The oldest land plants (2)
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.
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 carbondioxide from the air. The spines are sometimes also
called leaves. 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.
 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.
About 400 million years ago, in the beginning of the Devonian, there was
a kind of Yellowstonepark 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
photo shows a section of the stem of the most common plant, Rhynia.
The diameter of the stem is in reality 1,3 mms. By clicking on to 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.
A
second plant common in the Rhynie Chert is called Asteroxylon. It
is a very early member of the club-mossgroup. In the picture you can see
the woodvessels 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 on to the photo to see one. One spore has a diameter of about
50 µms.
The Rhynie Chert is also full of fungal hyphae (threads) and spores. This
means that already in the Early Devonian plantremains
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.
Further developments
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 plantfossils 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 Gosslingia
from the Brecon Beacons in Wales. This one is slightly younger than the plants
of the Rhynie Chert. This plant too lacks leaves: gasexchange was entirely
through the stems. The ultimate branchings of the twigs are coiled up in
spirals, a feature occurring in many primitive plants.
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 bij 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 Belgium Late Devonian this is not yet the case.
However the branches in a way resemble fernleaves. The sporangia of this
plant are growing in clusters with a diameter of about 2.5 cms.
Seadplants
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 on
to 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
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