The 15 living species of the
genus Equisetum comprise the plants commonly known
as horsetails. The genus name is derived from the Latin equis,
meaning horse and seta, meaning bristle, in reference to the
coarse black roots of Equisetumfluviatile which
resemble a horse's tail ( Hauke,
1993 ). The horsetails range in size from the
diminutive E. scripoides (stems averaging 12.9 cm tall and 0.5 -
1.0 mm diameter) (Hauke, 1963) to the
giant horsetails, E. giganteum and E. myriochaetum, reaching
heights of 8 or more meters (Hauke, 1963)
and stem diameters of perhaps 4 cm (see "How large
can giant horsetails become"). Equisetum species
are vascular plants which reproduce sexually by means of spores which
are borne on cones. Hence, together with the other spore-bearing
vascular plants, the Lycophytes (club mosses), Psilophytes
(whisk ferns) and Pterophytes (true ferns), Equisetum
species are classified as pteridophytes. The genus Equisetum
is the only remaining representative of the once abundant and diverse
subdivision Sphenophytina. However, recent
phylogenetic studies suggest
that perhaps should be classified within the true ferns (
Pryer et al., 2001 ). Remarkably,
Equisetum has a history stretching back to the Cretaceous and
possibly as far back as the Triassic ( Hauke, 1978 ).
As a result, Equisetum may perhaps be the oldest living genus
of vascular plants ( Hauke, 1963
).
Morphology
of Equisetum (for more detailed information, photographs, and
illustrations of Equisetum morphology and anatomy see Dr. David Webb's
excellent tutorial
on Equisetum)
All Equisetum species
are herbaceous perennials. The plants consist of upright
aerial stems which arise from a very extensive underground rhizome
system ( Hauke, 1963
). Morphologically, the genus Equisetum is characterized
by jointed aerial stems and jointed rhizomes. The stems
of horsetails are "anatomically
[...] unique among plants" (Niklas,
1997) although they have an external appearance somewhat reminiscent
of bamboo. The upright aerial stems exhibit a monopodial branching
pattern, having one main axis of growth. This is the pattern which
is also found in most gymnosperms and angiosperms ( Scagel, et. al. 1984
). Equisetum species also have small microphyllous
leaves that are arranged in true whorls (Rustishauser, 1999) and the leaves of
each whorl are fused together to form a cylindrical sheath around
each node (Hauke, 1993
). Some, but not all, species form whorls of lateral branches
at the nodes of the aerial stems ( Hauke, 1993
). Unlike many other vascular plants (such as gymnosperms, angiosperms
and some ferns) which produce branches in the axils of leaves,
the leaves of Equisetum alternate with branches at each node
( Scagel et. al., 1984
). Like other vascular plants, Equisetum produce
new branches and leaves from the apical meristem. However,
most of the stem lengthening is produced by intercalary meristems
above each node and this produces rapid lengthening of the stem ( Stewart and Rothwell, 1993 ).
The aerial stems, but not the rhizomes, of some species die back
seasonally, whereas other species are evergreen. The rhizomes
have the same general morphology as the upright stems, although the
rhizomes bear adventitious roots (i.e. roots arising from the stem rather
than from other roots) at their joints in addition to leaf sheaths and
branches. The plants range in size from the 8 m high tropical
species E. myriochaetum to the 4-5 cm tall temperate
species E. scirpoides (
Hauke, 1963 ).
Fossil history
of the sphenopsids
Equisetum is an ancient
genus and comprises the sole surviving representatives of the
class Sphenopsida (the only class of the subdivision Sphenophytina)
( Scagel et al., 1984
). Sphenopsids first appeared in the fossil record
of the late Devonian. The earliest unequivocal sphenopsid
that has been discovered is Pseudobornia ursina, a monopodial
arborescent clonal plant of the upper Devonian which grew up to
20 m tall with stems up to 60 cm thick ( Stewart and Rothwell, 1993
; Scagel et al., 1984
). Pseudobornia dominated clastic streamside
habitats during this time (Behrensmeyer,
1992 ). Later, during the early Carboniferous,
a greater diversity of distinctly sphenopsid plants became prominent.
