628 GENERAL COMPARISON OF THE FILICALES 



distal branchings. Moreover, the prevalence of dichotomy in the venation 

 of Ferns at large is to be taken into account in this connection. Lastly, 

 dichotomy is a common feature in the first leaves of Fern-seedlings, and 

 is seen, probably as an occasional reversion, abnormally in the later leaves 

 of many Ferns, being sometimes a persistent character of varietal forms. 

 These facts suggest the enquiry as to the branching of the leaves of the 

 early Ferns : it has been pursued by Potonie, who finds among the early 

 fossils, and even among those of Pecopterid-type, evidences of dichotomy 

 which lead him to conclude that the truly pinnate type of leaf-construction 

 in all its parts originated phylogenetically from the true dichotomy. 1 



Potonie strengthens his position by noting certain palaeontological facts. 

 The Archaeopterids from the Devonian, Culm, and lower Carboniferous 

 have no midrib in their ultimate pinnules, but are characterised by parallel 

 veins, forked in a fan-like manner. In higher strata, however, a midrib 

 with lateral veins is found. Reticulate venation was apparently absent from 

 the Ferns of the Culm, and appeared in the Middle Carboniferous, while 

 the higher type of reticulation, with areas of smaller meshes filling up the 

 meshes of a larger reticulate system, occurs first in the Mesozoic period. 

 The fossil record would thus support the early prevalence of dichotomy, 

 so far as venation is concerned. 



Before accepting Potonie's conclusion an examination of the development 

 of the apparently pinnate type in living Ferns is necessary. In 1875 Kny 

 showed that in Ceratopteris the lower pinnae arise alternately below the 

 leaf-apex, the branching being monopodial, and without individual relation 

 to the segments of the apical cell. 2 This origin of the lower pinnae has been 

 verified also in other Ferns, and may be taken as the usual type where the 

 leaves are elongated and the lateral parts numerous. But it is to be noted 

 in such cases .that the pinnae themselves may branch dichotomously, that 

 towards the apex of the leaf there may be a gradual transition to a dichoto- 

 mous branching, the pinnae being then produced sympodially after the 

 scheme seen in the pinnae of Allosorus (Fig. 248) ; and that in all cases 

 the pinnae arise in strict relation to the lateral wings or flanges of the leaf. 

 For, however much disguised by special developments at the base of the 

 leaf, or by the bulk of the leaf-stalk in proportion to the wings, still every 

 Fern-leaf is essentially a dorsiventral structure, with margins which may or 

 may not be developed as projecting wings, but can commonly be traced even 

 down to the base of the leaf-stalk ; and it is upon these that the pinnae 

 originate. The general facts may be summed up thus : that the branches 

 arise marginally on the flattened leaf; that where the leaf is massive and 

 greatly elongated the lateral parts are laid down monopodially, but where 

 the surface-growth predominates there is dichotomous branching without 

 the formation of a strong midrib ; but the one type may pass into the other 

 in the length of a single leaf. 3 



1 PalaeophytologiC) pp. 110-121. 2 Compare Kny, Parkeriaceen, Taf. xxiv. 



3 Compare Goebel, Organography , p. 317- 



