Horsetails (Equisetum) can be found in damp habitats, growing along side streams and rivers.
Like the ferns they are archaic plants, thriving in abundance during the time of the dinosaurs.
Today there are about 15 living species. They are common in the Northern hemisphere and in
temperate zones, rarer in the tropics and absent from Australia and New Zealand.
The stems shoot from subterranean rhizomes, dying back each year in cold climates. They may
also produce subterranean expansions of the roots or tubers that store starch to fuel the initial
growth each spring. The growth is monopodial (as in palm trees) - a single axis bears whorls of
side-branches. eventually, when ripe, the end of the stem terminates in a 'cone' or strobilus.
The leaves are very small and called microphylls. They form a sheath around the stem and are
visible just above each whorl of branches. The stem has a characteristic jointed appearance.
Above: Equisetum arvense, note the jointed stem with whorls
of side-branches and tiny sheaths of microphylls (each a
small toothed leaf) above the branches. If you like closely
then you will see ridges/grooves running along the stem.
Like ferns, horsetails produce spores but no seeds and no flowers. However, whereas ferns are
often classified separately or grouped together with the seed-bearing plants (gymnosperms)
and the flowering plants (angiosperms) as the pteridophytes, horsetails belong to the
Most horsetails grow in damp or marshy habitats, but some can grow in drier soil. Most are a
few inches to a foot or two in height, but the tropical Equisetum giganteum reaches 10 metres in
|Horsetails and a tail in Biomimetics
| Above: a cross-section through a young vegetative (sterile) stem of the common horsetail,
Equisetum arvense. The stem has 6-19 grooves producing vertical ridges running along its
length. These ridges provide the main support for the plant as they are packed with
sclerenchyma, which occurs in the outer cortex. The stems are hollow, with a central pith
cavity surrounded by a ring of vascular bundles. There are as many vascular bundles as there
are ridges and the vascular bundles are radially aligned with the ridges. The sterile stems are
up to 50-80 cm tall (up to 24 inches tall). The number of toothed leaves forming a sheath
around the node is the same as the number of ridges. Alternating with the vascular bundles,
and situated in the cortex, are large longitudinal air spaces, called vallecular canals. These
probably serve for both respiration (as the plant grows in damp soils which may be
water-logged) and to reduce weight and increase stem strength.
The chlorophyllous tissue is the main photosynthetic tissue and opens to the outside air via
stomata which are sunken beneath pores flanked by subsidiary cells. The tip of the microphyll
has a few hydathodes, or water-secreting pores. True stomata are confined to the grooves
overlying the vallecular canals.
The hollow central pith reduces the weight of the stem, and the chief supporting structures
(sclerenchyma principally, less so the vascular bundles and least of all the parenchyma) are
concentrated towards the outside, increasing the strength of the stem and its resistance to
buckling. The sclerenchyma ares strengthened by silica, in both the stem and branches. This
stiffens the structures and as horsetails often grow in groves, they tend to hold eachother up
with the help of the silica. This is one of the few examples where silicon performs an obvious
The xylem consists of tracheids. The first-formed protoxylem only persists as fragments
adhering to the sides of the carinal canal inside each vascular bundle. The later metaxylem
develops from two groups, one on either side of the phloem. In the node the tracheids run
horizontally, forming a ring which connects all the vascular bundles together. All the xylem is
Reproduction and Alternation of Generations
The main plant is the diploid sporophyte (diploid = contains one pair of each chromsome, 2n
where n is the number of chromosome pairs). In spring the rhizome produces short-lived fertile
stems, which are reddish-brown in Equisetum arvense and up to 25 cm tall. In contrast, in some
forms, like Equisetum palustre the same vegetative stems later become fertile. These end in a
terminal cone or strobilus. The strobilus is a scaly structure, each scale is actually a
sporangiophore or stalk bearing a ring of about 20 spore-producing sporangia (singular:
sporangium). The strobilus has no central pith cavity, being generally solid, and vascular
bundles running its length give off traces to the sporangiophores. The spores are produced
following meiosis, a cell division in which the number of chromosomes halves, so that each
spore contains only one set of chromosomes and is haploid (contains n chromosomes).
