More About Lycopsids
Ancient and Formerly Glorious Plants
Lycopsids are the oldest group of living vascular plants. Some of their extinct members were major players in the expansion of plants onto the land, and massive arborescent forms were the most important contributors to the great coal beds that fueled the Industrial Revolution. Today they are represented by only six genera belonging just three families: Lycopodiaceae (club mosses), Selaginellaceae (spike mosses) and Isoetaceae (quillworts). Theyre typically inconspicuous ground cover, epiphytic or aquatic components of modern floras.
Lycopsids are related to several extinct groups, including Cooksonia (some, but not all species), Asteroxlon, Drepanophycus, zoosterophylls and barinophytes, through the possession of sporangia (spore-bearing organs) that are stalked and kidney-shaped (reniform). Collectively known as lycophytes, these plants comprise one of the two great clades of vascular plants that probably diverged from the earliest vascular plants sometime from the Silurian or Early Devonian; the other clade, euphyllophytes, include the trimerophytes, ferns, sphenopsids (e.g., horsetails), progymnosperms (e.g., Archaeopteris), and seed plants (e.g., conifers, cycads, and angiosperms).
Lycopsids are distinguished from other lycophytes because they posses spirally arranged microphylls (narrow, spine-like leaves supplied with a single, unbranched vein) and adaxial sporangia. (Intermediate forms such as Asteroxylon and Drepanophycus are commonly referred to as pre-lycopsids.) The earliest known lycopsid, Baragwanathia, is known from the Middle or Late Silurian of Australia as well as the Early Devonian of Euramerica. Its a herbaceous plant that appears to be most closely related to the modern day club mosses (e.g., Lycopodium). Club mosses are homosporous (they produce uniformly-sized bisexual spores). This condition, which is believed to be primitive, was probably also present in the earliest lycopsids.
The Protolepidodendrales (e.g., Archaeosigillaria, Leclercqia and Protolepidodendron) are a group of lycopsids that extended from the Early to Late Devonian. Many were short, herbaceous plants, but some reached heights of at least 50 cm. They had rhizomous rooting structures, dichotomous branching and were probably homosporous. At least one species, Leclercqia complexa, possessed ligules (tiny flaps of tissues attached just above the microphyllous leaves), a feature also found in spike mosses, arborescent (tree-like) lycopsids and quillworts.
Spike mosses (Selaginellales) probably diverged from the Protolepidendrales sometime during the Middle or Late Devonian, but their fossils first appear in the Upper Carboniferous. They exhibit heterosporous reproduction (producing egg-like megaspores and sperm-like microspores) and endosporic develoment of the megaspore that approaches the seed habit of seed plants. They are represented today by a single genus, Selaginella.
Arborescent (tree-like) lycopsids also probably diverged from Protolepidendrales during the late Middle Devonian (Givetian). Once the dominant plants of Carboniferous coal swamps and important components of Late Paleozoic and Triassic wetlands, theyre represented today by only a single genus, Isoeteles. In addition to their arborescent habit, the more derived forms (i.e., after the Early Carboniferous) are notable for their cambial growth (thickening axial growth), production of secondary tissues (e.g., wood and secondary cortex), bipolar growth (regular upward and downward growth), microphyllous leaf abscission, heterosporous reproduction and endosporic development.
Several of the earlier arborescent lycopsids exhibit features that appear to be transitional between the Protolepidendrales and the more derived post-Devonian forms (Isoetales and Lepidodendrales). Some of the earliest taxa have been reported from China and Kazakhstan. Two Chinese genera, Chamaedendron and Longostachys, were probably 0.5-1.5 m high, and had dichotomous branching of rooting and branching structures. A lycopsid from Kazakhstan, Atasudendron, was estimated to be 2-3 m high and had a greater differentiation of stem tissues that did the Chinese species. Heterospory was found in Chamaedendron and the reproductive organ of a lycopsid from Kazakstan.