These Carboniferous sphenopsids are currently classified into
two orders, the Sphenophyllales and the Equisetales
( Stewart and Rothwell. 1993
). The Sphenophyllales, consisting of
a single genus Sphenophyllum , were herbaceous
plants with whorls of wedge-shaped leaves on a jointed stem.
Sphenophyllum species increased in abundance until the
Upper Carboniferous, but vanished by the end of the Permian.
The Equisetales include the major families Archaeocalamitaceae
, Calamitaceae , and Equisetacae . The
Archaeocalamitaceae were arborescent sphenopsids which persisted
from the Upper Devonian through the Lower Permian and were similar
to the much more numerous Calamitaceae ( Stewart and Rothwell, 1993
). The Calamitaceae, which has a single
genus,
Calamites (see also this beautiful reconstruction),
encompasses the now extinct arborescent woody sphenopsids, some
of which attained heights of up to 30 m and diameters of up to 30
cm ( Scagel et al., 1984
). Finally, the family Equisetaceae consists
of the living genus Equisetum as well as other extinct
herbaceous sphenopsids resembling Equisetum. Interestingly,
the Calamitaceae closely resembled the Equisetaceae
in having rhizomatous growth, fused leaf sheaths at the nodes, and
in many other respects. The chief differences between the two families
lie in cone morphology and in the lack of secondary (woody) growth
in the Equisetaceae in contrast to the presence of secondary
growth in the Calamitaceae ( Stewart and Rothwell, 1993
).
The Carboniferous represented
the peak of pteridophyte diversity and abundance ( Rothwell, 1996
). It was also during this period that about 75% of the world's coal
was formed. Hence, there is rich fossil evidence for the ecology
and biogeography of this period. The great Carboniferous
coal swamps were warm and humid and occupied the wet tropical low-lying
areas ( Pearson, 1995
). These swamps were dominated by giant arborescent
Lycopods in genera such as Lepidodenderon and Lepidopholios ( Stewart and Rothwell, 1993
). Sphenopsids, especially in the genera Calamites
and Sphenophyllum were common members of the flora
during the Carboniferous. The Pennsylvanian plant assemblages
are probably the best known plant assemblages of the Paleozoic,
and possibly the entire pre-Cretaceous. From palynological
and coal-ball analysis of Pennsylvanian floras, it is possible to
gain insight into the ecology of Carboniferous sphenopsids.
Sphenophyllum species were ground-cover plants which occurred
in nearly all lowland habitats (
Behrensmeyer et al., 1992 ). Calamites
were hydrophytes, like Equisetum , and grew on loosely
consolidated substrates such as sand bars, lake and stream margins,
and other unstable moist substrates ( Tiffney, 1985
). Therefore, it is probable that Calamites were
centered outside the comparatively stable coal swamps. Calamites
were the only Carboniferous lowland arborescent plants that
had the capability for extensive vegetative propagation
( Tiffney, 1985
). The rhizomatous growth of Calamites, like that
of modern Equisetum, allowed them to form extensive colonies
on disturbed wetland areas. However, Calamites and Sphenophyllum
were relatively minor components of the vegetation in
terms of overall biomass contribution ( Behrensmeyer et al,. 1992
; Tiffney, 1985
).
During the Carboniferous,
Laurussia and Gondwanaland collided and thus began the formation
of the supercontinent Pangea. In the Late Carboniferous,
there was widespread peat formation in the moist equatorial region
coal forests in what is now Europe and central and eastern North
America. However, climate changes in the late Pennsylvanian and
early Permian began to herald the demise of the great coal swamps.
During this time, the equatorial regions of Pangea became drier and
rainfall became more seasonal (
Parrish, 1993 ). The climate also became cooler
with extensive glaciation in the southern hemisphere. This
trend continued through the Triassic when arid to semiarid climates
prevailed ( Stewart and
Rothwell, 1993 ). This led to a worldwide
change from hydric conditions to mesic conditions which are less
favorable to sphenopsid growth. In addition, the inability
of sphenopsids to grow in the increasingly dry sites probably
reduced the their ability to compete with the increasingly successful
ferns, cycads, and conifers in these drier sites ( Koske et al., 1985
). These changes probably led to the extinction of
Calamites during the Lower Permian and the extinction
of the Sphenphyllales by the end of the Permian.