The inner wall of the sporangium forms a tapetum which dissolves to nourish the developing
spores and contribute to the formation of the spore coat. The outermost spore wall layer
almost entirely peels away to form four elongated strands or arms, called elaters or haptera,
that are wrapped around the spore when moist, but when the sporangium opens and the spore
mass dries, the elaters uncoil and thrash about as they dry (due to stresses placed in the
material by uneven thickenings) and this freshing breaks up the spore mass and assists spore
dispersal. The sporangium itself opens via on elongated slit, called a stomium, as the wall
contracts upon drying, the stresses being caused by the drying of specially thickened cells in
the sporangium wall.
The spores are of one size and type (horsetails are homosporous), contain chlorophyll and are
short-lived. Upon germination a projection protrudes from the spore, forming a chain or
filament of cells. Cell division in other planes converts this filament into a cushion of cells
anchored by filaments called rhizoids. The gametophyte derives from the spore cell by mitosis
and so is entirely haploid (n). Thus, like ferns, horsetails undergo an alternation of
generations, alternating between the dominant diploid sporophyte and the haploid
gametophyte. The gametophyte is small, usually no more than 1 cm across and 3 millimetres
thick. It is green and fleshy, somewhat leaf-like or liverwort-like and consists of several lobes,
which slant upwards and so are called aerial lobes.
The gametophytes are actually of two types. One remains small, is short-lived and produces
only the male organs, called antheridia. The antheridia produce a mass of sperm cells
(antherozoids or sprematozoids). The second type of gametophyte is larger and
longer-lived, producing female organs first, the flask-like archegonia, each containing an egg
cell, and if none of these are fertilised it then produces male antheridia, followed by another
crop of archegonia. This second type may live up to two years and produce several
The antheridia are typically produced towards the tips of the aerial lobes of the gametophyte,
the archegonia between the lobes. The antherozoid is a helical cell with a helical arrangement
of flagella at one end and requires the presence of a film of surface moisture, for its release
from the antheridium, in which to swim. When released, the antherozoids swim towards
receptive archegonia, which secrete a pheromone to attract the sperm. Once an egg is
fertilised by an antherozoid, it becomes a zygote which develops into a young sporophyte
attached to the gametophyte, upon which it initially depends for nourishment, until its own root
and photosynthesis can sustain it.
The sporophyte develops in a curious way. First a small axis is produced, which produces a
limited number of whorls of leaves and branches before its growth ceases. This first shoot axis
has the most primitive type of vascular cylinder, a protostele (see ferns). Next, a bud grows out
from below the first whorl of leaves and forms a larger secondary axis, which may have a more
advanced vascular cylinder, a siphonostele. This process repeats until the final mature shoot
of adult size and adult vascular cylinder type (the final type depending on species, so those
with more advanced types need to progress through more stages). This is possibly a case of
the developing plant reiterating its evolutionary history. A siphonostele is the least specialised
type of vascular cylinder, and the most poorly suited to supporting a large plant on dry land.
The most advanced (as far as primary growth is concerned) is like that shown in the mature
horsetail above - a cylinder of separate vascular bundles. The fact that the gametophyte
begins as a filament of cells, perhaps also echoes an ancestry as a filamentous alga or similar
The Mechanical Properties of the Equisetum Stem
The mechanics of the supporting stem of horsetails presents a curious story. Clearly,
parenchyma are important for supporting the stems of small herbaceous plants. Each
parenchyma cell when turgid (full of sap) acts like a pressurised cushion and neighbouring
parenchyma cells push against one-another and so give the tissue mechanical stiffness. In this
way parenchyma function as a pressurised (water-filled) cellular solid. This stiffness is of
course lost if the cells lose water and become flaccid, as occurs when a plant runs short of
water and wilts. Equisetum, however, has ways around this. It has been shown experimentally
that the endodermis contributes substantially to the mechanical support of the plant, even
when the parenchyma lose turgor pressure (Spatz and Emanns, 2004).