Early arborescent lycopsids have also been reported from Euramerica. For example, Protolepidodendropsis (Givetian, Spitzbergen) may have been a small tree with a dichotomous crown. Lepidosigillaria (Frasnian, New York) had a swollen (cormose) base and a trunk at least 5 m tall. It also had leaf cushions (the distinctive bases of abscised leaves) that were arranged linearly near the base but spirally at about 1 m in height. Clevelandodendron (Famennian, Ohio) was an unbranched cormose lycopsid that was about 1.5 m tall. Cyclostigma, an abundant and widespread lycopsid from the Late Devonian and Early Carboniferous, reached a height of at least 8 m. It had a pattern of dichotomous branching and leaf cushions that were similar to those of the Carboniferous Lepidodendrales, but it had a rooting structure with short, blunt lobes. Lepidodendropsis, another common lycopsid from the Late Devonian and Early Carboniferous, also had dichotomous branching and a lobed and furrowed rooting structure. Heterospory has been recorded from several Late Devonian Euramerican taxa, including Bisporangiostrobus, Barsostrobus, Cyclostigma, and Clevandodendron.
Arborescent lycopsids probably diverged into the Isoeteles and Lepidodendrales sometime during the Early Carboniferous. The Lepidodendrales (or scale trees) are distinguished from the Isoeteles (and the earlier arborescent lycopsids) primarily by the presence of certain anatomical details of the leaf cushion and the development of a stigmarian rhizomorph (an extensively branched root system similar in structure and growth to their aerial branches). They were a spectacular but relatively short-lived group. Some lepidodendralians became the tallest trees of the Paleozoic. The most famous scale tree, Lepidodendron, is estimated to have exceeded 50 m in height. Diaphorodendron, Lepidophiloios and Sigillaria are other important examples of the Lepidodendrales. These trees dominated the lowland swamps of the Upper Carboniferous and contributed most of the organic matter to the vast coal deposits that extend over much of North America and Eurasia. But the diversity and stature of these giants declined dramatically near the end of the Carboniferous (late Westphalian and Stephanian) and they became extinct early in the Lower Permian.
The Isoetales are far less spectacular than the Lepidodendrales, but their evolutionary history is considerably longer. They can generally be distinguished from their lepidodendralian relatives by their cormose (or lobed) rooting structure and their more modest growth habit. Most, if not all, isoetaleans apparently grew as unbranched axes (stems). Examples include Chaloneria (Late Carboniferous), Pleuromeia (Triassic) and Nathorstiana (Lower Cretaceous). Isoetes, the last surviving genus of the Isoetales and of probably all arborescent lycopsids, doesn't share their arborescent habit. Instead, it's a short, semi-aquatic currently plant found in a variety of freshwater and brackish habitats.
Convergent but Strange
Arborescent lycopsids provide a striking example of convergent evolution. Although they diverged from the euphyllophytes (e.g., ferns, horsetails, progymnosperms and seed plants) sometime in the Silurian or Early Devonian, they independently evolved secondary tissues, deciduous leaves, and heterosporous reproduction. Some grew to be giants. Some evolved a reproductive system that approached the seed habit. However, they were strange when compared to the seed plants (conifers, cycads ginkoes and angiosperms) that populate our world.
Secondary growth (thickening rather than upward growth) in arborescent lycopsids and seed plants is generated by a ring of tissue called the vascular cambium. This tissue is bifacial in seed plants. Tissue on the inside (i.e., the inside face) of the vascular cambium creates secondary xylem (wood) while that on the outside creates secondary phloem (a living tissue that conducts photosynthate to other tissues) and bark. As a seed tree grows, the axes increase in diameter by generating wood on the inside, while the living phloem remains near the surface where it can continue to receive oxygen. In contrast, lycopsids have a unifacial vascular cambium. The inside face produces wood, but the outside face doesnt produce secondary phloem. Arborescent lycopsids produced relatively little wood, which apparently functioned primarily to conduct water from the rooting organs to photosythetic tissues higher in the plant. Instead, structural support apparently depended on a massive cortex that extended from the outside face of the vascular cambium to the external layer of persistent leaf cushions. The outermost layer of this cortex developed into a dense bark-like tissue called the lycopod periderm, which was waterproof and decay resistant. We dont understand how cortical tissues developed, but we know that it didnt continue to grow outward as the tree grew upward.
