Comparisons With Leaves Of Other Plant Groups

The fern leaf, or pteridophyll, differs from the “true leaf” (euphyll) of the flowering plants in its vernation, or manner of expanding from the bud. In most ferns, vernation is circinate; that is, the leaf unrolls from the tip, with the appearance of a fiddlehead, rather than expanding from a folded condition. It also differs in its venation, which usually is free or simply reticulate rather than being highly complex and made up of areolae containing numerous branched, free-ending veinlets (except in certain specialized genera).

Fern leaves (except in the horsetails, Equisetum) differ from the leaves (sphenophylls) of conifers in that fern leaves usually display a well-developed central midrib with lateral vein branches rather than a dichotomous, midribless pattern or a simple vein in a narrow, needlelike, or straplike leaf. Although a few ferns that have narrow leaves also have only a single central vascular strand (e.g., certain species of Schizaea), they can usually be distinguished readily from the scalelike or awl-like leaves (microphylls) of club mosses on the basis of other characteristics, such as the position of the sporangia and the mode of leaf development. A few genera of ferns (e.g., sword ferns, Nephrolepis; Jamesonia; Salpichlaena; and climbing ferns, Lygodium) have members with more or less indeterminate (i.e., continuous) leaf growth accomplished by periodically quiescent buds. Fern leaves, however, are mostly determinate; that is, they stop growing when they reach maturity. In most ferns leaves grow from apical cells, and these delicate embryonic cells are protected by the curled-over spiral of the crosier (unrolling leaf tip) and by trichomes or scales. When the blade formation is complete, there is no longer an embryonic tip.

In overall length, mature leaves vary from 1 or 2 mm (0.04 or 0.08 inch) in certain filmy ferns (Hymenophyllaceae) to 30 or more metres (100 feet; family Gleicheniaceae). In terms of overall size, the most massive frond is that of the elephant fern (Angiopteris), with fronds more than 5 metres (16 feet) long and petioles 15 cm (6 inches) in diameter.