Lycopodiopsida is a class of vascular plants also known as lycopsids, lycopods, or lycophytes. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia on the sides of the stems at the bases of the leaves. Although living species are small, during the Carboniferous, extinct tree-like forms (Lepidodendrales) formed huge forests that dominated the landscape and contributed to coal deposits. The nomenclature and classification of plants with microphylls varies substantially among authors. A consensus classification for extant (living) species was produced in 2016 by the Pteridophyte Phylogeny Group (PPG I), which places them all in the class Lycopodiopsida, which includes the classes Isoetopsida and Selaginellopsida used in other systems. (See Table 2.) Alternative classification systems have used ranks from division (phylum) to subclass. In the PPG I system, the class is divided into three orders, Lycopodiales, Isoetales and Selaginellales.
Characteristics Club-mosses (Lycopodiales) are homosporous, but the genera Selaginella (spikemosses) and Isoetes (quillworts) are heterosporous, with female spores larger than the male. As a result of fertilisation, the female gametophyte produces sporophytes. A few species of Selaginella such as S. apoda and S. rupestris are also viviparous; the gametophyte develops on the mother plant, and only when the sporophyte's primary shoot and root is developed enough for independence is the new plant dropped to the ground. 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. Lycopodiaceae and spikemosses (Selaginella) are the only vascular plants with biflagellate sperm, an ancestral trait in land plants otherwise only seen in bryophytes. The only exceptions are Isoetes and Phylloglossum, which independently has evolved multiflagellated sperm cells with approximately 20 flagella (sperm flagella in other vascular plants can count at least thousand at most, but the number is generally much lower, and flagella are completely absent in seed plants except for Ginkgo and cycads). Because only two flagella puts a size limit on the genome, the largest known genomes in the clade are in Isoetes, as multiflagellated sperm is not exposed for the same selection pressure as biflagellate sperm in regard to size.
Taxonomy
Phylogeny The extant lycophytes are vascular plants (tracheophytes) with microphyllous leaves, distinguishing them from the euphyllophytes (plants with megaphyllous leaves). The sister group of the extant lycophytes and their closest extinct relatives are generally believed to be the zosterophylls, a paraphyletic or plesion group. Ignoring some smaller extinct taxa, the evolutionary relationships are as shown below.
As of 2019, 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:
Classification The rank and name used for the taxon holding the extant lycophytes (and their closest extinct relatives) varies widely. Table 1 below shows some of the highest ranks that have been used. Systems may use taxa at a rank lower than the highest given in the table with the same circumscription; for example, a system that uses Lycopodiophyta as the highest ranked taxon may place all of its members in a single subclass.
Some systems use a higher rank for a more broadly defined taxon of lycophytes that includes some extinct groups more distantly related to extant lycophytes, such as the zosterophylls. For example, Kenrick & Crane (1997) use the subdivision Lycophytina for this purpose, with all extant lycophytes falling within the class Lycopsida. Other sources exclude the zosterophylls from any "lycophyte" taxon. 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. Older systems have used either three classes, one for each order, or two classes, recognizing the closer relationship between Isoetales and Selaginellales. In these cases, a higher ranked taxon is needed to contain the classes (see Table 1). As Table 2 shows, the names "Lycopodiopsida" and "Isoetopsida" are both ambiguous.
Subdivisions The PPG I system divides up the extant lycophytes as shown below.
Class Lycopodiopsida Bartl. (3 orders) Order Lycopodiales DC. ex Bercht. & J.Presl (1 extant family) Family Lycopodiaceae P.Beauv. (16 extant genera) Order Isoetales Prantl (1 extant family) Family Isoetaceae Dumort. (1 extant genus) Order Selaginellales Prantl (1 extant family) Family Selaginellaceae Willk (1 extant genus) Some extinct groups, such as zosterophylls, fall outside the limits of the taxon as defined by the classifications in Table 1 above. However, other extinct groups fall within some circumscriptions of this taxon. 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. The orders included by Taylor et al. are:
Order †Drepanophycales (including Baragwanathia, Drepanophycus and Asteroxylon) Order †Protolepidodendrales Order †Lepidodendrales Order †Pleuromeiales Mauseth uses the order †Asteroxylales, placing Baragwanathia in the Protolepidodendrales. The relationship between some of these extinct groups and the extant ones was investigated by Kenrick and Crane in 1997. When the genera they used are assigned to orders, their suggested relationship is:
Evolution
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