GENERAL CONCLUSIONS 329 



in pH, or in the intensity of illumination, on opposite sides of the 

 zygote, may set in motion the reactions which determine the con- 

 spicuously polarised morphological development of the young plant. 

 So, too, in the enclosed zygotes of bryophytes, ptcridophytes, and seed 

 plants, the orientation of the embryo may be attributable to relatively 

 minor factors, though these become incident with a very high degree of 

 constancy during the development of each particular species. In this 

 connection the eusporangiate ferns are of special interest. In the genus 

 Botrychium, some species, e.g. B. hmaria, have no suspensor, whereas 

 others, e.g. B. obliquwn, have a conspicuous and well developed one. 

 In B. hmaria. Fig. 3 If, the zygote is still spherical or subspherical when 

 it divides to yield an epibasal segment contiguous with the archegonial 

 neck; but in B. obliquum. Fig. 31g, h, the zygote undergoes considerable 

 elongation and divides unequally by a transverse wall so that a large 

 suspensor cell lies next to the neck while the small embryonic cell is 

 thrust into the gametophyte tissue. Now here we have two species of 

 the same genus (or, at any rate, indisputably related species, should 

 Botrychium be split into two genera), in which the initial embryonic 

 developments are very different. But we cannot regard the genetical 

 differences between the species as being of great magnitude. Similar 

 evidence is afforded by the Marattiales. In Angiopteris evecta (Fig. 32) 

 in particular, the embryo is sometimes observed with a suspensor and 

 sometimes without one. There are thus grounds for the view that the 

 presence or absence of a suspensor is a phenomenon which may have 

 been rated above its true genetical or physiological importance. If this 

 conclusion is valid, we begin to see that, in the matter of their early 

 embryogeny, the phyletic gaps between bryophytes, ptcridophytes and 

 seed plants may be less considerable than has been supposed, 



TOWARDS A STATEMENT OF THE PRINCIPLES OF EMBRYOGENESIS 



The factors which determine the embryonic development must be 

 much more fully explored before embryological data can be used with 

 safety in taxonomy and phylogeny. In so far as formal and structural 

 features can be shown to be largely due to extrinsic factors, to that 

 extent they lose their value in phylesis. What seems to be most needed 

 at the present time are working hypotheses relating to embryogenesis 

 and a statement of the principles of embryonic development. 



In plants, with their 'continued embryogeny,' hypotheses relating 

 to morphogenesis and embryogenesis will largely, though not com- 

 pletely, overlap. General features in the development of embryos in all 

 clisses of plants have already been considered (p. 317) and the factors 

 which may be involved have been discussed (p, 319). In this section 

 an attempt is made to prepare the way for a statement of the principles 



