Selaginella apoda, commonly known as meadow spikemoss, is a perennial lycophyte native to much of the eastern United States and parts of northeastern Mexico. The life cycle is the shortest of the genus Selaginella, as well as one of the shortest among the lycophytes. Selaginella apoda is found primarily in damp soils in habitats such as swamps, wet fields, open woods and along stream banks. Selaginella apoda presents the potential for case studies involving the plant's adaptability to environmental toxins. It is closely related to Selaginella eclipes and S. ludoviciana, with both of which it has been reported to form hybrids. This group is characterized by relatively flat strobili and large megasporophylls which occur in the same plane as the lateral leaves. The plant was originally described, and named Lycopodium apodum by Carl Linnaeus in his Species Plantarum (1753).
Description
Selaginella apoda stems have smaller leaves in two rows as well as larger leaves in two rows. This species can be differentiated from a regular moss species based on its veins running along the leaves. The stem of S. apoda can best be described as leafy. Selaginella species possess rhizophores. Many species in the genus Selaginella also demonstrate dichotomous branching. Vegetative leaves on S. apoda are lined with small teeth, have a scale-like appearance, are short in length, and change in shape and size in the same node. Selaginella apoda branches can be a maximum of 15 centimeters in height and have a creeping style. The ligule on Selaginella species can be found rising on the adaxil surfaces of leaves at their base, as well as on sporophylls. The shape of the ligule is awl-shaped and occurs singly. The leaves of S. apoda contain a cuticle layer on their adaxial surfaces and they do not have hairs on their abaxial surfaces. The internodes of S. apoda branches can be used to classify the branches as reproductive or vegetative, as the internodes are extended on vegetative branches. Selaginella apoda adventitious and primary roots contain a root cap at their tips, have the ability to branch when growing, are white, and possess root hairs, located in close proximity to the tips. Selaginella apoda primary root system contains only three single root strands; as such, the system is ailing, for it also does not branch significantly. Adjacent to the axil, sporangia are created from artificial cells. While stomata can be found following the leaf margin on the lower surface of the plant's leaves, stomata on the upper surface of S. apoda leaves disperse entirely following the laminae. The dorsal leaves of S. apoda have acuminate apices. The primary leaves of S. apoda significantly contrast in shape from the cotyledons of the plant. Selaginella apoda leaf blades have borders with a margin that is light green in appearance or simply identical to each other. The strobilus of S. apoda is often restricted to a length of 1-2 centimeters. Typically, the strobili of S. apoda are in an erect position. Within the sporangia, the two gametophyte forms start development. Selaginella apoda sporangia can be distinguished from other species of Selaginella in how its wall is composed of two layers instead of three.
Taxonomy The species was first described by Carl Linnaeus in 1753 as Lycopodium apodum. The specific epithet apodum is the neuter form of an adjective apodus, coined by Linnaeus, meaning "footless". When in 1840 Antoine Frédéric Spring transferred it to the genus Selaginella, he did so under the name Selaginella apus, apus being a noun used in apposition rather than Linnaeus's adjective, on the grounds that in classical Latin, there is no such adjective as apodus. In 1915, Merritt Lyndon Fernald published the name Selaginella apoda, following Linnaeus in using an adjective as the specific epithet, in this case agreeing in gender with the feminine noun Selaginella. Both names have been used. Selaginella apoda falls under the order Selaginellales and the family Selaginellaceae, the biggest family of the lycophytes group and of which Selaginella is the single genus. The lycophytes contain over 1000 species across 10-15 extant genera. In the order Selaginellales, Selaginella also is the only living genus. Approximately 700-800 species comprise the genus Selaginella worldwide, of which about 320 can be found in the Western Hemisphere alone. Selaginella apoda is a synonym for Lycopodioides apoda as well as Lycopodium apodum. This plant species was named based on Linnaeus' observation that its strobili were stalkless. Selaginella apoda can be subcategorized into the subgenus Stachygynandrum. The genus Selaginella has been subjected to taxonomic treatments, including the arrangement of a plant's sporangia as well as the types of spores the plant species produces. In terms of phylogenetics, S. apoda falls under the S. pallescens OPHA clade, species that are native to the American continent and have one type of sporophyll in the form of a megaspore network. In the family Selaginellaceae, microsporangia are larger than the megasporangia and the strobili are quadrangular in shape or flat. Selaginella apoda, under the synonymous name of Lycopodium apodum, can be identified by stomata spread across the plane of the adaxial sides of its leaves, the leaf margins of the plant are all similar to each other, the diameter of their megaspores within the range of 0.29 – 0.35 millimeters, and the plant has acute to attenuate apices on at least 5 of their leaves.
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