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

332 EQUISETACEAE/EQUISETUM
DIVISION EQUISETOPHYTA
HORSETAILS
AThis is a very ancient group consisting of a single extant family. Fossil forms date to the Devonian
Period (408–360 million years ago), and the division reached its maximum diversity and
abundance in the Paleozoic Era. They were components of the Carboniferous Period’s (360–286
million years ago) swamp forests that formed present-day coal deposits; some reached the proportions
of trees (to 18 m tall) and were probably competitors of the tree Lycopodiophyta. The
largest living species is the tropical Equisetum giganteum L., which may exceed 5 m in height
(Bell & Woodcock 1983; Raven et al. 1986; Bold et al. 1987). The division is sometimes referred to
as the Arthrophyta (Woodland 1997) or the Sphenophyta (Raven et al. 1986). The morphologically
distinct Equisetophyta are characterized by whorled microphylls (= leaves with a single
vein) and hollow, jointed, green stems. Some species have numerous small branches and bear a
slight resemblance to a horse’s tail—this, however, is not the true derivation of the common or
scientific names as is often thought (note derivation of scientific name discussed in the generic
synopsis of Equisetum). The evolutionary relationships of the group have been unclear, with
recognition as a division reflecting the extreme morphological distinctiveness of Equisetum.
However, recent phylogenetic studies indicate that Equisetum is embedded within the ferns,
perhaps mostly closely related to the Marattiales (an ancient order extending back 340 mya,
and often considered to be the second most basal group of living living ferns) (Pryer et al. 2001;
Moran 2004). However, because of their morphological distinctiveness, and until this relationship
is confirmed, we are tentatively following traditional treatments and recognizing the
Equisetophyta as a separate division.
REFERENCES: Bell & Woodcock 1983; Raven et al. 1986; Bold et al. 1987; Wagner & Smith 1993;
Woodland 1997; Pryer et al. 2001.
EQUISETACEAE Michx. ex DC.
HORSETAIL FAMILY
AThe family is represented in the modern world only by the distinctive genus Equisetum. It
contains ca. 15 species (Hauke 1993) and is nearly cosmopolitan, being absent only from Australia
and New Zealand (Hoshizaki & Moran 2001). While most botanists accept only a single
genus (e.g., Hauke 1993), some (e.g., Skoda 1997) would segregrate a number of species into the
genus Hippochaete. That genus, more commonly recognized as subgenus Hippochaete (SCOUR-
ING-RUSHES), is distinguished by its sunken stomates, versus stomates flush with the epidermis
in subgenus Equisetum (HORSETAILS) (Hauke 1963, 1993; Skoda 1997). Both East TX species are
in subgenus Hippochaete.
FAMILY RECOGNITION IN THE FIELD: plant body consisting primarily of hollow, jointed, green
stems; leaves inconspicuous, scale-like, in whorls at the very distinct nodes; sporangia in small,
terminal cones.
REFERENCES: Correll 1949, 1956, 1966a; Skoda 1997, Hauke 1960, 1963, 1978, 1990, 1993; Guillon
2004.
EQUISETUM L. HORSETAIL, SCOURING-RUSH
Plants perennial, rhizomatous; stems hollow in center, with a series of smaller canals, jointed
with very distinct nodes, ridged, green and photosynthetic, usually unbranched (in East TX
species) but some lateral branches forming if the apex is injured; leaves small, inconspicuous,
whorled, scale-like, fused into sheaths surrounding the nodes but with tips free and tooth-like;
sporangia on the undersurface of peltate sporophylls arranged in discrete terminal strobili (=
cones); spores of 1 kind (plant homosporous).