EMBRYOGENESIS IN FLOWERING PLANTS 231 



fourth type is not. Soueges considers that the third and fourth types 

 may be the resuh of hybridisation, the second the resuU of adaptation 

 by condensation or convergence. He also recognises two major 

 divisions, or periods. In the first of these periods, i.e. types 1, 2, and 3 

 above, the developing embryo obeys the laws of embryonomy : in the 

 second period only the terminal cell of the two celled proembryo obeys 

 the laws, and the basal cell is omitted from further consideration. 



The first period comprises three series : in A, the terminal cell of the 

 two-celled embryo is divided by a longitudinal wall and is considered 

 the most primitive: in B, the wall of the terminal cell is longitudinally 

 oblique; in C, the proembryo consists of a linear series of four cells. 

 Three comparable series are also recognised in the second period. 

 Each period includes six megarchtypes, these being based on the 

 behaviour of the basal cell during the embryonic development. Johan- 

 sen has criticised this system on the grounds that it is based on a too 

 restricted survey of angiosperm embryos, and that the separation of 

 series A and B is debatable. His own scheme is that of types and their 

 variations. Soueges pays most attention to the terminal cell of the two- 

 celled proembryo : Johansen considers that both the terminal and basal 

 cells should be given equal consideration. In this work these matters of 

 embryo classification cannot be treated at length, but it is recognised 

 that their value may increase as our knowledge of the factors in embryo- 

 genesis increases. 



SOME GENERAL ASPECTS: LAWS OF EMBRYONIC DEVELOPMENT 



Johansen (1950) states that angiosperm embryology may be con- 

 sidered in terms of general embryology and special and comparative 

 embryology. General embryology might be further subdivided into 

 embryonic morphology (the static aspect, comprising the study of the 

 external form and internal structure), embryonic physiology (the 

 dynamic aspect, the emphasis being on growth), and embryogeny 

 (defined as the kinematic aspect, and comprising the major part of past 

 and contemporary embryological investigations). Embryogeny may 

 be further subdivided into embryogenesis (the origin of the embryo), 

 embryo-tectonics (the architecture, i.e. cellular pattern of the embryo), 

 embryogenergy (the destinations or functions of the embryo). In the 

 writer's view the several aspects of embryology should not be separated, 

 though, admittedly, it may be convenient to do so for some purposes. 

 At any particular stage, the form and structure of an embryo are the 

 result of antecedent gene-controlled metabolism, together with the 

 physical and other factors which become incident in the enlarging 

 embryo; and the way in which an embryo develops determines its 

 functional activities. 



