The oldest land plants (II) (I)
During the Early Devonian (405 - 385 million years ago) several other
groups of plants developed. They had the common feature of being rather small
(not higher than half a meter) and simply structured.
The compressed fossils of Devonian plants scarcely give any information. Determining these very old plants is extremely difficult as hardly anything shows on the fossils. Only stems, bifurcations and possibly spines. With luck you find sporangia. In that case it might be possible to detect the name of the plant. Sometimes the cuticle has stayed intact. After chemical treatment it is possible to make a microscope preparation of it in which e.g. cells and stomata are visible.
The discovery of a silicified moor
near the Scottish village of Rhynie, 40 km north-east of Aberdeen, has revealed
a lot about the structure of very old plants.
A second plant common in the Rhynie Chert is called Asteroxylon. It is a very early member of the clubmoss group. In the picture you can see the wood vessels of this plant, reinforced with annular and spiral thickenings.
Of a third plant, Horneophyton, the sporangia and the spores are found often. Typical of higher plants is that the spores are formed in clusters of four. For some time these often cluster in so called tetrads. Click the photo to see one. One spore has a diameter of about 50 µm.
From the Early Devonian the flora evolution gains momentum. Wereas the number
of species in the Late Silurian could be counted on the fingers of two hands,
in the Early Devonian this has become quite impossible. Yet at that moment
the number of species was still very limited and in many cases the vegetation
at a certain spot consisted of only one species or a very small number of
species. At the rare finding places of well-preserved Early Devonian plant
fossils one often tends to find one dominating species with the occasional
sparse occurrence of other species.
Another plant from the Zosterophyllum group is Gosslingia from the Brecon Beacons in Wales. This one is slightly younger than the plants of the Rhynie Chert. In Gosslingia the sporangia are sitting scattered along the stems and the tips of the stems are coiled.
The further the Devonian proceeds, the more forms appear and the higher some plant species become. In the Middle Devonian tree 'ferns' appear with a height of some meters. In the Late Devonian some plant groups develop the ability to form thicker stems by means of secundary development. Thus we see the appearance of woody stems enabling the plants to form trees. In the Late Devonian there are already rather high trees up to 8 meters.
During the Middle and the Late Devonian more species with leaves or leaflike structures appear. These developed through 'webbing' of finely branched twigs, i.e. the twigs became connected by intervening tissue. In the fernlike plant Rhacophyton from the Belgian Late Devonian this is not yet the case. However the branches in a way resemble fern leaves. The sporangia of this plant are growing in clusters with a diameter of about 2.5 cm.
The oldest seed plants date from the Late Devonian. The special feature of seeds is their being enveloped. In Moresnetia (so called after the town of Moresnet in Belgium) this envelope is not yet completely closed. It is placed around the seed like a sort of calyx, leaving the top of the seed visible. Moresnetia is one of the oldest seed plants in the world and the oldest in Europe. For the time being, for new discoveries are made regularly. In Belgium extraordinary well preserved specimens have been found, in which even the seeds are visible. Click the photo!
The oldest seed plants were gymnosperms for the seeds were not yet embedded in an ovary. From this kind of plants the many species of seed ferns developed that grew in the coalswamps during the Carboniferous. And a Moresnetia-like plant must have been the ancestor of all modern flowering plants. Surely a plant to treat with respect.