EMBRYOGENKSIS IN LEPTOS POR ANGI ATE FERNS 167 



evidence that the suspcnsor is an organ which, in difTerent fern groups, 

 may have been lost or eliminated in the course of descent. We should 

 note, however, that a suspensor is consistently present in the seed plants. 



Gametophytc ami Sporop/ivte. Considerations of the embryogeny 

 prompt a reference to similarities and differences in the development of 

 the gametophyte. Each of the alternating generations begins from a 

 single cell and affords evidence of polarity from the outset; in each 

 there is an orderly segmentation pattern, a distal apical meristem, or 

 growing point, being organised at an early stage. In the gametophyte, 

 however, the 'continued embryogeny' and physiological dominance of 

 the apical cell group are usually not retained as they are inthesporophyte. 

 The very different developments of the gametophyte and sporophyte 

 must be attributed to intrinsic differences. Yet it is not simply a question 

 of chromosome number: a diploid prothallus still has the characteristic 

 gametophyte morphology. Ultimately, as it seems, there must be some 

 over-riding metabolic difference, or difference in the biochemical 

 constitution or organisation, of the zygote as compared with the spore. 

 Any satisfying biochemical theory of organismal pattern must be able 

 to account for the very different developments of the gametophyte and 

 the sporophyte. 



Segmentation Pattern and Organogeny. In any general discussion of 

 embryonic development in ferns, the distinctive segmentation pattern, 

 especially during the early stages, naturally attracts attention, the more 

 so as the several primary organs can be related to the segmentation at 

 the octant stage. This indeed, led the earlier embryologists to postulate 

 a direct causal relationship between segmentation and organ formation ; 

 or, as Goebel (1918, p. 978) called it, a kind of 'mosaic-theory' of plant 

 embryogeny. In a critical analysis of this conception and its origins, 

 Bower (1923, p. 300) has pointed out that 'instead of accepting a general 

 embryology as based on cell cleavages, it would be more natural to 

 regard the embryo as a living whole; to hold that it is liable to be 

 segmented according to certain rules at present little understood : that 

 its parts are initiated according to principles also as yet only dimly 

 grasped: and that there may be, and sometimes is, coincidence between 

 the cleavages and the origin of the parts, but the two processes do not 

 stand in any obligatory relation one to the other.' That the developing 

 zygote should be envisaged as a whole seems indisputable. Its specific, 

 gene-determined reaction system, and the incident biophysical and 

 environmental factors, are responsible for the distribution of growth 

 and therefore for both the segmentation pattern and the pattern of 

 organ formation. These two phenomena will be broadly, and some- 

 times closely, coincident. As to the relative positions of the primary 

 organs, the foot may be inconstant in position, This led Bower (1923) 



