Onoclea sensibilis

Onoclea sensibilis L.
(Latin: sensibilis, sensitive, perceptible)

Local names: sensitive fern, bead fern

Rhizomes creeping; petiole bases with 2 vascular bundles; leaves conspicuously dimorphic, of 2 very different types, scattered along the rhizome, erect, glabrous; sterile leaves to ca. 1(–1.3) m tall, thin herbaceous, deciduous, broadly triangular to ovate in outline, deeply pinnatifid, the pinnae few, subopposite (especially the lowermost), undulate to irregularly deeply lobed, the margins entire, the veins reticulate and forming a series of linear to oblong areoles along the midvein (= costa), the rachis winged; fertile leaves persistent over winter, 2-pinnate, the blades greatly reduced, the ultimate blade segments (= smallest subdivisions of blade) rolled into globular, berry-like or bead-like structures concealing the sori, the whole fertile leaf superficially resembling a narrow panicle of small round fruits; 2n = 74 (Johnson 1993b). Swamps, low woods, and wet areas; Pineywoods and n edge of Gulf Prairies and Marshes w through Post Oak Savannah to Milam, Upshur (BRIT), Red River, Henderson, Limestone, Falls (Turner et al. 2003), and Wilson (TAES; Correll 1956) cos.; also disjunct w to Gillespie (O’Kennon 8367, Bear Mt., wet depression e of granite mine, BRIT), Burnet (Correll 1956), Uvalde, and Wilson (Turner et al. 2003) cos.; se Canada and throughout e U.S. w to ND, WY, CO, and TX; also e Asia (the Asian plants are often recognized as var. interrupta Maxim.). Sporophylls produced Apr–Oct, persisting through the winter and releasing the green spores in spring before expansion of the sterile leaves (Johnson 1993b). Both the specific epithet and the common name derive from the sensitivity of this species to cold— the sterile leaves die back with even a light frost (Johnson 1993b). The sterile leaves superficially resemble those of Woodwardia areolata, but that species has alternate basal pinnae, the pinnae lacking lobes (pinnae can be sinuate) but with minutely serrulate margins. The disjunct TX populations may represent survival since Ice Age times in tiny pockets of appropriate moist microhabitats; numerous TX plants display such disjunctions (see page 36). Sensitive fern is native to both e North America and e Asia (Hoshizaki & Moran 2001); see Fig. 34 and page 32 for a discussion of this interesting disjunct distribution pattern. Molecular evidence obtained by Gastony and Ungerer (1997) supported the recognition of North American plants as O. sensibilis var. sensibilis and those of e Asia are as O. sensibilis var. interrupta Maxim. Fifty-seven million year old Paleocene Epoch fossils virtually identical to modern members of this species provide evidence that fern species can remain essentially unchanged over millions of years (Rothwell & Stockey 1991; Serbet & Rothwell 1999). Unusual leaves, intermediate in form between typical sterile and fertile leaves, are known in this species (Thieret 1980), possibly as a result of some type of injury (e.g., frost, defoliation) (Lloyd 1971; Beitel et al. 1981). Because of the unusual appearance of the fertile leaves (greatly reduced blade tissue and bead-like pinnules), they are sometimes gathered for use in dried arrangements. The species has trophopods—petiole bases that are “specialized to accumulate food and to persist as a storage organ long after the frond to which it belonged has withered and decayed away” (Wagner & Johnson 1983). Sensitive fern is reported to be poisonous; horses are said to become unsteady and collapse upon ingesting large amounts of the plant. Other symptoms include walking in circles, difficulty in chewing (with food sometimes falling from the mouth), seizures, and death. The specific cause is not known with certainty but is thought to be thiaminase, an enzyme that destroys thiamine and causes Vitamin B1 deficiency (A number of other pteridophytes are well known to cause thiamine depletion including species of Cheilanthes, Equisetum, Marsilea, and Pteridium [Hodgdon 1951; Burlage 1968; Turner & Szczawinski 1991; Burrows & Tyrl 2001]). Sensitive fern also contains phytoecdysones, a class of hormone-like plant defense compounds that disrupt ecdysis (= molting) in insects—insects eating the plant thus have their developmental sequence altered (Swain & Cooper-Driver 1973).

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