GENERAL CONCLUSIONS 323 



EMBRYOLOGICAL SIMILITUDE 



Not only are the major phyletic lines or subdivisions of the Plant 

 Kingdom separated by wide 'phyletic gaps,' but gaps may also separate 

 taxonomic groups of apparently close affinity. It may therefore be 

 asked if the present survey indicates that the basic structural plans, or 

 organisational patterns, of species in the major groups are perhaps less 

 different than their adult conformations lead one to suppose. The 

 answer to this very difficult question appears to be in the affirmative: 

 in an earlier Section (p. 317) it has been seen that certain features are 

 common to the embryological development of all plants except the sim- 

 plest filamentous algae, colonial algae, fungi and lichens. To use 

 Woodger's terminology, the constructional plans (or Bauplans), as 

 manifested in the early embryogeny, in Oedogonium, Fiicus, Marchantia, 

 Funaria, Lycopodlum, Angiopteris, Pinus, Capsella and Lilium are more 

 alike than they are unlike; they have, in fact, a great deal in common. 

 In their initial development, the zygotes of these representative genera 

 react as if they shared a common basic substance; but later, when 

 various specific genetical factors begin to act, very divergent morpho- 

 logical developments become evident. In the alternative interpretation 

 — that the living substance is rather diff'erent in them all — the com- 

 parable embryonic developments might be attributed either to parallel 

 or convergent evolution, or to the impact of the same extrinsic factors. 

 There is general acceptance of the view that bryophytes, pteridophytes 

 and seed plants all evolved from green algal ancestors, and therefore it 

 may be assumed that all have some protoplasmic organisation and 

 constituents in common. For example, the photosynthetic, respiratory 

 and osmotic mechanisms appear to be common to all autotrophic 

 plants. It is therefore understandable why some developmental 

 features are common to species in diff'erent major groups. It is a 

 considerably more exacting task, however, to decide whether or not 

 the data of embryogeny enable us to discern affinities between major 

 divisions such as algae, bryophytes and pteridophytes, or ferns, 

 gymnosperms and flowering plants. 



In comparing early embryonic developments, it would be helpful 

 to know how mutations may affect the immediate post-zygotic develop- 

 ment. What, for example, is the nature and magnitude of the genetical 

 change that determines the loss of the suspensor in a group, or genus, 

 in which that organ is normally present ? In the zygote, envisaged as a 

 reaction system, it may be that a comparatively small constitutional 

 change determines the presence or absence of a suspensor; e.g. in the 

 Marattiaceae and Ophioglossaceae there are species with a suspensor 

 and related species without one; and among flowering plants we can 



