An Introduction to the Genus Equisetum and the Class Sphenopsida as a whole

(For information on the basic biology of the genus Equisetum , I recommend Dr. David Webb's excellent pages on Equisetum biology. See also "Introduction to the Sphenophyta")



Contents
:

1. Introduction
2. Morphology of Equisetum
3. Fossil history of the sphenopsids
4. Distribution and taxonomy of Equisetum
5. Ecology and reproductive biology of Equisetum




Introduction

    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 Equisetum fluviatile 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.

If you have any comments or questions, please contact the author, Chad Husby ( chad.husby@fiu.edu or husby.1@osu.edu )

© Chad E. Husby 2002

Last modified October 27, 2002

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