Fossil range: Silurian - Recent
| Lycopodiopsida - clubmosses |
The Division Lycopodiophyta (sometimes called Lycophyta) is a tracheophyte subdivision of the Kingdom Plantae. It is the oldest extant (living) vascular plant division at around 420 million years old, and includes some of the most "primitive" extant species. These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.
There are around 1,200 living species divided into three main groups within the Lycopodiophyta, sometimes separated at the level of order and sometimes at the level of class. These are subdivided at the class level here:
- Class Lycopodiopsida – clubmosses and firmosses
- Class Selaginellopsida – spikemosses
- Class Isoetopsida – quillworts
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.
The Lycopodiophyta are one of several classes of plants that expanded onto land during the Silurian and Devonian periods. Like all explorers it found new hazards that demanded new solutions. While the ocean currents blended oxygen, water and nutrients into a soup, the land had a layered structure with water and minerals in the soil and oxygen and light in the air. The intense sunlight presented a greater risk of genetic damage. Without water, pervasive desiccation became a possibility, and more structural support was required to resist gravity.
Many adaptations of the Lycopodiophyta can be explained as a response to these conditions. They continued the development and specialization of roots to extract nutrients from the soil and developed leaves for photosynthesis and gas exchange, using a stem for transport. A waxy cuticle helped retain moisture, and stoma allowed respiration. The vulnerable meiotic gametophyte is protected from radiation by its reduced size and often by the use of subterranean mycorhiza for its energy source instead of photosynthesis. Club-mosses are homosporous, but spike-mosses and quillworts are heterosporous. In heterospores the female spores are larger than the male because they store food for the new generation.
During the Carboniferous period, tree-like Lycopodiophyta (such as Lepidodendron) formed huge forests and dominated the land. 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 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 treatmen t for Alzheimer's disease.