A frond is the leaf- like structure of a fern or alga. The term is colloquially applied to the leaves of palms, cycads, and plants with pinnately compound leaves. A significant difference is that, unlike the leaves of the latter, fern fronds bear the reproductive structures (spore-bearing structures) of the sporophyte plant. Because many ferns grow fronds that are held more vertical than horizontal, the "upper" and "lower" surfaces of a frond are more correctly referred to as the adaxial and abaxial surfaces, respectively.
A fern frond consists of a stipe, the stem supporting the blade, and the blade consists of both a laminar (flattened) photosynthetic tissue and a rachis—that portion of the stem to which the laminar tissue is attached. The blades of fern fronds may vary from being simple (undivided) to being highly dissected, even "lace-like". If the leaf tissue is undissected, or the dissections do not reach to the rachis, the frond may be described as lobed or pinnatifid. Otherwise, the blade is compound and each large division of the laminar tissue arising from the rachis is called a pinna (pl., pinnae). The main vein or mid-rib of a pinna is known as a costa (pl., costae). Pinnae may be arranged along the rachis either directly opposite one another or alternating up the stem. The arrangement may change from the base of a blade to the tip, as in the example of Blechnum shown below (from base to tip: pinnae opposite to alternate, and pinnatisect to pinnatifid).
Many ferns have pinnae that are divided two or more times, and the level of division of the fronds is termed pinnate (or 1-pinnate), or twice-pinnate (2-pinnate), or the like. Each secondary division (division of a pinna) is termed a pinnule, and its mid-vein, a costule. A few species of ferns with divided fronds are not pinnate, but are palmate or bifurcate.
On some or all mature blades (usually on the abaxial surface) occur sporangia, which bear the spores. The sporangia are clustered in a sorus (pl., sori) or "fruit dot". Associated with each sporus in many species is a membranous structure called an indusium: an outgrowth of the blade surface that may partly cover the sporangial cluster. Fronds also may bear hairs or scales, glands, and, in some species, bulblets for vegetative reproduction.
Each frond arises from the stem or rhizome, which in most species is concealed in the ground or creeps along the ground (or branch or rock) surface. Growth of a fern frond differs from that of a leaf of a flowering plant. The fern frond unrolls from a tightly-coiled structure called a "fiddle-head" (see circinate vernation).
Some fern species feature frond dimorphism, in which fertile and sterile fronds differ in appearance and structure.