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. (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, we find the largest known genomes in the clade in Isoetes, as multiflagellated sperm is not exposed for the same selection pressure as biflagellate sperm in regard of 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. During the Carboniferous, tree-like plants (such as Lepidodendron, Sigillaria, and other extinct genera of the order Lepidodendrales) formed huge forests that dominated the landscape. Unlike modern trees, leaves grew out of the entire surface of the trunk and branches, but fell off as the plant grew, leaving only a small cluster of leaves at the top. The lycopsids had distinctive features such as Lepidodendron lycophytes, which were marked with diamond-shaped scars where they once had leaves. Quillworts (order Isoetales) and Selaginella are considered their closest extant relatives and share some unusual features with these fossil lycopods, including the development of both bark, cambium and wood, a modified shoot system acting as roots, bipolar and secondary growth, and an upright stance.

  • Introduction to the Lycophyta from the University of California Museum of Paleontology