700 PROFESSOR F. O. BOWER ON 



the juvenile leaves of Filicales {Trichomanes, Marsilia) ; it is common in Gymno- 

 sperms, and occurs in som-e Angiosperms. In all such cases the question arises 

 whether the simple structure is really primitive, or the result of reduction. The 

 fact that a consecutive series of ascending steps from these simple types to the 

 most complex, and that such steps are commonly illustrated in more or less complete 

 series in the individual life, as we pass from the juvenile leaf to the adult, seems to 

 indicate quite clearly that there is no difference in kind, but only in degree, between 

 the simple and the branched leaf of the sporophyte ; it also accords with the view 

 that primitive leaves were simple. But it is well to have in view a clear case of 

 simplicity by reduction. It is seen in Platyzoma* In this Fern Thompson has 

 shown that the leaves are dimorphic, the fertile being simply pinnate, the sterile 

 being apparently simple. But careful examination has shown various stages of 

 abortion of the pinnae in the sterile leaf, leading to cases where the pinnae are 

 wholly absent. Thus by abortion a simple leaf may be derived from a pinnate leaf. 

 The simple leaf of a Lycopod or an Equisetum may actually be primitively simple, 

 or the result of a simplification of a similar nature to that of Platyzoma. But 

 whether the one or the other, the simple leaf may be held to be just as much a 

 " phyllome " as are the more complex leaves of Ferns. We may conclude generally 

 that a leaf may be simple or branched. Branching, whether seen in form or only 

 in venation, is not of the essence of foliar nature, but is to be regarded as a means 

 of elaboration of the member, which may be subject again to simplification by 

 suppression of the branches.] 



III. In the great majority of cases the leaf is branched, and the relatively 

 primitive types show that the branching is primarily distal. This may affect the 

 outline of the leaf, and lead to its being more or less distinctly lobed ; or it may be 

 seen only in the venation, and the limb be entire. The relation of these states will 

 be considered later (V). It has already been seen that the opinion expressed in 

 the writings of Hofmeister, Sachs, Prantl, Goebel, and Potonie is that the 

 fundamental type of branching in Fern-leaves is dichotomous.^ The first result of 

 the observations here described is to establish the fact that from all the primitive 

 types of Filicales the branching in the juvenile leaves is dichotomous. In this 

 respect they accord with what is seen in the adult leaves of the Sphenophyllales and 

 of certain archaic Equisetales. Thus it follows that in all the primitive phyla of 

 Vascular Plants where branching occurs, distal dichotomy is the fundamental mode 

 of that branching. 



IV. Dichotomy may be apparent only in the venation, or it may characterise 

 also the configuration of the leaf. Within near circles of affinity both states may be 

 represented. A good example is seen on comparison of Todea superba (fig. 6) and 



* Thompson, Trans. Roy. Soc. Edin., 1916. 



t Compare Lignier, "Equisetales et Sphi'nophyllales," Bull. Soc. Linn, de Normandie, 5 serie, vol. vii, p. 93, 1903. 



I See paragraph on p. 659 as to the use of this term. 



