Frond

Frond refers to the leaf structure of ferns. The term is colloquially applied to the leaves of palmss, cycads, and other 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. A fern frond consists of a stipe: the stem supporting the blade and the blade consists of laminar (flattened) phoyosynthetic 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. Otherwise, the blade is compound and each large division of the laminar tissue arising from the rachis called a pinna' (pl., pinnae). Many ferns have pinnae that are divided two or more times, and the level of division of the fronds is termed pinnate, or twice-pinnate, or the like. In cases where the pinna is divided into lobes, it is termed pinnatifid. A few species of ferns with divided fronds are not pinnately divided, but are palmate or bifurcate. On some or all mature blades (usually on the underside) occur sporangia, which bear the spores. The sporangia are clustered in a sorus (pl., sori) or "fruit dot". Associated with each sorus in many species is a mebranous 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 surface (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).