232 EMBRYOGENESIS IN PLANTS 



Johansen (1950), following Soueges (1937), speaks of 'fundamental 

 laws of embryogeny' and states that there are 'certain characteristics 

 pertaining to embryos which partake of the nature of laws.'^ 'Since the 

 embryonomic laws are essentially specific, they provide the explanation 

 for the fundamental organisation of the embryo of a given species. . . .' 

 'when the laws of embryonomy for a given species have been determined, 

 they serve to define this species from the embryological standpoint.' 

 Soueges (1937) has made it clear that his laws of embryogeny are 

 'truly specific, proper to the species.' But once the primary, specific 

 laws have been established for a number of species, it becomes possible 

 to state general laws which serve to define embryonic types or gi"oups, 

 thus making possible a classification of embryos — 'the supreme 

 objective of all embryological studies.' It would be true to say that 

 Soueges' laws relate to the data of descriptive embryology rather than 

 to the causes of embryonic development. 



The laws of embryonomy are as follows : (Soueges 1937; Johansen, 



1950). 



The Law of Parsimony states that no more cells are produced by the 

 embryo than are absolutely necessary. 



The Law of Origin states that in any species the sequences of cell 

 formations are established in a particular way and with such regularity 

 that the origin of any cell can be specified in terms of, or related to, 

 the earlier units of the sequence. 



The Law of Numbers states that the number of cells produced by 

 different cell generations varies with the species and depends on the 

 rapidity of the segmentation in the cells of the same generation. 



The Law of Disposition states that in the course of normal embryonic 

 development, the cells are constituted by divisions in clearly determined 

 directions and appear to occupy positions in accordance with the role 

 which they must play. 



The Law of Destination states that, in the normal embryogeny of 

 any species, the cells of the proembryo give rise to clearly determined 

 parts, and always to the same parts of the embryo. 



Commentary. If these laws are intended, as Johansen (1950, p. 95) 

 states, to 'provide the explanation for the fundamental organisation of 

 the embryo of a given species,' it may be doubted if they do, in fact, 

 achieve this aim. The progressive organisation manifested in the 



1 The essential feature of a law, as generally understood in the natural sciences, is that it has 

 both a particular and a general application, and that it states the truth about related phenomena. 



Law: Scientific and philosophic uses: In the sciences of observation, a theoretical principle 

 deduced from particular facts, expressible by the statement that a particular phenomenon always 

 occurs if certain conditions be present. 



Laws of Nature: The order and regularity in Nature expressed by laws (1853). The con- 

 formity of individual cases to the general rule is that which constitutes a 'Law of Nature.' 

 (Shorter Oxford English Dictionary.) 