These extinctions left the remaining members of the Equisetales
as the only representatives of the Sphenophytina
( Stewart and Rothwell, 1993
).
By the Mesozoic, all sphenopsids
had the same basic body plan as present day Equisetum
( Behrensmeyer et al.,
1992 ). The remaining Equisetales
included the widespread Schizoneura , an upright herbaceous
genus, with stems up to two meters tall and two cm wide ( Behrensmeyer et al., 1992
), which first appeared in the Carboniferous and continued into
the Jurassic (Stewart and Rothwell,
1993 ). Schizoneura's large flat
leaves were a distinctive feature of this genus not commonly
found in the Equisetales (
Scagel et al., 1984 ). Another herbaceous
sphenopsid which survived from the Carboniferous to the Lower
Cretaceous was the genus Phyllotheca ( Stewart and Rothwell, 1993
). Both Schizoneura and Phyllotheca
were at first confined to Gondwanaland where they dominated the
broad swampy areas, but due to later migration attained almost cosmopolitan
distribution ( Hallam, 1973
). In addition, the genus Neocalamites, first
appeared in the Upper Permian and survived until the Lower Jurassic.
Neocalamites resembled small Calamites in
gross morphology (Stewart and Rothwell,
1993 ) with stems 10 to 30 cm thick and possibly 10
m high ( Behrensmeyer
et al., 1992 ). It was widely distributed during
the latter Triassic (Seward, 1959
). Equisetites, a genus which first appeared in the Carboniferous,
was the other major surviving genus of sphenopsids. Equisetites
were very similar to present day Equisetum and
there is some controversy as to whether they may actually have
been congeneric with present day Equisetum. If
Equisetites actually were Equisetum , then Equisetum
has existed since the Paleozoic and may indeed be the oldest extant
vascular plant genus ( Hauke,
1963 ). However, some Triassic and Jurassic Equisetites
were significantly larger than present day Equisetum,
reaching 8 to 14 cm in diameter (Stewart and Rothwell,1993
). Perhaps the largest Equisetites species,
E. arenaceus , lived during the Upper Triassic period ( Kelber and van Konijnenburg-van
Cittert, 1998 ). This remarkable species
had stems that averaged 25 cm in diameter and about 2.5-3.5 m in height
(Klaus-Peter Kelber, 2000, personal communication). Stewart
and Rothwell ( 1993
) hypothesized that large Equisetites may have had secondary
growth due to their size, but mention that there is no direct evidence
for this. Seward (1898
) mentioned interesting indirect evidence that E. areanceus
had secondary growth. Some bamboos have stems approaching
the diameter of E. arenaceus , yet lack secondary growth ( Judziewicz et al., 1999
). Bamboo stems are supported by extensive lignification ( Judziewicz et al., 1999
) and it seems possible that the large Equisetites likewise
had lignified support tissues. Although Spatz et al.( 1998 ) did not find
lignification in the supporting tissues of the E. giganteum
stems they examined, Speck et al. ( 1998 ) found slight
lignification in supporting tissues of E. hyemale .
The distribution and anatomy
of Mesozoic sphenopsids was consistent with primary colonization
of open or disturbed moist habitats. The sphenopsids as a
whole became less diverse and increasingly limited to herbaceous
forms during the Triassic (
Behrensmeyer et al., 1992 ). This trend was
probably due to increasingly arid conditions during the Triassic.
However, the surviving order Equisetales was widely distributed
and diverse during the Mesozoic. During the Jurassic, the
large Equisetites were present in nearly all parts of the
world. From the Jurassic, however, Equisetales
become smaller and less numerous ( Schaffner, 1930
). By the beginning of the Cenozoic, relatively small species
of Equisetum are all that appear ( Stewart and Rothwell, 1992
). This decrease in size and abundance during
the Cretaceous was probably also related to the rapid rise of
angiosperms to dominance and the resulting general decline in
the prominence of pteridophytes and conifers ( Schaffner, 1930
). However, despite this decline, during the Quaternary,
Equisetum species were found to be widely distributed in the
temperate zone ( Seward, 1959
).
