Equisetum

Equisetum, or horsetail, is the sole living genus in the family Equisetaceae. These vascular plants reproduce via spores, not seeds. The earliest fossils of modern horsetail species date to the Early Jurassic. The genus is nearly worldwide, though in some regions its species are considered weeds or invasive. The plants are toxic due to thiaminase, but have been used as food and in folk medicine.

Horsetails are perennial herbs. Most temperate species die back in winter, while tropical species and a few temperate ones—such as *E. hyemale*, *E. ramosissimum*, *E. scirpoides*, and *E. variegatum*—are evergreen. They typically grow 20–150 cm tall, though subtropical “giant horsetails” can be larger: *E. myriochaetum* has been recorded at 8 m, and *E. giganteum* at 5 m.

The leaves are greatly reduced, usually non-photosynthetic, and contain a single unbranched vascular trace—the defining feature of microphylls. However, research from 1999 suggests horsetail microphylls evolved from reduced megaphylls, unlike the ancestral microphylls of lycophytes. Leaves are arranged in whorls fused into nodal sheaths. The stems are green, photosynthetic, hollow, jointed, and ridged (with 3 to 40 ridges). Branches may or may not appear in whorls at the nodes, and they often emerge below the leaves from buds between leaf bases.

Spores are produced in strobili—cone-like structures at the tips of some stems. In many species, the cone-bearing shoots are unbranched; in some, like *E. arvense*, they are non-photosynthetic and appear early in spring. In others, such as *E. palustre*, they resemble sterile shoots, being photosynthetic and branched. Horsetails are mostly homosporous, though in field horsetail smaller spores produce male prothalli. Each spore has four elaters that act as moisture-sensitive springs, helping the spores crawl and hop after the sporangia split open. Spores are photosynthetic and usually live no more than two weeks, but germinate quickly in humid conditions to form a gametophyte.

All tested *Equisetum* species contain mixed-linkage glucan : xyloglucan endotransglucosylase (MXE) activity, a novel enzyme not found in any other plants. Their cell walls also contain mixed-linkage glucan (MLG), a polysaccharide once thought limited to the Poales. Because *Equisetum* and Poales are evolutionarily distant, each likely evolved MLG independently. The presence of MXE suggests *Equisetum* evolved both MLG and a mechanism for cell wall modification. Non-*Equisetum* land plants tested lack detectable MXE. A negative correlation between XET activity and cell age suggests XET loosens walls during cell expansion. The absence of MXE in Poales implies it has a different, unknown role there; the correlation with cell age suggests MXE may help stop cell expansion.

Plants of the World Online accepts 18 *Equisetum* species, though some researchers recognize 25. Living species fall into three lineages, usually treated as subgenera. The type subgenus *Equisetum* means “horse hair” in Latin; the other large subgenus, *Hippochaete*, means the same in Greek. Hybrids occur only within the same subgenus. Two plants sold as *E. japonicum* and *E. camtschatcense* are actually forms of *E. hyemale* var. *hyemale*, though they may be listed as separate varieties. The 2006 Smith et al. classification, based on molecular phylogeny, placed *Equisetum* and Equisetaceae within leptosporangiate ferns, a placement maintained in later classifications.

*Equisetum* is a living fossil, the only surviving genus of the subclass Equisetidae, which dominated forest understories for over 100 million years during the late Paleozoic. Some ancient equisetids were trees up to 30 m tall; the genus *Calamites* of the family Calamitaceae is common in Carboniferous coal deposits. The oldest modern horsetail remains are from the Early Jurassic: *E. dimorphum* from Patagonia and *E. laterale* from Australia. Silicified *E. thermale* from the Late Jurassic of Argentina shows all modern morphological traits. The split between *E. bogotense* and all other living *Equisetum* is estimated to have occurred no later than the Early Jurassic.

In subgenus *Paramochaete*, *Equisetum bogotense* (Andean horsetail) is found in upland South America up to Costa Rica; it sometimes includes *E. rinihuense* as a separate species. Previously placed in subg. *Equisetum*, Christenhusz et al. (2019) moved it here because *E. bogotense* appears sister to all other living species. Subgenus *Equisetum* includes *Equisetum arvense* (field horsetail).