Further support is provided by the outer cortex of sclerenchyma cells (sometimes the
cortex is described as being composed of collenchyma) which are hardened by silica. Being
situated at the outer perimeter of the stem, in the longitudinal strengthening ridges, this tissue
presumably resists bending of the stem when it is dynamically loaded (such as by wind).
What matters when resisting bending is not the cross-sectional area of the stem, but its
moment of area. The moment of area reflects the distribution of material around and away
from the central axis. Increasing the amount of material in cross-section and increasing the
distance of this material away from the central axis increase resistance to bending. The central
central cavity (pith cavity) is hollow, since this contributes little to resistance to bending and
adds to the weight (self-loading) of the stem. This is one benefit of having a hollow stem, a
hollow trunk in a woody tree, or a hollow long-bone in the limbs of an animal. The only
constraint is that the walls must have a certain minimum thickness to resist buckling (caving
inwards) and so the same amount of material can not be spread too far from the central axis (in
a ring which is too wide and hence too thin-walled).
We also need to consider the role of the vallecular canals. These clearly serve to reduce the
weight of the stem (and hence lower its self-loading - we want a stem that is easily able to
support its own weight in order to support the branches attached to it!).The Equisetum stem
essentially consists of two supporting rings, the inner endodermis and the outer
sclerenchyma/collenchyma joined together by struts of parenchyma tissue. This structure is
very strong and stiff, but also light, and has been mimicked by engineers (Milwich et al. 2006).
The struts of stiffened (turgid) parenchyma will resist compression and wall buckling.
The field of engineering which mimics biological structures is called biomimetics and is an
increasingly important field. It would be interesting to see whether or not air pressure inside the
vallecular canals contributes significantly to resistance to bending. Certainly, the canals may
become stretched or compressed when the stem is pushed into an oval contour, such as when
beginning to buckle, and at each node a diaphragm of tissue closes off the ends of the
vallecular canals and also interrupts the central cavities of adjacent internodes (Leroux et al.
2011). Clearly, these are not tightly sealed conduits, since they are continuous with the normal
extracellular air spaces of the living tissues which constitute the gas transport system of the
plant. However, air would likely find it hard to escape rapidly and if it becomes compressed
then the canal will stiffen due to the increased air pressure (much as air contributes to the
strength of foam). It would also be interesting to see whether swaying of the stem affects air
flow through the intercellular spaces and hence gas exchange.
Finally, the vascular bundles should contribute to support. Xylem vessels need strong and
stiffened walls in order to conduct xylem sap under negative and sometimes positive pressures.
The position of the vascular bundles in a ring at some distance from the central axis (around
the central cavity) increases their moment of area and hence their contribution to stem
In summary, the Equisetum stem is exquisitely designed to withstand mechanical stresses due
to dynamical loading.
Function of the Carinal Canals
It has been suggested that these canals, which form when the early-formed protoxylem
collapses, may have a role in water conduction (Leroux et al. 2011). During developmental
growth, the internodes rapidly elongate and the elastic protoxylem eventually ruptures, forming
the carinal canals. However, these canals have particular materials lining their inner surface,
including extensin, and it has been suggested that they may for a while carry over the
protoxylem's role in water conduction until the metaxylem develops and takes over this role in
the mature plant. These canals also connect to the rings of xylem at the nodes and so xylem
sap could theoretically travel along the internodes in the carinal canals and then across the
nodes in the xylem rings, in the developing plant.
Leroux, O., J.P. Knox, B. Masschaele, A. Bagniewska-Zadworna, S.E. Marcus, M. Claeys, L.
van Hoorebeke and R.L.L. Viane, 2011. An extensin-rich matrix lines the carinal canals in
Equisetum ramossimum, which may function as water-conducting channels. Ann. Bot. ? : ?
Milwich, M., T. Speck, O. speck, T. Stegmaier and H. Planck, 2006. Biomimetics and technical
textiles: solving engineering problems with the help of nature's wisodm. Am. J. Bot. 93:
Spatz, H.-CH. and A. Emanns, 2004. The mechanical role of the endodermis in Equisetum plant
stems. Am. J. Bot. 91: 1936-1938.