5.1 A dominant, branched sporophyte #
Vascular plants have lignified tracheids and phloem, and a dominant, branched, independent sporophyte, so one fertilisation can supply many sporangia. Silurian and Early Devonian Cooksonia is a forked axis with terminal sporangia, no leaves, and no roots; some specimens have tracheids. Rhynia, from the Rhynie chert, has a protostele and a cuticle, and fungi in that deposit already show mycorrhizal-like partnerships. Aglaophyton conducts without true tracheids. Roots and leaves came later, and more than once.
A homosporous gametophyte is free-living but small: green, or, in many lycophytes, subterranean and fungus-fed. Heterosporous gametophytes are often endosporic, developing inside the spore wall, as they do in seed plants.
5.2 Lycophytes and euphyllophytes #
Lycophytes (clubmosses, spikemosses, quillworts) have microphylls, leaves with one vein, not associated with a leaf gap. The classical view derives them from enations that later gained a vein. Sporangia are axillary or adaxial, often in a strobilus. Lycopodium in the broad sense is generally homosporous. Selaginella and Isoetes are heterosporous. Isoetes, a corm with quill-like leaves, is the living neighbour of the Carboniferous tree lycophytes, closer to them than a modern clubmoss is. Size is not phylogeny.
Euphyllophytes (ferns in the broad sense, plus seed plants) have megaphylls, typically with branching veins. Zimmermann’s telome theory derives a megaphyll from a flattened, webbed branching system. It is a homology hypothesis, and fern and seed-plant megaphylls were assembled separately. The practical contrast is one vein and no gap, against a leaf trace leaving a more complex stele. It is not “small leaf versus big leaf”.
Monilophytes include leptosporangiate ferns, marattioid ferns, ophioglossoid ferns, and horsetails. Molecular trees place Equisetum among the ferns. Psilotum looks rootless and almost leafless and was taught as a living Rhynia. It is a reduced fern, related to the ophioglossoid line. Its simplicity is a loss. Horsetails have jointed stems, leaf sheaths, and sporangia on sporangiophores; sperm are multiflagellate. A leptosporangium develops from one initial, has a one-cell-thick wall, and usually an annulus. A eusporangium (marattioid ferns, Ophioglossum, lycophytes, seed plants) develops from several initials, has a thicker wall, and holds many more spores.
Wood is not tied to the seed. Late Devonian Archaeopteris, a progymnosperm, was a tree with secondary xylem and fern-like fronds, neither a fern nor a modern conifer. Seeds are also Late Devonian, older than the Carboniferous forests below. A trunk and a dispersal unit are separate inventions.
Teaching skeleton, with the uncertainties already stated: bryophytes sister to vascular plants; vascular plants split into lycophytes and euphyllophytes; euphyllophytes split into monilophytes and seed plants.
5.3 The fern life cycle in full #
The study of The fern life cycle in full provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Gymnosperm Life Cycle
5.3.1 Sporophyte, sorus, annulus #
The familiar plant is the sporophyte: rhizome, adventitious roots, and fronds. Many leptosporangiate fronds unroll as a crozier (circinate vernation). Ophioglossum and horsetails do not. Sporangia are often abaxial, in a sorus. An indusium is the cover: kidney-shaped in Dryopteris, linear in Asplenium, absent in Polypodium, replaced by a reflexed margin (a false indusium) in bracken. The indusium does not throw the spores.
As a leptosporangium dries, water leaves the annulus cells, the annulus coils, the wall tears at the stomium, and a further snap throws the spores. The device is hygroscopic, like a moss peristome. A typical leptosporangium yields about 64 spores from 16 spore mother cells. Most ferns are homosporous. The water ferns (Marsilea, Salvinia, Azolla) are heterosporous and endosporic. Azolla carries a cyanobacterium in cavities of the leaf. It is still not a seed plant.
5.3.2 Prothallus and the next sporophyte #
A homosporous spore grows a prothallus: in the standard temperate case a green heart a few millimetres to about a centimetre across, rhizoids and antheridia below, archegonia near the notch, no vascular tissue, easily dried. Fern sperm are multiflagellate and swim in a surface film. Without that film there is no fertilisation, so the sexual stage stays tied to moisture even when the sporophyte later tolerates drier ground. Bracken’s rhizome spreads where a new prothallus would rarely succeed. The two generations have different ecologies.
The zygote stays in the archegonium. A foot absorbs from the prothallus until the young sporophyte has a root and a leaf and the prothallus withers. Meiosis is in sporangia on the mature fronds, years later. Compared with a moss, both have swimming sperm, a retained embryo, and spores from meiosis. The fern sporophyte becomes the whole independent plant and can live for years. Neither has a seed. Tree ferns are tall by a fibrous trunk and a crown of fronds, not by conifer wood.
5.4 Carboniferous forests #
Carboniferous wetland forests of tropical Euramerica were dominated by plants with no living match in size. Tree lycophytes (Lepidodendron, Sigillaria) reached tens of metres on trunks built mostly of thick periderm, not of an oak-like cylinder of wood. Their spreading root systems are Stigmaria. Tree-sized horsetail relatives (Calamites), tree ferns, and pteridosperms (seed plants with fern-like leaves, not ferns) grew with them. Coal of this age is compressed peat from those wetlands, where burial outran decay. Coals of other ages exist.
Height had already been solved in more than one way, and the earliest seeds are Late Devonian, older than these forests. Burial of that peat is part of the long-term carbon cycle. A fossil tree fern is not the ancestor of a named living genus. A living quillwort is related to those tree lycophytes and is still a small aquatic plant. The coal is peat from extinct side branches.
5.5 Key terms, summary, and self-check #
Key terms. Microphyll; megaphyll; lycophyte; monilophyte; leptosporangium; eusporangium; sorus; indusium; annulus; stomium; prothallus; endospory; progymnosperm; pteridosperm.
Summary. Vascular plants combine lignified conducting cells with a branched independent sporophyte; roots and leaves were added later and more than once. Lycophytes bear microphylls; monilophytes, including horsetails and reduced Psilotum, bear megaphylls and are the living non-seed euphyllophytes. A fern throws spores with a drying annulus, grows a prothallus that still needs external water for swimming sperm, and then becomes an independent sporophyte. Carboniferous coal forests included bark-trunked tree lycophytes and true seed plants. They show that height and the seed had already been achieved. They are not a set of ancestors-by-resemblance, and they do not come with an invented atmospheric figure.
Self-check.
1. Why is Psilotum a poor model of the earliest vascular plants?
2. How does a microphyll differ from a megaphyll if size is not the definition?
3. Where does meiosis occur in a fern, and what does the prothallus still require?
4. What does the annulus do, as distinct from the indusium?
5. What made up most of a Lepidodendron trunk, and why is a seed fern not a fern?
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