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The oldest land plants (II)  (I)

Spiny plants

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. Sawdonia
Thus the plant Sawdonia ornata had no leaves, but it did have spines. The function of the spines was probably to enlarge the surface for the assimilation of carbon dioxide from the air. They were not needed for defense for the first vertebrate land animals (the amphibians) did not appear before the Middle Devonian. Possibly the plants formed thick shrubberies using the spines as a hold.
Another spiny plant from the Early Devonian is Drepanophycus spinaeformis. This one had a thicker stem with firm spiny leaves. The stems are often found without the spines. In this case the scars will be visible.

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.

Rhynie Chert      

RhynieThe 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.
About 400 million years ago, in the beginning of the Devonian, there was a kind of Yellowstone park with vulcanos and geysers at that location. Some of the geysers spouted boiling, siliceous water with intervals of a couple of years which caused the vegetation, including the underlying peat layers, to silicify. The fossils in this so called Rhynie Chert are so perfect that the plants can be studied within the accuracy of cell structure level. It is possible to make very thin slices of the fossil bearing chert, which can be viewed under the microscope.

The Rhynie Chert was discovered in 1912 by the geologist Dr. W. Mackie. The paleobotanists R. Kidston and W.H. Lang have published a very extensive description of the plants and the fungi in the chert from 1917 to 1922. In the last decades many new discoveries have been made.

Rhynia The photo shows a section of the stem of the most common plant, Rhynia. The diameter of the stem is in reality 1,3 mm. By clicking the photo you can see a stoma of this plant as well, complete with two closing cells. The state of conservation is extremely good.
Rhynia is very close to the primeval plant from which most of the modern plants have originated.

Asteroxylon 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.

Sporangium of HorneophytonOf 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.

The Rhynie Chert is also full of fungal hyphae (threads) and spores. This means that already in the Early Devonian plant remains Hyphae
decomposed by fungal activity. Further many little animals have been found. Mostly they are under 1 mm. In most cases it concerns mites and springtails.
At the moment research is still in full progress. The Rhynie Chert is of equal importance for the understanding of the oldest land plants as the Burgess Shale for animal life.
(click here for more photos of the plants and animals of the Rhynie chert)

Further developments

Zosterophyllum 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.
The plants still have a relatively simple structure, like Zosterophyllum from the Forfar area in Scotland. This plant lacks leaves: gas exchange was entirely through the stems. The ultimate branchings of the twigs are coiled up in spirals, a feature occurring in many primitive plants. The sporangia are placed in long, loose spikes.

Some stems of Zosterophyllum are found showing K- or H-like branching. These are probably underground stems (rhizomes). Click the photo to see the details.

GosslingiaAnother 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.

RhacophytonDuring 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.

Seed plants

MoresnetiaThe 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.

Hans Steur