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The lycophytes (Lycopodiophyta) are vascular plants that are the sister group to all the other vascular plants (the non-lycophyte clade of vascular plants is known as Euphyllophyta and is composed of Monilophyta [the ferns, horsetails, and whisk ferns] and Spermatophyta [the seed plants, i.e., the angiosperms and gymnosperms]). Based on the fossil record, lycophyte diversity is believed to have reached a maximum several hundred million years ago. Lycophytes from the Carboniferous period included fast-growing trees reaching 10 m in height, the remains of which contributed greatly to the formation of the coal that has fueled modern technology (and global warming) over the past couple of centuries.

There are three extant groups of lycophytes: the lycopods (or "club mosses", family Lycopodiaceae), the quillworts (Isoëtes), and the spikemosses (Selaginella). Like the other vascular plants (the euphyllophytes), lycophytes have a dominant and complex sporophyte generation. Lycophytes release haploid spores from the sporophyte and have a gametophyte generation that develops independently of the sporophyte, in some cases in association with symbiotic fungi.

Lycopods are homosporous (i.e., they produce only one type of spore), whereas quillworts and spikemosses are heterosporous, producing megaspores and microspores. Heterospory is believed to have evolved multiple times among land plant lineages.

(Raubeson and Jansen 1992; Duff and Nickrent 1999; Qiu et al. 2007; Banks 2009; Banks et al. 2011)

The phylogenetic relationships and taxonomy of the lycopods (Lycopodiaceae) were reviewed by Wikström (2001), who recognized four genera (together including no more than 400 to 500 species) within the Lycopodiaceae: Lycopodium, Lycopodiella, Huperzia, and Phylloglossum. Lycopodium and Lycopodiella, which together make up the "strobilate clade", each include no more than around 40 species. Huperzia includes 300 to 400 species, the majority of them growing as epiphytes in low- to mid-altitude montane rain forests of South America, Africa, and Southeast Asia, or as ground dwellers in open habitats of the upper montane neotropical rain forests and high altitude alpine vegetation of the Andes. Around 85 to 90% of all living Huperzia species (all species groups except the H. selago group) are included in a tropical epiphytic clade. Phylloglossum is monotypic (i.e., it includes just a single species, P. drummondii) and is found only in Australia and New Zealand. (Wikström 2001)

Isoëtes, the sole quillwort genus, is a cosmopolitan group of heterosporous lycophytes that includes as many as 200 or more species. Quillworts are strikingly homogeneous morphologically, although the genus has a global distribution and exhibits diverse ecological adaptations, with species ranging from ephemeral seasonal terrestrials to evergreen aquatics. The taxonomic challenges resulting from conserved (i.e., relatively invariant through the course of evolution) morphology are compounded by numerous examples of hybridization and allopolyploid speciation. Quillworts usually appear as tufts of linear leaves arising from an underground, corm-like rootstock. Ellipsoidal sporangia occur in expanded leaf bases.  (Taylor et al. 2004).

Just a single genus of spikemosses (Selaginella, with around 700 species) is generally recognized, but spikemosses are found across the globe and can be found from tropical to arctic habitats, including the well known drought-tolerant desert species S. lepidophylla (the "resurrection plant"), which can revive after years of dehydration. Banks (2009) reviewed the biology of Selaginella.

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