Leaf Shapes

The leaf (also known as frond) of ferns is the part of the plant most readily visible to observation. The leaf plan in practically all ferns is pinnate—that is, featherlike with a central axis and smaller side branches—and this is considered to be the primitive condition because of its widespread occurrence. From this basic type a broad diversity of forms evolved. Some ferns have palmate leaves (with veins or leaflets radiating from one point), and some, such as the staghorn ferns, have secondarily evolved falsely dichotomous leaves. In some genera (e.g., Lygodium and Salpichlaena) the main leaf axis (rachis) twines about on shrubs and small trees, sometimes reaching 20 metres (65 feet) in length.

Whether a given leaf is divided into segments (compound) or is undivided (simple) is of considerable value in identification of similar fern species. The difference between divided and undivided leaves is not a profound one, however, and closely related species commonly differ from one another in this respect.

The extent of division in fern leaves, or fronds, ranges from those in which the leaf margins are merely so deeply lobed as to have narrow-based segments to those having obviously stalked leaflets, or pinnae. The pinnae themselves may also be lobed or truly divided with stalked segments; and the resulting segments, the pinnules, may also be lobed or divided. Depending on the degree of cutting, fronds are described as simple, once divided, twice divided, thrice divided, and so on. Some ferns are known in which the fronds are five times compound, making them exceedingly delicate, with segments so small as to be almost hairlike.