250 EMBRYOGENESIS IN PLANTS 



investigations have demonstrated the exactitude and specificity of the 

 processes of embryonic growth, cell division and organogenic develop- 

 ment in many species. In these phenomena they perceive the working 

 of embryonomic laws. In each species, every cell division, as it were, 

 is meaningful in a high degree; and this indeed is true can we but 

 discover the meaning. Because of the specificity and regularity of the 

 embryonic segmentation pattern in each species, and, further, the 

 regularity with which cells in certain positions give rise to particular 

 organs or tissues — all visible expressions of the orderly working of the 

 genetical constitution — a classification based on embryology seems 

 reasonable and feasible. It is reasonable to argue that species with the 

 same general hereditary constitution will be closely comparable in their 

 embryonic development ; conversely, close similarities in the embryonic 

 development are an indication of taxonomic affinity. These proposi- 

 tions are borne out by the Cruciferae. Closely related families may also 

 be expected to have much in common in their embryonic development, 

 whereas unrelated families may show histological differences. The 

 basic question is: Are these arguments supported by the observed 

 facts? 



In diff'erent species, because of diff'erences in their genetical con- 

 stitutions, the distribution of growth, and concomitantly the seg- 

 mentation pattern, may be different. In some species the zygote is 

 divided transversely into two almost equal cells; in others, the two 

 cells are very unequal in size and very different in their subsequent 

 development, i.e. in their metabolic activity. In general, the basal cell 

 contains substances which are osmotically active, whereas in the 

 terminal cells such substances are probably utilised in the synthesis of 

 protoplasm. Now, as a fact, we still know very little about the 

 constitution of biological reaction systems and the way they work. 

 Differently constituted reaction systems may yield closely comparable 

 segmentation patterns; and, on the other hand, comparatively small 

 metabolic differences may be attended by considerable diff'erences in 

 the early segmentation pattern. Thus quite small genetical differences 

 may cause considerable differences in the reaction system and, con- 

 sequently, in the cellular pattern. Until much more is known about 

 organismal reaction systems, and the effect on them of genie mutation 

 and other genetical changes, and on the resulting segmentation pattern, 

 conceptions of the kind advanced by Soueges and Lebegue must be 

 viewed with caution and reserve. For all we know, quite small and 

 perhaps relatively unimportant genetical differences may produce 

 considerable differences in the early embryogeny. If the developing 

 zygote is thought of as a specific reaction system, and if it is borne in 

 mind that the embryo grows as a whole, and that physical factors are 



