660 PROFESSOR F. O. BOWER ON 



wachsthum). Where the growth is apical the origin of the pinnae may be mono- 

 podial, as already shown in Ceratopteris by Kny, and exemplified by Goebel in 

 Adiantum Edgeivorthi* Where the marginal growth has supervened the branch- 

 ing becomes dichotomous. This has been summed up by Goebel in the following 

 words : " In all Ferns the lateral primordia of pinnae appear on the primordium of 

 the leaf, and if the leaf is a greatly elongated one the lateral parts are laid down in 

 rapid succession ; but where surface-growth predominates, then there is dichotomous 

 branching, and there is no formation of a strong leaf-spindle or midrib." 



Such facts suggest a phyletic meaning. Bearing on this question Goebel 

 recognises three stages of leaf-development in Ferns : (i) marginal growth and 

 dichotomous branching ; (ii) apical growth, with monopodial branching, but later 

 marginal growth and dichotomous branching ; (iii) persistent apical growth, forming 

 long-growths and short-growths, the latter limited, as in Gleichenia and Lygodium. 

 He remarks that " whether this series forms an ascending or a descending one, or 

 whether we must recognise it as simply one construction, cannot certainly be 

 determined. What is certain is that the configuration of the primordium of the 

 leaf is connected with that of the mature condition in the manner that has been 

 indicated above." The facts and comparisons to be brought forward in this memoir 

 will help to resolve this question. It may here be briefly stated that they are 

 believed to indicate that the marginal growth and dichotomous branching represent 

 the primitive foliar development ; that the stronger apical growth with prevalent 

 monopodial branching is a derivative state, which is absent from many juvenile 

 leaves, and, though commonly present in the adult, it lapses into the primitive 

 dichotomy in the distal region. Already there is much evidence from anatomical 

 sources which indicates that in Fern leaves the apex and base show more primitive 

 characters than the middle region. It will be important to keep this conception in 

 mind when examining the leaf-constructions to be detailed below, f 



Goebel describes the juvenile leaves as arrested members (Hemmungsbildungen)4 

 But if the comparative evidence leads to the conclusion above stated, this would 

 appear to be an inversion of the true evolutionary conception of the leaf, as 

 exemplified in the ontogeny. A more probable thesis would be that the juvenile 

 leaf is not arrested, but is something similar to the primitive and fundamental type ; 

 while the adult leaf is a promoted condition, and its higher development is consequent 

 upon a phyletic advance in complexity comparable to that seen in the ontogeny. 

 But the later and more elaborate leaves may be subject to arrest due to various 

 causes, which throws them back to such a state as is seen in the juvenile leaves. 

 This is the theoretical position which will be entertained in this memoir, and it 

 will be seen later how far it is in accord with the facts to be described. 



* Organography, Engl. ed.,ii, p. 317, fig. 20. 



t E. W. Sinnott, Ann. of Hot, 1911, p. 167 ; and other writers. 



J Flora, 1 889, p. 28 ; Vergleichende Entwickelungsgeschichte, p. 254 ; Organographie, 2te Autlage, p. 378. 



