Dipteridinae. (Dipteris.)

The genus Dipteris, formerly included in the Polypodiaceae, has been assigned to a separate family partly on account of the slight obliquity of the vertical annulus ([fig. 231], G) and on other grounds[702]. The four species Dipteris conjugata, D. Wallichii, D. Lobbiana (= D. bifurcata), and D. quinquefurcata ([fig. 231]) are characterised by a creeping rhizome bearing fronds reaching a length of 50 cm.; in D. conjugata and D. Wallichii the lamina is divided by a median sinus into two symmetrical halves, while in other species the leaf is dissected into narrow linear segments. The main dichotomously branched ribs are connected by lateral branches and these by tertiary veins, the delicate branches of which end freely within the square or polygonal areolae ([fig. 231], A′, E). The naked sori are composed of numerous sporangia and filamentous hairs: while in some species the soral development conforms to that characteristic of the Mixtae, it has been shown that in one species, D. Lobbiana (= D. bifurcata[703]), the sporangia develope simultaneously as in the Simplices. Dipteris occurs in company with Matonia on Mt Ophir and elsewhere in the Malay peninsula; it extends to the Philippines, Samoa, New Caledonia, China, New Guinea, and the subtropical regions of Northern India.

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The impossibility of drawing a hard and fast line between the divisions adopted in any system of classification is well illustrated by the ferns. In the main, the three-fold grouping suggested by Bower is probably consistent with the order of evolution of the true ferns. The Polypodiaceae, which are now the dominant group, are in all probability of comparatively recent origin, while the Gradatae and Simplices represent smaller subdivisions with representatives in remote geological epochs. The genera Loxsoma, Matonia and Dipteris afford examples of ferns exhibiting points of contact with more than one of Bower’s subdivisions: they are generalised types which, like many relics of the past, are now characterised by a restricted geographical range.

RECENT FERNS

Fig. 232. Davallia aculeata. (⅖ nat. size.)

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].