Equisetophytes

Equisetum, Calamites †, Sphenophylls †, and Pseudobornia †

The equisetophytes are a group of living and extinct spore-bearing plants that exhibit whorled growth in branching and leaf insertion. The modern forms, Equisetum spp., known as the horsetails or scouring rushes are herbaceous plants with whorled photosynthetic branches, and reduced whorled leaves. Ancient members of this group, such as Calamites, grew to heights over 30 meters. They produced wood to reach these heights, and displayed laminate leaves for photosynthesis. Other equisetophytes, such as the Sphenophyllales and Pseudobornia had distinct nodes and internodes, like modern horsetails, but lacked other features such as hollow stems as in Sphenophyllum, or whorled appendages as in Psuedobornia.

Ecology and Form

    • Most groups live(d) in wetland environments.

Stems

Morphology

    • Whorled or opposite branches

    • Distinct nodes and internodes

    • Pseudo- or true monopodial growth

Anatomy

    • These plants are hollow in the internodes, and solid in nodes.

    • Modern Equsietum has a unique stelar pattern with carinal and vallecular canals

      • Sometimes referred to as an equisetostele or a eustele

    • Ancient forms had a cambium that produced secondary xylem

Leaves

    • Dichotomous branches with lamina in Pseudobornia

    • Whorled leaves at nodes in Equisetales and Sphenophyllales

    • Highly reduced and non-photosythetic in living forms

    • photosynthetic in extinct forms

Roots

    • Present in Equisetales

    • Unknown for Pseudobornia

Reproduction

    • Spore-bearing plants

    • Sporangia tend to be whorled on recurved stalks called sporangiophores

    • Sporangia aggregated into a cone (=strobilus)

Geologic Range

Diversity

  • The Equisetophytes (or Equisetidae) are composed of three orders: the Pseudoborniales †, the Sphenophyllales †, and the living Equisetales

    • Only one genus is extant from this group: Equisetum

    • Ancient forms grew to tree-size in Paleozoic (e.g. Calamites)

Incertae sedis taxa

Protohyenia †

  • Pseudomonopdoial axes bearing dichotomous laterals

  • Laterals terminated by erect oval sporangia

Estinnophyton (?) †

  • Estinnophyton might have had a closer affinity with the protolepidodendralean lycopsids

  • E. gracile (Fairon-Demaret 1979)

  • E. wahnbachense (Kräusel et Weyland; Fairon-Demaret 1979)

    • Originally described as Protolepidodendron wahnbachense (Kräusel et Weyland 1932) from the Siegenian of Germany

    • Narrow axes with persistent leaves, which are bifurcated and spirally arranged.

    • The vegetative and fertile leaves bifurcate twice and the four resulting segments are spread in various planes

      • The fertile leaves support two pairs of stalked elongate sporangia which are directed towards the axis

  • E. yunnanense (Hao et al. 2004)

    • Lower Devonian (upper Pragian) Posongchong Formation at Zhichang village, Gumu Town, Wenshan District, Yunnan Province, China.

    • This plant possesses aerial stems with isotomous branching

    • The leaves are slender, once- or twice-bifurcated, and arranged in low spirals or pseudo-whorls

    • Fertile leaves are morphologically identical to the vegetative ones, clustered on the axis, forming a loose strobilus-like structure

      • There are three to six fertile leaves in each gyre

      • Two ovoid-elongate sporangia are singly attached to the adaxial surface each subtended by a recurved stalk, either at the base of each segment in a once-bifurcate fertile leaf or below the points of successive divisions in a twice-bifurcate fertile leaf

      • E. yunnanense differs from E. gracile and E. wahnbachense, mainly in possessing two single stalked sporangia on each fertile leaf rather than two pairs of sporangia, as well as in the dimensions of its leaves.

Estinnophyton

Above: Line drawing of the fertile leaves of E. yunnanense

Above: Phylogenetic tree of the equisetophytes, including living and fossil taxa (From Fig. 3, Elgorriaga et al. 2018)