Northern Ontario Plant Database

Plant Description

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The Genus Equisetum

Equisetaceae (Horsetail Family)

Although prehistoric relatives of the horsetails, like woody, tree-sized Calamites, were once the dominant plant type during the Carboniferous (about 350-300 million years ago), only one herbaceous genus, Equisetum, now remains. Horsetails have a perennial rhizome that gives rise to jointed above-ground stems, while adventitious roots anchor the rhizome to the ground. The dominant generation of the horsetail life cycle is the sporophyte (described below); gametophytes are bisexual, grow on the soil surface, and are photosynthetic.

The horsetails have undergone a number of changes in recent years in the interpretation of their anatomy and their classification. Horsetails, formerly classified in the Phylum Sphenophyta, were also once called "fern allies," along with the clubmosses (Lycopodium, etc.), quillworts (Iso�tes), and spikemosses (Selaginella). But recent work by researchers like Kathleen Pryer et al. (2001, 2004), Des Marais et al. (2003), and Alan R. Smith et al. (2006) has shown that the horsetails actually belong with the ferns in a subcategory currently known as the Monilophytes. This group does not yet have a formal taxonomic name, but would be placed between Phylum (or Division) and Class, in a rank such as subphylum, infradivision, or superclass. Until the International Code of Botanical Nomenclature (ICBN) makes a ruling on the various taxonomic ranks and names that have been proposed for this group, we will refer to them by the common name Monilophytes (horsetails + ferns), which is a category situated between the Phylum Tracheophyta (vascular plants) and Class Equisetopsida (horsetails).

Stems
Horsetails are characterized by green, photosynthetic aerial stems, jointed at the nodes, and with vertical ridges or ribs on the internodes; stems may be simple and unbranched, or bear whorls of branches. The reduced leaves contain no chlorophyll, so photosynthesis is carried out by the stems and branches, if present. The aerial stems, which may persist for 1-3 years, arise from perennial, horizontal rhizomes. Horsetails are also called scouring rushes because the epidermal cells contain hard silica deposits, which made them useful for scouring pots and pans before the invention of steel wool scouring pads. Stomates usually occur in vertical rows within the furrows or valleys of the ribbed stems. Beneath the ridges, collenchyma cells, with unevenly thickened primary walls, provide strength to the stems.

Horsetails also have a unique internal anatomy. In the cortex, the vascular tissue forms separate vascular strands beneath each vertical ridge. Most species, except for E. scirpoides, have a hollow centre, called the central canal (or pith canal), and two series of smaller canals, which extend the length of the internode. These canals are interrupted at each node by a solid layer of tissue, which extends across the node and contains a cylinder of xylem (siphonostele). The canals in the outermost series, located in the outer cortex, beneath the valleys between ridges, are called the vallecular canals (or cortical canals). Interior to these, and associated with each vascular strand, are the smaller carinal canals (or vascular canals). These mark the position of the earliest vascular tissue, the protoxylem, which is destroyed as the stem grows, leaving a cavity (the carinal canal) in its place.

Some species of Equisetum have dimorphic shoots, with separate, morphologically different fertile and sterile stems. The fertile stems of these species usually lack chlorophyll and branches, and wither soon after spores are shed, while the sterile shoots are photosynthetic and bear whorls of branches. In a few species, like E. sylvaticum, the fertile shoot will become green and develop branches after the spores have been shed. Horsetail species with monomorphic shoots have fertile and sterile shoots that look the same. The only difference is that a fertile shoot will bear a strobilus at the top of a simple or branched photosynthetic stem. After spores are shed, the strobilus will wither, but the stem remains functional.

Branches
In some species, whorls of photosynthetic branches arise from each node of the jointed stems, emerging through the base of the leaf sheath. These branches may be simple (unbranched) or bear secondary branches, as in E. sylvaticum. Some species, like E. hyemale, are characterized by unbranched stems, but if the growing tip is damaged, one to several new (but smaller) aerial shoots may be produced.

Leaves
Horsetails leaves were once considered to be microphylls (and are still listed as such in many web sites), since each leaf has only one leaf trace, but now horsetail leaves have been shown to be reduced megaphylls. A whorl of several small, scale-like leaves occurs at each node. The small leaf blades are connate (fused together), except at their tips, and form a sheath around each node with teeth along the upper margin.

Sporangia
In contrast to the strobilus of a clubmoss, which is composed of a series of reduced sporophylls (sporangia-bearing leaves), the terminal strobilus of Equisetum is composed of tightly packed whorls of very modified 6-sided structures, called sporangiophores, that look like thick, flat umbrellas. The sporangiophores are attached perpendicular to the axis of the strobilus. Attached to the bottom of each sporangiophore are 5-10 peltate sporangia. When the spores are mature, the strobilus elongates slightly, separating the sporangiophores and allowing the spores to be released. The green, photosynthetic spores of Equisetum bear 4 elaters, which are long strap-like structures that aid in spore dispersal.

Optional Links
For more images and information of the Equisetaceae and its members, see these excellent websites:

Images of the Equisetaceae, from David Webb's Non Flowering Plant Family Access Page, at the University of Hawaii.

Images of the Equisetaceae, from www.PlantSystematics.org, a Diversity of Life website from Cornell University.

Horsetails, part of the Pteridophytes of Wisconsin: Ferns & Fern Allies, a website by Gary Fewless, at the Cofrin Center for Biodiversity, University of Wisconsin-Green Bay.

References
Des Marais, D.L., A.R. Smith, D.M. Britton, and K.M. Pryer. 2003. Phylogenetic relationships and evolution of extant horsetails, Equisetum, based on chloroplast DNA sequence data (rbcL and trnL F). International Journal of Plant Sciences 164 (5): 737 751.

Pryer, K.M., H. Schneider, A.R. Smith, R. Cranfill, P.G. Wolf, J.S. Hunt and S.D. Sipes. 2001. Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature 409: 618 622.

Pryer, K.M., E. Schuettpelz, P.G. Wolf, H. Schneider, A.R. Smith, and R. Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91: 1582 1598.

Smith, A.R., K.M. Pryer, E. Schuettpelz, P. Korall, H. Schneider, and P.G. Wolf. 2006. A Classification for Extant Ferns. Taxon 55(3): 705-731. Back to species list