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