Club-mosses (Lycopodiales) are homosporous, but the genera Selaginella (spikemosses) and Isoetes (quillworts) are heterosporous, with female spores larger than the male.

[3] Many club-moss gametophytes are mycoheterotrophic and long-lived, residing underground for several years before emerging from the ground and progressing to the sporophyte stage.

[9][10][11] (multiple branches, incertae sedis) living lycophytes and their extinct close relatives ferns & horsetails spermatophytes (seed plants) As of 2019[update], there was broad agreement, supported by both molecular and morphological evidence, that the extant lycophytes fell into three groups, treated as orders in PPG I, and that these, both together and individually, are monophyletic, being related as shown in the cladogram below:[11] lycopodiales Isoetales Selaginellales The rank and name used for the taxon holding the extant lycophytes (and their closest extinct relatives) varies widely.

[12] In the Pteridophyte Phylogeny Group classification of 2016 (PPG I), the three orders are placed in a single class, Lycopodiopsida, holding all extant lycophyte species.

Taylor et al. (2009) and Mauseth (2014) include a number of extinct orders in their division (phylum) Lycophyta, although they differ on the placement of some genera.

Tetragonostachys) Isoetales (Isoetes) †Lepidodendrales (†Paralycopodites) The Lycopodiopsida are distinguished from other vascular plants by the possession of microphylls and by their sporangia, which are lateral as opposed to terminal and which open (dehisce) transversely rather than longitudinally.

Phylogenetic analysis shows the group branching off at the base of the evolution of vascular plants and they have a long evolutionary history.

Devonian fossil lycopsids from Svalbard, growing in equatorial regions, raise the possibility that they drew down enough carbon dioxide to change the Earth's climate significantly.

[23] During the Carboniferous, tree-like plants (such as Lepidodendron, Sigillaria, and other extinct genera of the order Lepidodendrales) formed huge forests that dominated the landscape.

Quillworts (order Isoetales) and Selaginella are considered their closest extant relatives and share some unusual features with these fossil lycopods, including the development of both bark, cambium and wood, a modified shoot system acting as roots, bipolar and secondary growth, and an upright stance.

The Lycopodiopsida had their maximum diversity in the Pennsylvanian (Upper Carboniferous), particularly tree-like Lepidodendron and Sigillaria that dominated tropical wetlands.

[25] In Euramerica, tree-like species apparently became extinct in the Late Pennsylvanian, as a result of a transition to a much drier climate, giving way to conifers, ferns and horsetails.

Nevertheless, lycopodiopsids are rare in the Lopingian (latest Permian), but regained dominance in the Induan (earliest Triassic), particularly Pleuromeia.

Shiraia sp Slf14 is an endophytic fungus present in Huperzia serrata that produces Huperzine A, a biomedical compound which has been approved as a drug in China and a dietary supplement in the U.S. to treat Alzheimer's Disease.

[29] This fungal endophyte can be cultivated much more easily and on a much larger scale than H. serrata itself which could increase the availability of Huperzine A as a medicine.