Distribution
and taxonomy of Equisetum
Present day Equisetum
species are naturally distributed throughout much of the world,
although they are notably absent from Australia and New Zealand
( Scagel et al., 1984
) and from the islands of the central Pacific, Indian, and
South Atlantic islands (Schaffner,
1930 ). The diversity of species increases from
the equator to the temperate zone in the northern hemisphere,
whereas there are only four species in the Southern Hemisphere
( Hauke, 1963
; Hauke, 1978 ).
The present day species of
the genus Equisetum are divided into two distinct
subgenera: subgenus Equisetum , with eight
species and subgenus Hippochaete, with seven species.
There are several primary differences between the two subgenera.
Species in subgenus Equisetum have stomata that are flush
with the epidermal surface, whereas members of the subgenus Hippochaete
have stomata that are sunken below the epidermal surface. The stems
of the subgenus Equisetum are short-lived, relatively
soft, and tend to be regularly branched, whereas the stems of the
subgenus Hippochaete , with few exceptions, tend to be long-lived,
hard, fibrous, and unbranched or irregularly branched ( Hauke, 1963 ; Hauke, 1969a ).
In addition, four of the species of the subgenus Equisetum
demonstrate stem dimorphism between non-photosynthetic, unbranched,
coniferous stems and photosynthetic, branched, vegetative stems
( Hauke, 1978
). No such dimorphism occurs in the subgenus Hippochaete
( Hauke, 1963
). Although the chromosome number (n=108) is the same for
all Equisetum species, the subgenus Hippochaete
has larger chromosomes than those of subgenus Equisetum ( Hauke, 1978
).
The subgenus
Hippochaete includes the Equisetum species
often called "scouring rushes" (although also known generally
as horsetails) due to their rough, silica-impregnated epidermis.
The rough silicaceous stems of plants of this subgenus were used
by American pioneer settlers for scouring dirty cookware and polishing
wood ( Scagel et al., 1984
). The seven species in this group are E. giganteum
, E. myriochaetum , E. ramosissimum, E. laevigatum
, E. hyemale , E. variegatum and E. scirpoides
. This group contains the two largest Equisetum
species, E. giganteum and E. myriochaetum .
With the exception of E. laevigatum , and some varieties
of E. ramosissimum , all of the species in this subgenus
have evergreen stems ( Hauke,
1963 ). This group is very widespread
with species distributed over large areas of every continent,
except for Australia and New Zealand. The Old World species
E. ramosissimum , which ranges from 60º North latitude
to 30º South latitude, has the widest latitudinal range of
any Equisetum species ( Schaffner,
1930 ). The subgenus Hippochaete, as
a whole, ranges as far north as Ellsmere Island (greater than 80º
North latitude) and as far south as Argentina (approximately 40º
South latitude) ( Hauke, 1963
).
The subgenus Equisetum
contains the species commonly known as "horsetails." The
eight species of this group are E. arvense , E. pratense,
E. sylvaticum , E. fluviatile , E. palustre
, E. bogotense , E. diffusum , and E. telmateia
. The species in this group tend to be regularly branched
and hence can resemble bushy horse tails. Certain members of
this subgenus are found from 80º North latitude to 40º South
latitude. Only one species of this subgenus, the diminutive E. bogotense
of Central and South America, has a range that extends to the Southern
Hemisphere. The other seven species of this group are found in
the Northern Hemisphere ( Hauke,
1963 ). Most species of subgenus Equisetum
are temperate, with a few extending their ranges into the subtropics
and only E. bogotense ranging into the tropics.
The aerial stems of all of these species, except for E. bogotense
and E. diffusum , (the two most southerly species), are
annual ( Hauke, 1978
).
Ecology and reproductive
biology of Equisetum
Equisetum
species grow in wet places such as moist woods, ditches, wetlands,
and in road fill where sufficient groundwater is available.
