Selaginella
Selaginella is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses. 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. Description. "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) 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. The plants are  heterosporous with spores of two different size classes, known as megaspores and microspores. 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. 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. Taxonomy. 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" , and "Lycopodioides" . "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 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 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" and others, and the other centered on the broad "Stachygynandrum" group. Walton & Aston classification. In the "Manual of Pteridology", 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: Weststrand & Korall, 2016 classification. genus: "Selaginella" Zhang & Zhou, 2015 classification. genus: "Selaginella" Species. There are about 750 known species of "Selaginella". 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. Cultivation. A number of "Selaginella" species are popular plants for cultivation, mostly tropical species. Some of the species popularly cultivated and actively available commercially include: