664 CONCLUSION 



On the other hand, the examination of the embryos of various types of 

 Pteridophytes has shown that the occurrence of a suspensor is variable 

 within near phyletic limits, that the form of the embryo itself is in high 

 degree plastic, and that a certain correspondence can be traced between 

 biological requirement and the proportions, or even the actual position 

 of the parts relatively to one another, and to the parent prothallus. Thus 

 the haustorium or foot is found to be inconstant in position, and may 

 be present or absent in plants of near affinity ; the root may be entirely 

 absent, or vary in its position ; the cotyledons also may vary in number 

 and in position as well as in form and dimensions. Such irregularities, 

 together with a certain degree of physiological reasonableness which may 

 often be seen to underlie them, led not unnaturally to the conclusion 

 drawn by Goebel 1 that " root, shoot, and haustorium are laid down in the 

 positions that are most beneficial for their function." This implies that 

 all parts are opportunist growths. To those who accept this thesis 

 as true, embryology cannot form a secure basis for general comparisons 

 or for phylogenetic argument. For them comparative embryology would 

 be little better than a study of the more or less immediate biological 

 adaptations of the embryos themselves : there would be no common ground 

 from which the comparison could start. 



But it may be questioned whether this extreme position is fully 

 justified. The endeavour must be made to recognise and isolate those 

 characters of the embryo which are variable, and to see whether there is 

 not some element of constancy in shape or in construction which underlies 

 the fluctuating features, and runs through all the different forms. This 

 has been greatly facilitated by recent discoveries; for now the embryos 

 of all the leading types of living Pteridophytes are fairly well known, with 

 the exception of the Psilotaceae though possibly these are, for comparative 

 purposes, the most important of them all. 



A revision of the embryology in the whole series of Pteridophytes 

 described above leads to the conclusion that the form is not so inchoate 

 or immediately plastic as Goebel's statement implies : comparison shows 

 that there is one point comparable in them all (where fully investigated) 

 which does not appear susceptible of disturbance on a basis of opportunism, 

 viz. the position of the apex of the axis relatively to the primary 

 segmentation ; or, expressed in other words, the relation of the polarity of 

 the embryo to its first cleavages. 



Of this primary segmentation there are two types, according as a 

 suspensor is present or absent; otherwise it shows that remarkable constancy 

 of cleavage which led earlier writers to construct the theory of octants, 

 now no longer to be upheld. It has been shown that these two types 

 may appear in the same phylum (Lycopodiales, Ophioglossales) and even 

 in the same genus ' (Botrycftiwri) ; and there is accordingly reason to 

 believe that, however important biologically, they do not mark such 



1 Organography, ii. p 246. 