Rhizomatous clonal growth is a universal feature of the genus and
is very important in its ecology and its ability to utilize ground
water. A single rhizome system may cover hundreds of square feet
( Hauke, 1963
). The rhizomes an penetrate to soil depths of four meters
in some circumstances ( Page,
1997 ). This deep rhizome growth gives the plants
the ability to survive environmental disturbances such as plowing,
burial, fire and drought. The extensive rhizome system also
allows the Equisetum plants to supply themselves with water
and mineral nutrients from deep underground and hence allows them to grow
in habitats, such as road fill, which appear dry on the surface ( Hauke, 1966
).
A remarkable characteristic of
Equisetum species is their ability to take up and accumulate
silicon in their tissues. This element appears to be necessary
for growth. Silica accumulates on the epidermis of the plants,
giving the epidermis a rough texture ( Parsons and Cuthbertson,
1992 ). This characteristic is probably very
important in explaining the seeming absence of insect and fungus
interactions with horsetails (
Hauke, 1969a ). Recent research on the protective
value of silica seems to indicate that silica solutions when applied
to plants can provide effective protection
from fungal diseases and from insect attack. This
would explain why gardeners have long used horsetail extract to
protect plants against pathogens and predators ( Quarles, 1995
).
As in other pteridophytes,
sexual dispersal in Equisetum occurs by means of
spores. Equisetum spores are green, spherical, and have
thin spore walls (Hauke, 1963).
Each Equisetum spore has four unique strap-like
structures called elaters
attached to the spore surface at a common point. These elaters
are hygroscopic (i.e. they expand and contract with changes in humidity)
and probably function to help disperse the spores (Hauke, 1963). Equisetum
spores are short-lived and can germinate within 24 hours of release
from the cone. After 5-17 days, depending on humidity, they
are no longer capable of germination ( Hauke, 1963 ).
In nontropical species (the majority of Equisetum ), the
spores are produced over a short period of time during the growing
season ( Duckett, 1985
). Equisetum gametophytes appear to require a substrate
of recently exposed bare mud in order to become established ( Duckett and Duckett,1980
). Like pioneer species, they rapidly attain
sexual maturity and are adversely affected by competition from
bryophytes and vascular plants ( Duckett and Duckett, 1980
; Duckett, 1985
). The resulting inefficiency of spore germination
and gametophyte reproduction in non-pioneer situations probably limits
gene flow and leads the high degree of genetic divergence found between
Equisetum populations ( Korpelainen and Kolkkala, 1996
). Therefore, sexual reproduction in Equisetum is
limited to rather narrow ecological conditions and this limits
the dispersal ability of Equisetum via spores.
The uniform
chromosome number throughout the genus (n = 108) facilitates
hybridization between Equisetum species (Scagel et al., 1984
). Hybridization is also favored by the relatively narrow ecological
requirements of gametophytes which encourages the formation of
mixed populations of gametophytes on suitable sites ( Hauke, 1978 ).
These mixed populations increase the probability of cross fertilization
between gametophytes of different, but compatible, species.
In areas where environmental conditions are especially conducive to
spore germination and gametophyte establishment, Equisetum
hybrids are particularly frequent and widespread. In Britain
and Ireland, for example, Equisetum hybrids are particularly
successful ( Page, 1985
). This success appears to be due primarily to the moist
temperate oceanic climate and relatively low competition from other
plants, conditions which favor both gametophyte and sporophyte generations
of Equisetum (Page,
1985 ). Equisetum hybridization is especially
frequent within the subgenus Hippochaete where five common
hybrids are known. Within the subgenus Equisetum,
there is only one common hybrid ( Hauke, 1978 ).
There are many more known hybrids within each subgenus, but these
hybrids tend to be much less common ( Hauke, 1978
). No hybrids between the two subgenera have yet been reported
and this adds further evidence that the two subgenera are naturally
distinct (Krahulec et al., 1996
).
Equisetum
species have a remarkable ability to reproduce vegetatively.
This helps to compensate for the inefficiency of spore reproduction.
An extensive rhizome system allows Equisetum species
to rapidly colonize disturbed areas ( Hauke, 1963 ).
