keys to the vascular plants of east texas - Botanical Research ...

350 DRYOPTERIDACEAE/POLYSTICHUM
(Turner et al. 2003), and Wilson (Correll 1956; TAES) cos.; also e Edwards Plateau; se Canada
and throughout e U.S. w to ND and TX. Sporophylls produced Apr–Oct, persisting through the
winter and releasing the green spores in spring before expansion of the sterile leaves (Johnson
1993b). The common name is in reference to the sensitivity of the leaves to even a light frost
(Johnson 1993b). The sterile leaves superficially resemble those of Woodwardia areolata, but
that species has alternate pinnae with minutely serrulate margins. SENSITIVE FERN is native to
both e North America and e Asia (Hoshizaki & Moran 2001); see generic synopsis of
Brachyelytrum (Poaceae) for a discussion of this interesting disjunct distribution pattern. Fiftyseven
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, possibly as a result of some
type of injury (e.g., frost, defoliation) (Lloyd 1971; Beitel et al. 1981). This species 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 (Burlage 1968; Turner & Szczawinski 1991; Burrows & Tyrl 2001). �
POLYSTICHUM Roth CHRISTMAS FERN, SWORD FERN, HOLLY FERN
AA cosmopolitan genus of ca. 180 species (Wagner 1993). Cyrtomium is related to Polystichum
and some authorities lump it into Polystichum (e.g., Mabberley 1997). (Greek: poly, many, and
stichos, row, presumably in reference to the rows of sori on each pinna)
REFERENCES: Wagner 1993; Noodén & Wagner 1997; Schneider & Carlquist 1997.
Polystichum acrostichoides (Michx.) Schott, (resembling Acrostichum, leather ferns, another genus
of ferns with many crowded sori—Nelson 2000), CHRISTMAS FERN, DAGGER FERN. Stems
(rhizomes) erect; leaves evergreen, clustered, to 70 cm long, the blades elliptic-lanceolate to lanceolate
in outline, 1-pinnate; pinnae mostly alternate, auricled basally, the auricle on the
acroscopic side (= side toward the leaf apex) of the pinna, the margins bristle-toothed; petioles
densely scaly; leaf blades partially dimorphic, the proximal pinnae (those near blade base) sterile,
the distal pinnae (those near blade tip) of some blades fertile and conspicuously contracted
(but blade tissue still evident); sori round, crowded in 2–4 rows, medial, often confluent at maturity;
indusia peltate, entire, persistent. Rich wooded slopes, moist areas; Pineywoods and Post
Oak Savannah w to Anderson (Turner et al. 2003), Waller, Wood (Correll 1956; TAES), and Red
River (BAYLU, BRIT) cos.; also n Gulf Prairies and Marshes; se Canada and throughout e U.S. w
to MN and TX. Sporulating May–Nov. According to Correll (1956), “In some areas the evergreen
fronds are gathered for decorative greens at Christmas time, hence the common name.” Dunbar
(1989) also noted that the prominent “ear” (auricle) at the base of each of the pinnae causes the
pinnae, when held vertically, to resemble Christmas stockings. Experimental evidence (Noodén
& Wagner 1997) indicated that the leaves being green in winter is beneficial. The two hypotheses
for this benefit are: 1) extension of the period of photosynthesis into the winter, and 2) nutrient
storage in the old leaves requiring only a single transfer of nutrients to the new leaves (versus
two transfers if storage is in the rhizomes).
TECTARIA Cav. HALBERD FERN
AA mostly tropical genus of ca. 200 species (Moran 1993b); some are cultivated as ornamentals.
(Latin: tectum, roof, and aria, a suffix, in reference to the roof-like indusium of some species—Moran
1993b)
REFERENCES: Morton 1966; Moran 1993b.