There are several consequences to the unifacial cambium. One is determinate growth. Another is that the tree had to establish its final trunk girth early in its development. In other words, it essentially started out as a "thick stump" that grew into similarly thick pole covered with leaves. It would continue growing as a thick pole until it reached a pre-determined height. If a branching lycopsid (e.g., Lepidodendron), it would then branch a pre-determined number of times, produce sporangia (spore producing organs), reproduce, and then die; non-branching forms would simply skip the branching stage.
Another consequence of the unifacial cambium is the lack of phloem with which to transport the products of photosynthesis. This has led some authorities to suggest that the living tissues in rooting structures depended on photosynthetic rootlets, which were essentially modified leaves. Indeed, photosynthate distribution in the aerial branches appeared to be very limited. Lycopsid leaves, also known as microphylls, contain only a single bundle of vascular tissue with little if any phloem. (In contrast, the leaves of ferns and seed plants, also known as megaphylls, contain multiple vascular strands and considerable amounts of phloem.) One consequence of this localized distribution of photosynthate is the pattern of microphyllous leaf abscission. As the tree grew upward, the lower parts of the trunk would shed their deciduous leaves and essentially become dead tissue.
Arborescent lycopsids adopted what could be considered a quick and cheap lifestyle. Due to the inherit characteristics of a unifacial cambium, they invested few metabolic resources in the transport of photosynthate and in wood production.
One advantage of their developmental system is very rapid growth. The giants of the coal swamps apparently achieved maturity in only a few years. Once they reached their final height, they would develop and release their spores and then die. This resulted in remarkably short generation times and very rapid turnover. Its not surprising that they generated a tremendous amount of biomass, much of which ultimately became coal. Rapid growth apparently facilitated the re-establishment of dominance in favorable habitats following short-term disturbances (e.g., storms). On the other hand, relatively short life-spans may have inhibited their recovery following longer-term disturbances (e.g., climatic change). It may have contributed to their demise at the end of the Carboniferous; the return of favorable lowland habitats during the Stephanian of Euramerica was not accompanied by the return of the great lycopsid swamp forests. Instead, these habitats became dominated by tree ferns (e.g., Psaronius) and pteridosperms (seed ferns).
A degree of convergence also occurred in the reproductive of seed plants and arborescent lycopsids. Heterospory, the production of specialized sperm-like microspores and egg-like megaspores is widely regarded as a precursor to the seed habit. It evolved in several lineages (i.e., barinophytes, zygopterid pre-ferns, stauropterid pre-ferns, sphenopterids, water ferns, Archaeopteris, spike mosses and arborescent lycopsids). However, there are two elaborations of heterospory that are widely seen as additional precursors to the seed habit. These are the reduction of the total number of functional megaspores in a sporangium to a single functional spore and endosporic development (the fertilization of the haploid megagametophyte and initial development of diploid sporophyte occurs within the spore wall). Both of these elaborations are found in spike mosses and (at least some) arborescent lycopsids. Moreover, some of the lepidodendrid lycopsids surrounded their megagametophyte with a protective integument analogous to the seed coat in seed plants (e.g., the form genus Lepidocarpon). However, even though reproduction in some arborescent lycopsids approached the seed habit, it still depended fertilization by free-swimming sperm entering the shed megasporangium via a horizontal slit in the protective integument. This is thought to have occurred in the standing waters that typically surrounded the base of these trees.
- Francis Cardillo's web page on Isoetales (Quillworts):
- Hans Kerp's web pages on Carboniferous forests and lycopsid images:
- M. & H. Hieb's Plant Fossils of West Virginia:
- Palaeos.com's web page on the Lepidodendrales: www.palaeos.com/Plants/Lycophytes/Lepidodendrales.html
- Ralph Taggart's web pages on arborescent lycopsids and Carboniferous forests:
- U.C. Museum of Paleontology's introduction to the Lycophyta:
- U.C. Museum of Paleontology Virtual Lab web page on lycophytes: www.ucmp.berkeley.edu/IB181/VPL/Lyco/LycoTitle.html
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- Image Credits: