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 Potonié, 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.! 
Potonié 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 Alosorus (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? 
1 Palacophytologie, pp. 110-121. 2? Compare Kny, Parkeriaceen, Taf. xxiv. 
>Compare Goebel, Organography, p. 317- 
