The Division Lycopodiophyta is a tracheophyte subdivision of the Kingdom
Plantae. It is the oldest extant vascular plant division at around 410
million years old. These species reproduce by shedding spores and have
macroscopic alternation of generations, although some are homosporous while
others are heterosporous. Members of Lycopodiophyta bear a protostele, and
the sporophyte generation is dominant. They differ from all other vascular
plants in having microphylls, leaves that have only a single vascular trace
rather than the much more complex megaphylls found in ferns and seed
plants. Classification
There are around 1,200 living species of Lycopodiophyta which are generally
divided into three orders; in addition there are extinct groups. There is some
variation in how the extant orders are grouped into classes: they may be put
into a single class; they may be put into two classes, with the Isoetales and
Selaginellales combined into one class; or they may be put into three classes,
one order in each. The system which uses two classes for extant species is:
Class Lycopodiopsida – clubmosses and firmosses
Class Isoetopsida – quillworts, scale trees, and spikemosses
Class † Zosterophyllopsida – extinct zosterophylls.
The following phylogram shows a likely relationship between Lycopodiophyta
orders. Evolution
The members of this division have a long evolutionary history, and fossils are
abundant worldwide, especially in coal deposits. In fact, most known genera are
extinct. The Silurian species Baragwanathia longifolia represents the
earliest identifable Lycopodiophyta, while some Cooksonia seem to be related.
Lycopodolica is another Silurian genus which appears to be an early member of
this group. Fossils ascribed to the Lycopodiophyta
first appear in the Silurian period, along with a number of other vascular
plants. Phylogenetic analysis places them at the base of the vascular plants;
they are distinguished by their microphylls and by transverse dehiscence
of their sporangia. Sporangia of living species are borne on the upper surfaces
of microphylls. In some groups, these sporophylls are clustered into strobili.
During the Carboniferous Period, tree-like Lycopodiophyta formed huge
forests that dominated the landscape. The complex ecology of these tropical
rainforests collapsed during the mid Pennsylvanian due to a change in
climate. Unlike modern trees, leaves grew out of
the entire surface of the trunk and branches, but would fall off as the
plant grew, leaving only a small cluster of leaves at the top. Their remains
formed many fossil coal deposits. In Fossil Park, Glasgow, Scotland,
fossilized Lycopodiophyta trees can be found in sandstone. The trees are marked
with diamond-shaped scars where they once had leaves.
The Lycopodiophyta had their maximum diversity in the Upper Carboniferous,
particularly tree-like Lepidodendron and Sigillaria, that dominated tropical
wetlands. In Euramerica these became apparently extinct in the Late
Pennsylvanian, as a result of a transition to a much drier climate, to
give way to conifers, ferns and horsetails. In Cathaysia tree-like
Lycopodiophyts survived into the Permian. Nevertheless, lycopsids are
rare in the Lopingian, but regained dominance in the Induan, particularly
Pleuromeia. After the worldwide Permian–Triassic extinction event
Lycopodiophyta pioneered the repopulation of habitats as
opportunistic plants. The heterogeneity of the terrestrial plant communities
increased markedly during the Middle Triassic when plant groups like
sphenopsids, ferns, pteridosperms, cycadophytes, ginkgophytes and conifers
resurfaced and diversified quickly. Characteristics
Club-mosses are homosporous, but spike-mosses and quillworts are
heterosporous, with female spores larger than the male, and gametophytes forming
entirely within the spore walls. The spores of Lycopodiophyta are highly
flammable and so have been used in fireworks. Currently, huperzine, a
chemical isolated from a Chinese clubmoss, is under investigation as a
possible treatment for Alzheimer's disease.
Gallery References
External links Introduction to the Lycophyta from the
University of California Museum of Paleontology
Lycophytes Fossil Groves
Paleo Plants