Lepidodendrales

Ecology and Form

• Arborescent forms (up to 30m long and 2m wide); small shrubs; sprawling forms living in organic-rich, swampy environments

• Lepidodendrales had 3 basic habits:

  1. Pole-like stem that branched repeatedly to form a crown only late in an individual’s determinate life span, during which it underwent a progressive developmental shutdown (Andrews and Murdy 1958; Eggert 1961)

  2. Large-diameter trunk but one on which were borne small, deciduous lateral branches in two opposite rows; the reproductive organs terminated these branches

     • These trees also had a determinate crown, and the lateral branches continued to be produced within that crown (Hirmer 1927; Phillips and DiMichele 1992; DiMichele et al. 2013; Chomicki et al. 2017; DiMichele and Bateman 2020); thus, they were functionally polycarpic)

  3. Arboreous Sigillaria, cones were borne directly on the trunk in successive whorls, probably representing paedomorphically reduced lateral branches, and the crown underwent comparatively few apical dichotomies prior to developmental shutdown

Stems

• e.g. Lepidodendron, Sigillaria, Diaphorodendron

• Pseudo-bipolar growth

  • Root decays early in development

  • Early embryonic shoot splits with one "branch" becoming the "root" or rhizomorph

  • The other shoot become upright stem

• Exarch primary growth in upright portions, which is different from rhizome system

• Unifacial (one-faced) cambium in trunk or upright stems

  • Small amount of secondary xylem (wood) produced to the inside (centripetal)

  • No secondary phloem found in the trunk, therefore sugars from photosynthesis could not be transported down to root-like rhizomorph system

  • See rhizomorph description (below) for more details about leaves

• Large amounts of bark (phellem), which was a major structural support for trunk...

  • ...although D'Antonio & Boyce (2020) indicate that periderm was never greater than 15 cm for observed specimens

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• These trees were probably not as durable as modern trees, with trunk that served the primary purpose of elevating spores during reproduction

  • After spores dispersal, trunks may have fallen over into swamp with strong winds or storms.

  • All of their reproductive organs were produced in the crown, thus rendering them monocarpic or nearly so (Hirmer 1927, fig. 263; DiMichele and Phillips 1985; Chomicki et al. 2017)

Roots (Rhizomorph)

• The rooting system consisted principally of two morphologically and developmentally distinct components, the main axes or “rhizomorph” and the appendages generally referred to as “rootlets.” (DiMichele et al. 2022)

  • The rooting system is similar to their closest living relative, Isoetes (Stewart 1947; Hetherington et al. 2016; Hetherington and Dolan 2017)

  • The isoetalean rhizomorph is morphologically distinct from the “rhizophore” of the Selaginellales, a unique root-producing organ in that plant group (e.g., Lu and Jernstedt 1996; Mello et al. 2019);

• The rhizomorphs (Stigmaria) act like the main root system

  • Endarch primary growth in rhizomorphs, which is different from trunk

  • No true roots in this group; adventitious roots grew from rhizomorph

• Modified leaves on rhizomorph acts like secondary roots to absorb water and minerals

• Leaves on the top portions of rhizomorph were probably still green and photosynthetic to provide sugars to growing roots since the trunk lacks secondary phloem

  • Rhizomorph has a rhizotaxis (pattern of root insertion) due to modified leaves growing in a spiraling fashion

  • Modern plants display a phyllotaxis (pattern of leaf insertion) such as the Fibonacci pattern, but modern roots do not have a rhizotaxis

Protolepidodendropsis pulchra

• Høeg; Berry & Marshall 2015

• early Frasnian (Late Devonian) of Svalbard

• Cormose bases and small ribbon-like roots

• Their height is unknown but estimated to be around 2 to 4 m. They grew 15–20 cm (6–8 in) apart in wet soils

Protostigmaria eggertiana

• Cormose rooting organ with multiple lobes