Stigmaria: A Review of the Anatomy, Development, and Functional Morphology of the Rootstock of the Arboreous Lycopsids
Rhizomorphic lycopsids constitute the most derived lycophyte clade and some of the world’s best-known plant fossils. Arboreous taxa within the clade evolved independently of other nonlycophyte trees. Their rootstocks (rhizomorphs) have distinctive morphologies and anatomies and unusually canalized ontogenies, leading to debates regarding homology and functional morphology. Traditionally referred to form genus Stigmaria, rhizomorphs of arboreous genera are repeatedly isotomous, producing a helical rhizotaxy of abundant determinate appendages (rootlets). Regarded as leaf homologues, reexamination of fossil and living Isoetes “rootlets” suggests some shared genetic and morphological traits with true roots, while maintaining many leaflike phenotypic traits. This logic cannot automatically be extended to the radially symmetrical, rootlet-producing rhizomorph axes, which are inferred to combine developmental programs that distinguish root from shoot in nonlycophytes. Comparative embryogeny of several fossil genera plus Isoetes suggests shared ontogeny and heterochronically driven divergences. The first-formed microphylls and root support the juvenile plant until the rhizomorph is produced laterally as a sui generis organ that rapidly co-opts the role of the first-formed root, transitioning from radial to bilateral to radial symmetry. We reevaluate the conventional view that the rootlets were abscised, despite their superficial origin. Rootlets are common in areas of comparatively low resistance, which suggests low penetrating power. In addition to dissolved minerals, rootlets may have gathered soil CO2 for use within the rhizomorph and/or shoot system, and exposed upwardly directed roots may have been photosynthetic. In soil, rootlets improved anchorage, whereas in open water, largely hollow mature roots may have enhanced stigmarian system buoyancy and nucleated floating peat mats. Examination of hundreds of in situ stumps has failed to locate an uprooted specimen; hollowed-out dead stumps were conduits for materials into potentially dysoxic levels in the substrate. We cannot identify close modern analogues for the ecological role played by these unique trees.