Onoclea sensibilis, the sensitive fern, also known as the bead fern, is a coarse-textured, medium to large-sized deciduous perennial fern. The name comes from its sensitivity to frost, the fronds dying quickly when first touched by it. It is sometimes treated as the only species in Onoclea, but some authors do not consider the genus monotypic.
Description The sterile and fertile fronds of Onoclea sensibilis have independent stalks originating from the same rhizome, quite different from other ferns. The bright, yellow-green trophophylls (sterile fronds) are deeply pinnatifid and are typically borne at intervals along the creeping rhizome. The sterile fronds are deciduous with trophopods, swollen bases, that serve as over winter storage organs. The sterile fronds of O. var. sensibilis have a length of 1–1.3 m (3.3–4.3 ft) with 5–11 pinnae, leaf pairs, evenly spaced along the stipe. O. var. interrupta Maxim. fronds are shorter, 20–50 cm (7.9–19.7 in) long, with fewer pinnae, only 5–8 pairs.
The sporophylls (fertile fronds) are smaller, 20–45 cm (7.9–17.7 in) in length, non-green at maturity and have very narrow pinnae. They are persistent, standing 2–3 years. The sori comprise clusters of sporangia (spore cases) 2–4 mm (1/10–1/6 in) in diameter, like beads, on upright fertile fronds, hence the common name Bead fern. Sori are typically bilaterally symmetrical, though leaf forms have been observed with pinnae fertile only on a single side of the rachis. This form, named O. sensibilis L. F. hemiphyllodes (Kiss & Kümmerle, 1926) and a second, O. sensibilis L. F. obtusilobata having flat pinnules (not curled or bead shaped), were deemed to be variations not meriting taxonomic recognition (J. M. Beitel et al. 1981).
The fiddleheads have a pale reddish color.
Morphology The spermatogenesis process spans formation of spermatogenous cells to the release of spore. In homosporous ferns, like O. sensibilis L., developing spermatids are surrounded by two different walls at specific development stages, as opposed to a single wall reported in other species. Other differences include a delayed formation of the osmiophilic crest and during sperm release the cap cell removes intact, as opposed to forming a pore or collapsing altogether. Spores are monolete with the antheridium, or sporangium, containing either 32 or 64 sperm spores, usually being 64. Regardless of the number, the capsule's volume remains nearly the same. The mechanics of spore release and its timing are controlled by springtime humidity. The small fertile margins, that in live-form held spore in tightly rolled structures, maintain their dry, leathery shape over winter. These pinnules respond to spring's higher humidity by opening, releasing their spore into the air. Subsequent gametophytes are unisexual in early development, favoring cross-fertilization, later becoming bisexual to ensure species survival.
Taxonomy
Taxonomic history
Species The genus Onoclea was cast by Carl Linnaeus in 1751, separating from the fern's prior association with the Angiopteris genus. The binominal name, Onoclea sensibilis, was published in his 1753 Species Plantarum.
Varieties Onoclea sensibilis has two geographically disjunctive varieties. Onoclea sensibilis var. sensibilis is native to North America; Canada's central and eastern regions and the United States' north, central and eastern regions. Onoclea sensibilis var. interrupta Maximowicz (aka Maxim.) is native to Southeast Siberia, Japan and China. The varietal difference is their ultimate height, O. var. interrupta Maxim. only reaching half the height of its sister.
Alternate names Regional colloquial names for Onoclea sensibilis, the sensitive fern, focus on its characteristics.
bead fern, an alternate name based on its fertile beaded pinnae (leaflets) bolletjesvaren; Dutch, meaning "ball fern" druebregne; Danish, meaning "grape fern" dwa’hũdes gananitsga’kwaʼ; Cayuga, meaning "deer, what they lie on" harilik pärljalg; Estonian, meaning "common pearl leg" helmisaniainen; Finnish, meaning "mother of pearl" pärlbräken; Swedish, meaning "pearl bracts" unì·suwεkwaʼ; Onondaga, meaning "bait"
Etymology Onoclea sensibilis' name was descriptive. Onoclea comes from the Greek onos, meaning a vessel, and kleio, meaning to close, describing the closely rolled sori on its fertile fronds. Its species, from the Late Latin sensibilis, means sensitive, describing its high sensitivity to autumn's first frost and to drought.
Distribution and habitat Onoclea sensibilis is native to Northern Hemisphere temperate regions; the Russian Far East, China and Eastern Asia, and a wide native distribution in Northern America. It ranges from Newfoundland south to Florida and west to Texas, the Rocky Mountains, North and South Dakota, Quebec, and Manitoba. It has become naturalized in western Europe and New Zealand. Onoclea sensibilis can be found at elevations from sea level up to 1,500 metres (4,900 ft) in fresh water habitats, not brackish, as its spore germination ceases at salt (NaCl) levels ≥ 0.6%, moderately saline water and higher. Onoclea sensibilis grows best in moist shaded or partially shaded areas, dwelling in a variety of swamp and wood habitats: wet meadows, thickets and bogs, as well as stream and riverbanks and roadside ditches. It tolerates extremely wet soils, appearing in soggy ground or at the very edge of water in shade or sun. The plant can tolerate dryer conditions in shade. It prefers acidic (pH <6.8), loose, sandy to loam, limestone-based soils.
Ecology
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