Selaginella is a genus of plants in the family Selaginellaceae, the spikemosses. Many scientists still place the Selaginellales in the class Lycopodiopsida (often misconstructed as "Lycopsida"). This group of plants has for years been included in what, for convenience, was called "fern allies". S. moellendorffii is an important model organism, and its genome was sequenced by the United States Department of Energy's Joint Genome Institute.
Selaginella species are creeping or ascendant plants with simple, scale-like leaves (microphylls) on branching stems from which roots also arise. The plants are heterosporous (megaspores and microspores), and have structures called ligules, scale-like outgrowths near the base of the upper surface of each microphyll and sporophyll. Under dry conditions, some species of Selaginella roll into brown balls (a phenomenon known as poikilohydry). In this state, they may be uprooted. Under moist conditions the brown balls become green, because of which these are also known as resurrection plants (as in Selaginella bryopteris).
Unusually for the lycopods, each microphyll contains a branching vascular trace.
Generic division 
Some modern authors recognize three generic divisions of Selaginella: Selaginella, Bryodesma Sojak 1992, and Lycopodioides Boehm 1760. Lycopodioides would include the native North American species S. apoda and S. eclipes, while Bryodesma would include the native S. rupestris (as Bryodesma rupestre). Stachygynandrum is also sometimes used to include the bulk of species.
The first major attempt to define and subdivide the group was by Palisot de Beauvois in 1803-1805. He established the genus Selaginella as a monotypic genus, and placed the bulk of species in Stachygynandrum. Gymnogynum was another monotypic genus, but that name is superseded by his own earlier name of Didiclis. This turns out, today, to be a group of around 45-50 species also known as the Articulatae, since his Didiclis/Gymnogynum genus was based on Selaginella plumosa. He also described the genus Diplostachyum to include a group of species similar to Selaginella apoda. Spring inflated the genus Selaginella to hold all selaginelloid species four decades later.
Phylogenetic studies by Korall & Kenrick determined that the Euselaginella group, comprising solely the type species, Selaginella selaginoides and a closely related Hawai'ian species, Selaginella deflexa, is a basal and anciently diverging sister to all other Selaginella species. Beyond this, their study split the remainder of species into two broad groups, one including the Bryodesma species, the Articulatae, section Ericetorum Jermy and others, and the other centered around the broad Stachygynandrum group.
In the Manual of Pteridology, the following classification was used by Walton & Alston:
- subgenus: Euselaginella
- group: selaginoides
- group: pygmaea
- group: uliginosa (Ericetorum)
- group: rupestris (Tetragonostachys or Bryodesma)
- subgenus: Stachygynandrum
- series: Decumbentes
- series: Ascendentes
- series: Sarmentosae
- series: Caulescentes
- series: Circinatae
- series: Articulatae
- subgenus: Homostachys
- subgenus: Heterostachys
There are about 700 species of Selaginella, showing a wide range of characters; the genus is overdue for a revision which might include subdivision into several genera. Better-known spikemosses include:
- Selaginella apoda - meadow spikemoss (eastern North America)
- Selaginella arizonica Maxon (west Texas to Arizona and Sonora, Mexico)
- Selaginella asprella
- Selaginella bifida (Rodrigues Island)
- Selaginella biformis
- Selaginella bigelovii
- Selaginella braunii - Braun's spikemoss (China)
- Selaginella bryopteris - sanjeevani (India)
- Selaginella canaliculata - clubmoss (southeast Asia, Maluku Islands)
- Selaginella carinata
- Selaginella cinerascens
- Selaginella densa - lesser spikemoss (western North America)
- Selaginella eclipes - hidden spikemoss (eastern North America)
- Selaginella elmeri
- Selaginella eremophila Maxon
- Selaginella hansenii
- Selaginella kraussiana - Krauss's spikemoss (Africa, Azores)
- Selaginella lepidophylla - resurrection plant, dinosaur plant, and flower of stone (Chihuahuan Desert of North America)
- Selaginella martensii - variegated spikemoss
- Selaginella moellendorffii
- Selaginella oregana
- Selaginella poulteri
- Selaginella pulcherrima
- Selaginella rupestris - rock spikemoss, festoon pine, and northern Selaginella (eastern North America)
- Selaginella rupincola Underw. west Texas to Arizona and Sonora, Mexico
- Selaginella selaginoides - lesser clubmoss (north temperate Europe, Asia and North America)
- Selaginella sericea A.Braun - Ecuador
- Selaginella serpens
- Selaginella uliginosa Australia
- Selaginella umbrosa
- Selaginella uncinata - peacock moss, peacock spikemoss, blue spikemoss
- Selaginella underwoodii Hieron. west Texas to Wyoming and west into Arizona
- Selaginella wallacei
- Selaginella watsonii
A few species of Selaginella are desert plants known as "resurrection plants", because they curl up in a tight, brown or reddish ball during dry times, and uncurl and turn green in the presence of moisture. Other species are tropical forest plants that appear at first glance to be ferns.
A number of Selaginella species are popular plants for cultivation, mostly tropical species. Some of the species popularly cultivated and actively available commercially include:
- S. kraussiana: "golden clubmoss"
- S. moellendorfii: "gemmiferous spikemoss"
- S. erythropus: "red selaginella" or "ruby-red spikemoss"
- S. uncinata: "peacock fern"
- S. lepidophylla: "resurrection plant"
- S. braunii: "arborvitae fern"
- "Selaginella moellendorffii v1.0". Joint Genome Institute. United States Department of Energy. 2007. Retrieved 2009-04-08.
- Palisot de Beauvois (1805): Prodrome des cinquième et sixième familles de l'Æthéogamie, les mousses, les lycopodes.
- Korall, P. & Kenrick, P. (2002), "Phylogenetic relationships in Selaginellaceae based on rbcL sequences", American Journal of Botany 89 (3): 506–17
- Korall & Kenrick (2004): The phylogenetic history of Selaginellaceae based on DNA sequences from the plastid and nucleus: extreme substitution rates and rate heterogeneity. Molecular Phylogenetics and Evolution, Volume 31, Issue 3, June 2004, Pages 852-864
- Verdoorn, F., ed. (1938): Manual of Pteridology: J. Walton and A. H. G. Alston, Lycopodinae, pp. 500-506. Martinus Nijhoff, The Hague. 640pp, HB.