Selaginella is the sole genus in the family Selaginellaceae, the spikemosses or lesser clubmosses, a kind of vascular plant.
This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having spores of two types. They are sometimes included in an informal paraphyletic group called the "fern allies". S. moellendorffii is an important model organism. Its genome has been sequenced by the United States Department of Energy's Joint Genome Institute.[1] The name Selaginella was erected by Palisot de Beauvois solely for the species Selaginella selaginoides, which turns out (with the closely related Selaginella deflexa) to be a clade that is sister to all other Selaginellas, so any definitive subdivision of the species into separate genera leaves two taxa in Selaginella, with the hundreds of other species in new or resurrected genera.
Selaginella occurs mostly in the tropical regions of the world, with a handful of species to be found in the arctic-alpine zones of both hemispheres.[2] Fossils assignable to the modern genus are known spanning over 300 million years from the Late Carboniferous to the present.[3]
Selaginella species are creeping or ascendant plants with simple, scale-like leaves (microphylls) on branching stems from which roots also arise. The stems are aerial, horizontally creeping on the substratum (as in Selaginella kraussiana), sub-erect (Selaginella trachyphylla) or erect (as in Selaginella erythropus). The vascular steles are polystelic protosteles. Stem section shows the presence of more than two protosteles. Each stele is made up of diarch (having two strands of xylem[4]) and exarch (growing outward in) xylems. The steles are connected with the cortex by means of many tube-like structures called trabeculae, which are modified endodermal cells with casparian strips on their lateral walls. The stems contain no pith.
In Selaginella, each microphyll and sporophyll has a small scale-like outgrowth called a ligule at the base of the upper surface.[5]: 7 The plants are heterosporous with spores of two different size classes, known as megaspores and microspores.[6]
Unusual for the lycopods, which nearly always have microphylls with a single unbranched vein, the microphylls of a few Selaginella species contain a branched vascular trace.[7]
Under dry conditions, some species of Selaginella can survive dehydration. In this state, they may roll up into brown balls and be uprooted, but can rehydrate under moist conditions, become green again and resume growth. This phenomenon is known as poikilohydry, and poikilohydric plants such as Selaginella bryopteris are sometimes referred to as resurrection plants.
Some scientists still place the Selaginellales in the class Lycopodiopsida (often misconstructed as "Lycopsida"). Some modern authors recognize three generic divisions of Selaginella: Selaginella, Bryodesma Sojak 1992, and Lycopodioides Boehm 1760. Lycopodioides would include the North American species S. apoda and S. eclipes, while Bryodesma would include 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[8] 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 genus Didiclis/Gymnogynum 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[9][10] determined that the Euselaginella group, comprising solely the type species, Selaginella selaginoides and a closely related Hawaiian 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 on the broad Stachygynandrum group.
In the Manual of Pteridology,[11] the following classification was used by Walton & Alston:
genus: Selaginella
However, this is now known to be highly paraphyletic in most of its groupings. Two recent classifications, employing modern methods of phylogenetic analysis, are as follows:
genus: Selaginella
genus: Selaginella
subgenus Selaginella
subgenus Boreoselaginella
subgenus Gymnogynumsection Megalosporum
section Myosurus
section Lyallia
section Articulatae
section Lepidophyllae
section Homeophyllae
subgenus Pulviniella
subgenus Heterostachyssection Tetragonostachyae
section Heterostachys
section Auriculatae
section Homostachys
sectionS. braunii subclade
S. willdenowii subclade
S. pennata subclade
S. pervillei subclade
S. siamensis subclade
S. delicatula subclade
Oligomacrosporangiatae subgenus Stachygynandrumsection Plagiophyllae
section Circinatae
section Ascendentes
section Proceres
section Pallescentes
section Austroamericanae
section Heterophyllae
There are about 750 known species of Selaginella.[14] They show a wide range of characters; the genus is overdue for a revision which might include subdivision into several genera. Better-known spikemosses include:
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:
Curled up Selaginella tamariscina 
Wallace's Selaginella (Selaginella wallacei) Selaginella is the sole genus in the family Selaginellaceae, the spikemosses or lesser clubmosses, a kind of vascular plant.
This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having spores of two types. They are sometimes included in an informal paraphyletic group called the "fern allies". S. moellendorffii is an important model organism. Its genome has been sequenced by the United States Department of Energy's Joint Genome Institute. The name Selaginella was erected by Palisot de Beauvois solely for the species Selaginella selaginoides, which turns out (with the closely related Selaginella deflexa) to be a clade that is sister to all other Selaginellas, so any definitive subdivision of the species into separate genera leaves two taxa in Selaginella, with the hundreds of other species in new or resurrected genera.
Selaginella occurs mostly in the tropical regions of the world, with a handful of species to be found in the arctic-alpine zones of both hemispheres. Fossils assignable to the modern genus are known spanning over 300 million years from the Late Carboniferous to the present.
