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.
In size Ferns have a wide range: at the one extreme we have the filmy fern Trichomanes Goebelianum[705], growing on tree stems in Venezuela, with leaves 2·5 to 3 mm. in diameter, and at the other the tree ferns with tall columnar stems reaching a height of 40 to 50 feet and terminating in a crown of fronds with a spread of several feet. A common form of stem is represented by the subterranean or creeping rhizome covered with ramental scales or hairs: the remains of old leaves may persist as ragged stumps, or, as in Oleandra, Polypodium vulgare and several other species, the leaf may be cut off by the formation of an absciss-layer[706] leaving a clean-cut peg projecting from the stem. As a rule the branches bear no relation to the leaves and are often given off from the lower part of a petiole, but in a few cases, e.g. in the Hymenophyllaceae, it is noteworthy that true axillary branching is the rule[707]. In the typical tree-fern the surface resembles that of a Cycadean trunk covered with persistent leaf-bases and a thick mass of roots. Among epiphytic ferns highly modified stems are occasionally met with, as in the Malayan species Polypodium (Lecanopteris) carnosum and P. sinuosum[708].
The leaves of ferns are among the most protean of all plant organs; as Darwin wrote, “the variability of ferns passes all bounds[709].” The highly compound tri- or quadripinnate leaves of such species as Pteris aquilina, Davallia and other genera stand for the central type of fern frond; others exhibit a well-marked dichotomy, e.g. Lygodium, Gleichenia, Matonia, etc., a habit in all probability associated with the older rather than with the more modern products of fern evolution. Before attempting to determine specifically fossil fern fronds, it is important to familiarise ourselves with the range of variability among existing species and more especially in leaves of the same plant. A striking example of heteromorphy is illustrated in [fig. 233]. Reinecke[710] has figured a plant of Asplenium multilineatum in which the segments of the compound fronds assume various forms. In Teratophyllum aculeatum var. inermis Mett., a tropical climbing fern believed by Karsten[711] to be identical with Acrostichum (Lomariopsis) sorbifolium,—an identification which Goebel[712] questions,—the fronds which stand free of the stem supporting the climber differ considerably from the translucent and much more delicate filmy leaves pressed against the supporting tree. From this fern alone Fée is said to have created 17 distinct species. In this, as in many other cases, differences in leaf-form are the expression of a physiological division of labour connected with an epiphytic existence. Some tropical species of Polypodium (sect. Drynaria), e.g. P. quercifolium ([fig. 234] and [fig. 231], D), produce two distinct types of leaf, the large green fronds, concerned with the assimilation of carbon and spore-production, being in sharp contrast to the small slightly lobed brown leaves which act as stiff brackets (fig. 234, M) for collecting humus from which the roots absorb raw material. Similarly in Platycerium the orbicular mantle-leaves differ widely from the long pendulous or erect fronds fashioned like the spreading antlers of an elk. In Hemitelia capensis, a South African Cyatheaceous species, the basal pinnae assume the form of finely divided leaves identified by earlier collectors as those of a parasitic Trichomanes ([fig. 235]). In a letter written by W. H. Harvey in 1837 accompanying the specimen shown in [fig. 235], he says, “Apropos of Hemitelia, be it known abroad that supposed parasitical Trichomanes ... is not a parasite, but a part of the frond of Hemitelia.” The delicate reduced pinnae remain on the stem and form a cluster at the base of the fronds[713].
Fig. 233. Polypodium Billardieri Br. (¼ nat. size.) Middle Island, New Zealand. From specimens in the Cambridge Herbarium.
Fig. 234. Polypodium quercifolium. (Much reduced: M, Mantle-leaves.)
In many species the sporophylls are distinguished from the sterile fronds by segments with little or no chlorophyllous tissue, as in Onoclea struthiopteris[714] in which, each year, the plant produces a funnel-shaped group of sterile leaves followed later in the season by a cluster of sporophylls; or, as in many other genera, the fertile leaves are distinguished also by longer petioles and thus serve as more efficient agents of spore-dissemination. In Ceratopteris the narrow segments of the taller fertile leaves are in striking contrast to the broader pinnules of the submerged foliage leaves. Leaf-form is in many cases obviously the expression of environment; the xerophilous fern Jamesonia[715] from the treeless paramos of the Andes[716] is characterised by its minute leaflets with strong revolute margins and a thick felt of hairs on the lower surface; in others, xerophilous features take the form of a covering of overlapping scales (Ceterach), or a development of water-tissue as in the fleshy leaves of the Himalayan fern Drymoglossum carnosum. In the Bracken fern Boodle[717] has shown how the fronds may be classed as shade and sun leaves; the former are spreading and softer, while the latter are relatively smaller and of harder texture ([fig. 236], a and b). Even in one leaf six feet high, growing through a dense bush of gorse and bramble, the lower part was found to have the features of a shade leaf, while the uppermost exposed pinnae were xerophilous.