This ability gives Equisetum a distinct advantage over species
requiring seed establishment or which have slow-growing rhizomes
(Hauke, 1969a ).
For instance, the widespread creation of roadside ditches in America
has created significant new habitat for some Equisetum
species. This is because the soil in ditch habitats tends to
be moist and the rhizomatous growth of Equisetum species allows
them to survive and thrive under the conditions of sediment accumulation
that are characteristic of ditches ( Rutz and Farrar, 1984
). The ability of Equisetum to survive and spread in
areas of heavy sediment accumulation was dramatically demonstrated after
the 1912 eruption of Katmai Volcano in Alaska. In studies of vegetational
recovery from the volcanic tephra (ash and silt) deposited by
this eruption, E. arvense was found to be the most successful
herb. It was able to penetrate as much as one meter of tephra,
more than any other herbaceous species, and colonize large areas
via rapid rhizomatous growth (
Bilderback, 1987 ). The remarkable ability of
Equisetum to prosper under disturbed conditions was also
demonstrated after the eruption of Mount St. Helens in 1980 when Equisetum
formed almost monotypic stands in the newly deposited tephra ( Rothwell, 1996 ).
The deep rhizome system of Equisetum also allows these plants
to survive fire and rapidly recolonize burned-over sites ( Beasleigh and Yarranton, 1974
). It is probable that the vigorous and extensive
rhizomatous habit of Equisetum has been very important to
the long term survival and spread of the genus ( Hauke, 1969a ).
Fragmentation
of rhizomes and stems allows Equisetum to disperse readily
in suitable habitats where there is sufficient moisture. Even
the aerial stem fragments can sprout and form new colonies ( Wagner and Hammitt, 1970
; Schaffner, 1931
; Praeger, 1934 ).
Hence, vegetative reproduction allows Equisetum clones to persist
and spread even in the absence of sexual reproduction ( Hauke, 1963 ).
Vegetative
reproduction probably accounts for the widespread occurrence
and persistence of common Equisetum hybrids even where
one or both of the parents are absent ( Hauke, 1963 ).
This is because hybrids are generally sterile and hence are without
means of sexual reproduction. The rhizome system of a vigorous
hybrid clone theoretically has the ability to maintain dense colonies
within limited areas for long periods. Fragmentation and transport
of rhizomes and stems then has the potential to disperse the clone
from the site of the original hybridization ( Hauke, 1963 ).
This would account for the abundance of Equisetum
hybrids even if hybridization is a relatively uncommon occurrence
( Hauke, 1963
).
The distribution
and ecology of the giant Equisetum species of the American
tropics, E. giganteum and E. myriochaetum,
and E. x schaffneri provides another interesting case
study in the importance of vegetative persistence of hybrids in the
genus. These three species are largely confined to the upper
elevations between 150 and 3000 meters. Equisetum giganteum
is a giant species which grows up to 5 m in height. It is
the most widespread horsetail in Latin America, ranging from Guatemala
to Brazil, Argentina and Chile as well as on Hispaniola, Jamaica
and Cuba ( Hauke 1969a
; Hauke, 1963 ). Equisetum
myriochaetum is also a giant species and is known to grow to
8 m in height. Equisetum myriochaetum has a more limited
range and is distributed from southern Mexico to Peru ( Hauke, 1963 ).
There is also a widespread hybrid, E. x schaffneri , between
these two giant horsetails which ranges from Mexico to Peru ( Hauke, 1963 ).
Although E. x schaffneri is sterile, it persists via vegetative
reproduction and may form large colonies ( Hauke, 1967 ).
This hybrid is found throughout the region of overlap between its
parent species, but it is also found in Mexico, where E. giganteum
is not known to occur, and in Venezuela, where E. myriochaetum
is not known to occur. This unexpectedly extensive distribution
may be due to vegetative dispersal or to the production of an occasional,
rare, viable spore ( Hauke, 1963
). Viable spores have been observed for other Equisetum
subg. Hippochaete hybrids ( Krahulec et al., 1996
), so this hypothesis appears plausible. Equisetum
x schaffneri once again demonstrates the remarkable frequency
and persistence of Equisetum hybrids.