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Lycopsids, are a group of very ancient vascular plants that are only distantly related to other land plants. They have a long history stretching back possibly to the late Silurian period. Their living representatives include club mosses and quillworts, but in past ages they dominate the landscape with huge forests.
Lycopsids have roots, simple or branched stems and small, spirally arranged leaves (microphylls). The sporangium is either borne on a fertile leaf (a sporophyll) or is associated with one; it is thick-walled, and is either homosporous (producing only one kind of spore) or heterosporous (producing two kinds of spore). The sperm cells are mobile and have two or many flagella. Gametophytes are complex, with multicellular gametangia.
The earlier forms, such as Baragwanathia from the late Silurian / early Devonian of Gondwanaland, were small soft-bodied, low-growing plants which reached a typical height of 25 centimetres.
The group became especially successful and important during the late Carboniferous period, when the great Lepidodendrales formed huge swamp forests, with trees such as Lepidodendron reaching over 40 metres in height.
The drying out of the climate during the latest Carboniferous was catastrophic for the lycopsids, which require moist conditions to reproduce. All the giant "pole trees" like Lepidodendron died out, leaving only a number of small herbaceous types.
Today the Lycopsids are represented by five living genera with about 1100 species, all small creeping plants such as Lycopodium which reach only about 5 cm in height.
The following is a suggested classification
Class Lycopsida ("club-mosses') Order Drepanophycales (ancestral types) Order Protolepidodendrales Order Lycopodiales (Club "mosses") Order Selaginellales (Spike "mosses") Order Lepidodendrales = Lepidocarpales (Scale trees) Order Miadesmiales Order Pleuromeiales (intermediate forms) Order Isoetales ("quillworts')
Introduction to the Lycophyta - evolutionary history of the group - best on the web
History of Palaeozoic Forests - Fossil and Extant Lycophytes - description and links to photos of recent and extinct forms.
Lycophytes - readable palaeobotanical intro to the Lycopsids