The Pteridophyta constitute one of the major divisions of the plant kingdom.
Unlike the Bryophyta, they are plants which have supporting and conducting tissue and they have true stems, leaves and roots. However, they have not evolved to a point where seed production takes place. Hence, they are a group rather lower than seed plants in the evolutionary scale. They are a rather diverse group which can be split fairly convincingly into three large sub-groups, each with its own characteristic features.
These are the clubmosses. They have the following characteristics:
- The aerial shoot system of the sporophyte generation has, in most species, a dichotomously branching axis.
- The axis is differentiated into root and shoot.
- The leaves are small, termed microphylls; each has a single strand of conduction tissue, but this has no leaf gap where it diverges from the conducting tissue of the stem.
- Sporangia are borne in or near the axils of the microphylls.
- Heterospory is common.
- In living species, the gametophytes live upon the soil surface or underground. Little or nothing is known about fossil gametophytes; they are such small and delicate structures that they are very unlikely to be preserved as fossils.
- The spermatozoids have two flagella (only known in living species).
Living amongst the earliest land plants were some which already showed features deemed to be ancestral to the clubmosses. These are sometimes placed into a smaller sub-group, the Prelycopods. Individual genera may not have all of the characters of the main group but there are normally sufficient to justify their inclusion here. They are all extinct.
Asteroxylon, Drepanophycus, Kaulangiophyton, Baragwanathia, Protolepidodendron, Estinnophyton, Leclercqia, Colpodexylon, Archaeosigillaria, Sugambrophyton, Lycopodites, Phytokneme, Protolepidodendropsis, Cyclostigma, Prolepidodendron.
Lepidodendron, Lepidophloios, Bothrodendron, Duisbergia, Sigillaria, Pleuromeia, Valmeyerodendron.
Later evolved members of the Lycopsida developed into substantial trees. Some specimens exceed 38m in height. Other genera remained of modest size.
Isolated root systems are classified in the genus Stigmaria. Detached cones are named as species of Lepidostrobus or Sporangiostrobus.
Some fossil Lycopsida produced single, large spores (megaspores) in their megasporangia. Examples of these are Lepidocarpon and Achlamydocarpon. They are of interest as they provide information about the way in which seeds probably evolved in plants.
This group comprises the horsetails, some species of which are living today, and other fossil genera which are now extinct. The main features of the group are as follows:
- One main vertical axis with branches arising in whorls between the leaves at the nodes.
- Leaves microphyllous.
- The main stems are usually ridged.
- Underground rhizomes are commonly found.
- The sporangia are grouped in terminal cones (strobili).
- Heterospory uncommon.
- The gametophytes grow on the soil surface (only known in living species).
- The spermatozoids have many flagella (only known in living species).
As was the case with the clubmosses, some early genera showed affinities with this group and are included here:
Archaeocalamites, Pseudobornia, Eviostachya, Ibkya.
The name of this sub-group is derived from their wedge-shaped leaves. The narrow ends of the wedges forming the points of junction with the stems.
Fossil example: Sphenophyllum
Detached cones may be assigned to: Bowmanites, Litostrobus, Peltastrobus, Sphenostrobus, Cheirostrobus.
These are the true horsetails; included in this sub-group is the living genus Equisetum. Members of fossil genera were very substantial in size, growing up to 20m tall.
Fossil examples: Calamites, Annularia, Arthropitys, Phyllotheca, Schizoneura.
Detached cones may be assigned to: Calamostachys, Palaeostachya, Mazyostachys, Cingularia.
This is by far the largest group within the Pteridophyta. The ferns are still widely represented in the Earth’s flora. Some, such as the tree ferns may attain a height of 15m and may thus be very conspicuous in their habitat. Although known as tree ferns they have no true trunks. Instead, their stems are thickly clothed with massed rootlets which simulate trunks and have sufficient strength to support the apical crown of leaves.
Ferns are characterized by the possession of, usually, large leaves or megaphylls as they are called. The megaphyll appears to have evolved from an entire, branching, shoot system with most of its parts flattened into leaf laminae. The group as a whole is large and diverse with possibly as many as 10,000 species living at the present day. The range of plant structure exhibited is extreme. Some ferns are very small plants floating on water whilst others live deep in rock crevices where the light intensity is reduced to very low levels. The leaves of these plants are quite translucent; a characteristic that has caused these ferns are known as the filmy ferns. Conversely, some species, as we have seen, may be as big as trees and many others are quite stout plants commonly seen in parks and gardens.
It is beyond the scope of this book to attempt a full classification of the entire group. Instead, a list of features is followed by some fossil examples. As with the Lycopsida and Sphenopsida, some genera considered to be ancestral to the Filicopsida are given first.
- Leaves often megaphyllous. Many species with the stem as an underground rhizome.
- Sporangia usually on the undersides or margins of leaves; sometimes on fertile leaves.
- Heterospory very uncommon.
- The gametophytes grow on the soil surface (only known from living genera).
- The spermatozoids are multiflagellate.
Fossil ancestral examples: Hyenia, Pseudoporochnus, Calamophyton, Rhacophyton, Cladoxylon.
Stauropteris, Zygopteris, Corynepteris, Metaclepsidropsis, Psaronius, Scolecopteris, Pecopteris, Botryopteris, Anachoropteris, Ankyropteris, Osmundacaulis, Weichselia, Tempskya.
This was the point, then, about 350 million years ago when in the history of plants, the major groups had appeared. Some spore-bearing ones had progressed to producing two types of spores as described in Chapter 2 and even seed-bearing ones evolved.
As it turned out, plant life was about to show a vast explosive development that would have considerable consequences for mankind quite apart from promoting the development of many new large forms that must have looked extremely impressive at the time.