Frond
A frond is the
breast- 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 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 photosynthetic tissue and a rachis—that portion of the stem to which the laminar tissue is attached.
Encyclopedia
A
frond is the
breast- 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 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 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 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 . The main vein or mid-rib of a pinna is known as a
costa . 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 .
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. Each secondary division 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 occur
sporangia, which bear the
spores. The sporangia are clustered in a
sorus or "fruit dot". Associated with each sorus 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 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" .
