Lycopodiales

(botany) The type order of Lycopodiopsida.

An order of the class Lycopsida (clubmosses) that is evolutionarily positioned between two extinct orders. The Lycopodiales are more advanced than the Asteroxylales because their sporangia are unequivocally positioned on the upper surface of modified leaves, but less advanced than the Protolepidodendrales because they lack a uniquely lycopsid feature termed the ligule. All have steles with metaxylem toward the center of the axis, but otherwise show considerable variation. All are perennial nonwoody herbs. See also Asteroxylales; Protolepidodendrales.

The fossil record of lycopodialeans begins with Baragwanathia. Its earliest occurrence, in Australia, has been controversially dated as Late Silurian or Early Devonian. Unlike most other lycopsid orders, the range of variation observed among the fossils is well reflected by the extant genera. Eight genera (of approximately 350 extant species) are recognized: the widespread Huperzia, Phlegmariurus, Lycopodium, Diphasiastrum, Pseudolycopodiella, Lycopodiella, and Palhinhaea, together with the enigmatic antipodean endemic Phylloglossum.

Huperzia and Phlegmariurus are the most primitive of the living genera. They have non-rhizomatous aerial axes with undivided, star-shaped actinosteles. Sporangia are simple in construction and positioned on sporophylls that resemble leaves; they do not form cones. In contrast, the remainder of the genera have rhizomatous growth and possess thin-walled sporangia and sporophylls that are aggregated into cones. Lycopodium and Diphasiastrum gametophytes share an annular meristem, and the stem vascular system is partially dissected into a plectostele. In Pseudolycopodiella, Lycopodiella, and Palhinhaea, the vascular system is further dissected into an annulus of apparently discrete but actually interconnected strands, and axial branching is more complex. Because gametophytes are lobed and occur on the soil surface, they are able to photosynthesize, thus reducing their dependence for nutrition on symbiotic mycorrhizal fungi. Also, the deciduous sporophytes of Lycopodiella and Palhinhaea develop from a protocorm and produce globose sporangia.

Phlegmariurus species grow hanging from other plants; Pseudolycopodiella and Lycopodiella are semiaquatics. The remaining genera are dominantly terrestrial. Lycopodialeans tend to prefer habitats where interspecific competition is low; few clubmoss species form a major proportion of the biomass in their preferred communities. Nonetheless, together they have a global distribution. Species-level diversity is greatest in the tropics, although a few species reach the Arctic Circle.

Lycopodialeans have a large and long-lived diploid sporophyte, which alternates with the wholly physiologically independent, smaller, and generally shorter-lived haploid gametophyte. The sporophyte produces spores that are released from the sporangium and germinate to produce gametophytes, which in most genera are subterranean and rely for nutrition on mycorrhizal fungi. See also Lycophyta; Lycopsida.