mer is terns of Ferns as a Phylogenetic Study. 341 
On the leaf, when mature, the more or less largely developed 
wings may be traced upwards from the extreme base (e.g. 
various species of Trichomanes , Aspidium , P ter is, Cyathea , 
and in fact the majority of leptosporangiate Ferns), and 
along the whole margin of the frond, and extending over all 
the pinnae and pinnules; among the higher terms of the 
Filicineous series, however, this continuity, as seen in the 
mature state, appears at first sight to be broken, especially 
in the lower portions of the frond, where the phyllopodium 
often appears cylindrical ( Osmunda , Todea , Marattia, and 
Angiopteris , etc.), there being only such slight traces of the 
winged character as those noted as occurring on certain 
rhizomes. Again, there are formed in connection with the wings, 
at the base of the leaves of some of the higher Ferns, sheath- 
like and ‘stipular’ growths (Osmundaceae, Marattiaceae) : 
these also are more or less clearly continuous with the wings, 
and are to be regarded as the result of their peculiar develop- 
ment. That these basal growths, as well as those lateral 
lines, often discontinuous and but slightly marked, which are 
seen running up the leaf-stalk, are really metamorphosed 
wings, cannot be doubted, when it is seen that they may be 
traced upwards, and be seen to merge gradually into the 
typical wings of the upper portion of the leaf. Here also, 
as in the leaves of Angiosperms, a certain rough correlation 
is to be recognised : where the development, be it of leaf 
or axis, is robust, as a supporting organ, the wings are of 
a relatively reduced type: where the central phyllopodium 
is less bulky, there the wings are as a rule more largely 
developed. 
The prevalence of wings on the leaves of Ferns is to be put 
in relation with what is known of the development : the leaf 
is a bilateral structure, and these wings are its margins : they 
are derived in the leptosporangiate Ferns from a marginal 
series of cells which is continuous over the apex, and thus at 
least in the simplest Ferns (the Hymenophyllaceae) the leaf is 
typically but one layer of cells in thickness at its apex : but 
immediately below the apex begins that thickening which 
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