Equisetum ×ferrissii

Equisetum ×ferrissii Clute
(for James H. Ferriss, 1849–1926, Illinois journalist and field biologist with interests in land snails, fossils, ferns, and cacti; at one time president of the Amer. Fern. Soc.; see Clute 1926)

Local names: Ferriss’ horsetail, Ferriss’ scouring-rush, intermediate horsetail

Plant vegetatively variable but typically intermediate between the two parental species; aerial stems with basal part over-wintering, monomorphic, unbranched (unless damaged or abnormal), 20–180 cm tall, slightly roughened; leaves 14–32 per node (number evident as teeth of sheaths), the teeth promptly shed or persistent and light in color; cones typically failing to open, the apex somewhat pointed, typically with a small apiculum; spores misshapen, white (in contrast to the green spores of the other three taxa of Equisetum in TX). Stream banks, other moist areas; scattered primarily in w 2⁄3 of TX (solid dots on the county distribution map represent specimens identified by J. Peck and G. Diggs [e.g., Randall Co., L. Pace 149; Tarrant Co., J. Quayle et al. 749; Uvalde Co., L. & C. Taylor 33749, all BRIT] or for which spores are clearly aborted [e.g., Culberson, El Paso, and Jeff Davis cos., TEX-LL], while open circles are herbarium and literature records identified by other authorities—see discussion under E. hyemale); widespread in the U.S. and s Canada except absent from much of se U.S.; also n Mexico. Cones present late spring-early summer but not shedding spores. [E. hyemale var. elatum (Engelm.) C.V. Morton, E. hyemale var. intermedium A.A. Eaton] Hauke (1960, 1993) considered this taxon to be a hybrid between E. hyemale and E. laevigatum; molecular evidence confirms its hybrid origin (Des Marais et al. 2003). The cones often fail to open apparently because the central axis does not elongate to expose the sporangia; the sporangia therefore do not dry, split, and release the misshapen, white, abortive spores (PlantSystematics.org 2008). This hybrid has sometimes been treated as a variety of E. hyemale or has been mistaken for either E. laevigatum or E. hyemale; although sterile sexually, it can be found without the parental species and is apparently dispersed vegetatively (Hauke 1963, 1993). Des Marais et al. (2003) described it as “widespread and prolific.” In Mexico it is reported to be “nearly as common as its parents” (Mickel & Smith 2004) and in AR it has been found in every county (Peck 2011a). Likewise in TX, E. ×ferrissii is widespread and common; in fact, we have seen more specimens of what are apparently the hybrid than of the parental species. However, young stems, anomalous regrowth, or incomplete specimens of E. hyemale and E. laevigatum can often not be definitely separated from E. ×ferrissii (when lacking spores and cones); populations in the wild are much more easily identified since complete material is available. This hybrid is most easily distinguished from the two parental species (with greenish spherical spores and cones that open) by its white misshapen spores and cones which fail to open; it can also be distinguished by its slightly rough stems (when a fingernail is scraped lengthwise along the stem), sheaths of intermediate length (between the more elongate sheaths of E. laevigatum and the shorter “stubby” sheaths of E. hyemale), by the somewhat pointed cone apex typically with a small apiculum, and by the often persistent sheath teeth that are rather light in color. Yatskievych and Windham (2008c) noted that “The morphology of the more-or-less evergreen aerial stems is quite variable, but they are generally at least slightly rough to the touch and the stem sheaths sometimes resemble those of E. hyemale toward the stem base and those of E. laevigatum toward the tip.” Both parental species are poisonous, thus this hybrid should be considered so as well.

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