxonomic 

 units occur, we must begin with that stage which is common to all, i.e. 

 the single germ cell, and we must study its constitution, environment, 

 growth, assumption of form, and structural differentiation. An 

 adequate study of embryos thus requires an integration of the data of 

 the several main branches of botanical science, i.e. morphology, 

 physiology and genetics. On these grounds, a substantial, indeed a 

 unique, position may be claimed for embryological studies. 



During recent years, investigators of morphogenesis have devoted 

 much time to the study of shoot apices. Since this primary morpho- 

 genetic region is as yet inadequately investigated, it clearly merits all 

 the attention that can be given to it. But we should not lose sight of 

 the fact that, in experimental morphogenesis, the perennially embryonic 

 shoot apex has often been pressed into service as a substitute for the 

 young embryo, because of the manipulative difficulties which the latter 

 presents by reason of its small size, delicacy and inaccessibility. Even 

 though they may show similar growth developments, the meristematic 

 cell masses of the apex and of a developing zygote are not really strictly 

 comparable: the environmental and histological relationships of the 

 two are vastly different. In this tendency to avoid embryos as experi- 

 mental materials and to interpret morphogenetic processes in terms of 

 factors considered to be at work in the shoot apex, it may well be that 

 important phenomena are being neglected and important opportunities 

 missed. Studies of embryogenesis, therefore, not only carry their own 

 interest ; they should be regarded as the primary approach to the study 

 of morphogenesis. 



As an organism develops, its characteristic organisation becomes 

 evident. This is seen in its overall size, shape and structure, in the 

 relationships of its members, and not least, in the harmoniously 

 regulated development of the whole. Moreover, the several tissue 

 systems, whether in a vascular plant or a thallophyte, constitute a 

 characteristic and sometimes highly distinctive pattern, e.g. a cross- 

 section of the rhizome of the common bracken {Pteridium aqidUnum). 

 As a fact, our knowledge of the factors which determine the tissue 

 pattern in plants is still very slight; but this at least can be said: the 

 process has its inception in the embryogeny. 



