It is noteworthy that while certain vegetative features may in some cases be cited as family-characters, such features are not usually of much value from a taxonomic point of view. While the typical tree ferns are practically all members of the Cyatheaceae, a few members of other families, e.g. _Todea barbara_ (Osmundaceae) and the monotypic Indian genus _Brainea_ (Polypodiaceae), form erect stems several feet in height; but these differ in appearance from the Palm-like type of the Cyatheaceous tree ferns. On the other hand, the thin, almost transparent, leaf of _Hymenophyllum tunbridgense_ and other filmy ferns is a character shared by several species of _Todea_, _Asplenium resectum_, and _Danaea trichomanoides_ (Marattiaceae); the filmy habit is essentially a biological adaptation. The form of frond represented by certain species of _Gleichenia_, characterised by a regular dichotomy of the axis and by the occurrence of arrested buds, is on the whole a trustworthy character, though _Davallia aculeata_ (bearing spines on its rachis) (fig. 232) and _Matonia sarmentosa_ have fronds with a similar mode of branching and also bear arrested radius-buds. A limited acquaintance with ferns as a whole often leads us to regard a certain form of leaf as characteristic of a particular species, but more extended enquiry usually exposes the fallacy of relying upon so capricious a feature. The form of leaf illustrated by _Trichomanes reniforme_ is met with also in _Gymnogramme reniformis_ and is fairly closely matched by the leaf of _Scolopendrium nigripes_. The fronds of _Matonia pectinata_ (figs. 227, 228) bear a close resemblance to those of _Gleichenia Cunninghami_, _Adiantum pedatum_, and _Cheiropteris palmatopedata_[704]. =The habit, leaf-form, and distribution of Ferns.= The full accounts of the structure and life-history of the common Male Fern, given by Scott in his _Structural Botany_ and by Bower in the _Origin of a Land Flora_, render superfluous more than a brief reference to certain general considerations in so far as they may facilitate a study of fossil types.