328 EMBRYOGENESIS IN PLANTS 



of the processes involved in embryogenesis and of the nature and 

 magnitude of the factors producing particular morphological results, 

 we are in no position to say how wide the differences are between the 

 configurations which morphologists and taxonomists use as criteria of 

 comparison. Woodger (1945) has summarised the position by saying 

 that when we are able to replace a classification based on adult con- 

 figurations by one of zygote types, 'phylogenetic speculation will be 

 transferred from the plane of taxonomic transformations between adults 

 to the plane of actual or possible genetic transformations between 

 zygotes.' If we ever reach this position, the seemingly impassable or 

 unbridgeable gaps in the phylogenetic system may be found to have no 

 real existence. 



In concluding this Section, the question may perhaps be asked if it 

 is reasonable to expect to find similarities in embryos of distantly 

 related phyletic lines. If the hypothetical common ancestral group is 

 of great antiquity, as it seems to be in the case of the vascular plants, 

 it is hardly to be expected that the embryos of the living forms will be 

 closely comparable. But it is reasonable to suppose that some general 

 basic features may be common to them all, and this, in fact, is what we 

 find. In short, while there may have been more or less close embryo- 

 logical similitude at an early stage in the divergence of the several major 

 phyletic lines from the common ancestral stock, it is scarcely to be 

 expected that this would still be found in the contemporary correlatives. 



EXOSCOPIC AND ENDOSCOPIC EMBRYOGENY 



When a suspensor is present the embryogeny is typically endoscopic, 

 the suspensor and foot being formed adjacent to the neck of the arche- 

 gonium and the shoot apex being directed inwards. In exoscopic 

 embryos the sporophyte apex is directed towards the neck of the 

 archegonium. The embryos of bryophytes, Psilotales, Equisetales and 

 of Isoetes are exoscopic; those of the Lycopodiales, Marattiales, 

 gymnosperms and flowering plants are endoscopic; those of the lepto- 

 sporangiate ferns are lateral. The presence or absence of a suspensor 

 has been accorded an important place in comparative embryology. 

 We may, nevertheless, ask if the difference is truly one of fundamental 

 importance. Relatively minor, though consistent, differences in zygotic 

 reaction systems, or zygotic environments, may account for the presence 

 or absence of a suspensor, i.e. may determine the inception of more 

 active protein metabolism at one pole. As we have seen, the fact that 

 the apical region of an endoscopic embryo is thrust into the prothallial 

 tissue makes no important difference to its axial development and 

 organisation, as compared with an exoscopic embryo. In Whitaker's 

 experimental studies of Fucus eggs, it was found that small differences 



