670 CONCLUSION 



Ophioglossum mlgutum (Figs. 260 and 260 fa's, p. 466) and in Botrychium 

 (Figs. 261, 262, p. 468) the apex originates from approximately the centre 

 of the epibasal hemisphere, and notwithstanding that there is an early 

 displacement owing to the precocious development of the first root. The 

 facts thus suffice to support the general statement, that whatever the other 

 fluctuations of form of the Pteridophyte embryo may be, all the exactly 

 investigated types show the apex of the axis to originate in close relation 

 to the intersection of the epibasal octant-walls. 



It is accordingly recognised that the very first step that can be 

 observed in the embryogeny involves the definition of its polarity, and 

 that the apex of the shoot bears a constant and close relation to the 

 centre of the epibasal hemisphere. The base of the primitive shoot thus 

 defined is the suspensor where that part occurs ; where it is absent the 

 centre of the hypobasal hemisphere may be held to mark the base of the 

 primitive axis. 1 The whole embryo thus appears from the first as a 

 radially constructed spindle upon which appendages may be borne : these 

 are of the nature of leaves and of accessory roots, and they may vary in 

 number, and in position and time of origin, causing thereby marked 

 variations in the early structure, which are for the most part open to 

 biological explanation. The embryo is, however, subject also to early 

 distortion in various ways, through the formation of swellings of the nature 

 of haustoria, or of storage tubers : or it may be that modifications of 

 form arise in relation to the precocious development of some one 

 appendage and the correlative delay or diminution of another, or even 

 of the axis itself. Though such modifications are probably secondary, 

 yet they have produced such peculiarities of form and aspect in the 

 embryos in which they appear that the originally radial form of ihe shoot 

 is disguised, and its morphology has consequently been misunderstood. 

 Examples will now be quoted illustrating these various points. 



The origin of the cotyledon in Ferns is constant in time and place : 

 this is probably related to the prone position of the embryo, and to its 

 importance for early nutrition in replacing the supply derived from the 

 small and evanescent prothallus. But in other cases there is less constancy : 

 in Eqitisctnm there may be sometimes two, though usually three cotyledonary 

 leaves in the first whorl. In L. Scln^o and r/ilcgmaria one cotyledon takes 

 precedence, soon followed by a second leaf (Figs. 183, 184, 185), but 

 in /. clavatinn two equal cotyledons are formed (Fig. 186). Again, in 

 Sela^inclla Martensii two equal cotyledons appear very early ; in S. 

 spinulosa Bruchmann specially notes that though the two cotyledons may 



'Some previous writers have held the primary axis to run from the stem apex 

 obliquely to the apex of the first loot. Reasons will he shown below for regarding the 

 root as an accessory part, commonly lateral, and not determinate in position. Its 

 growth may in certain cases approximate t<> the original axis of polarity of the shoot, 

 as it does in the embryos of Ferns ; hut this is held to be an occasional and accidental 

 rather than an inherent character, as is indicated by a comparison with the more bulky 

 embryos of Eijnisi-lnin and the Ophioglossaceae. 



