Horsetails

Family Equisetaceae

Equisetum is known as a horsetail or scouring rush, and is a readily identifiable plant due to the whorled appearance of its branches and nodes. The horsetails have a tail-like appearance because of their whorled side branches, but the scouring rushes generally lack these side branches. Almost all species inhabit wetlands and damp places. Equisetum has the ability to absorb macro-molecules from the soil, such as silica, and use it as a strengthening agent. This attribute gives them a rough feel, in which early American settlers used to their advantage when cleaning (or scouring) pots. This group has a long ancestry, originating in the Devonian Period, but is only represented by a single living genus, Equisetum.

Above: Equisetum arvense, the common horsetail, displaying whorled side branches

Above: Equisetum hyemale, the scouring rush, which generally lacks side branches

Ecology and Form

  • Live in wetland environments: rivers, bogs, marshes, ponds

  • Absorb silica, and other macromolecules from soil

Sporophyte (spore-bearing phase)

Vegetative features

Stems

  • Pseudomonopodial upright growth

    • Upright stem and side branches formed from unequal division of an apical cell

  • Whorled branches in horsetails, but absent in scouring rushes

  • Distinct nodes and internodes

    • Internodes are hollow; Nodes are solid

    • Bamboo-like appearance,

    • Intercalary meristems at the nodes create "telescoping" growth

Leaves

  • Whorled leaves at nodes

  • Highly reduced, and mostly non-photosynthetic

Roots

  • Present in all species, emerging from rhizome

Reproductive features

  • Eusporangiate sporangia (larger spore cases with many spores) aggregated into a cone (=strobilus)

  • Sporangia are attached in a whorled arrangement. Sporangia stalks, called sporangiophores, are recurved back toward the cone axis

  • Spores have arm-like elaters for dispersal over wet areas

    • Elaters spread out when in dry, non-humid conditions allowing for prolonged suspension in air

    • Elaters wrap around spore when in humid conditions allowing the spore to drop in wet environments

Gametophyte (gamete-forming phase)

  • Few millimeters to a few centimeters in size

  • Photosynthetic

  • Dioecious and monecious gametophytes

Geologic Range

Diversity

Single living genus (Equisetum), and approximately 15 living species with worldwide distribution (Hauke, 1963, 1978; Husby, 2013; PPG I, 2016)

  • E. arvense; E. bogotense; E. diffusum; E. fluviatile; E. giganteum; E. hyemale; E. laevigatum; E. myriochaetum; E. palustre; E. pratense; E. ramosissimum; E. scirpoides; E. sylvaticum; E. telmateia; E. variegatum

Classification

└Embryophytes

└Eutracheophytes

└Euphyllophytes

└Equisetophytes

└Equisetales

└Equisetaceae

Above: Largest horsetail, E. myriochaetum

Above: E. telmateia

Above: close-up of E. fluviatile cone

Above: strobilus of Equisetum showing whorled and recurved sporangia on sporangiophore stalks

Above: Equisetum spores with elaters

Additional Resources