Stigmaria and stumps of clubmoss trees I II
In the vast marshes of the Late Carboniferous, an exuberant vegetation of herbaceous plants and trees grew. The giant, up to 45 m high, clubmoss trees Lepidodendron and Lepidophloios, as well as Sigillaria, up to 20 or 25 m high, dominated it. It is estimated that they supplied about 60 to 90% of the plant material from which coal was later formed. One wonders how these forest giants could have survived storms in such a swampy environment.
The classical answer to this question is that they had an extensive, horizontally growing root system of four repeatedly forking axes (see the image on the right from Heimans (1911) : Uit ons Krijtland). Nevertheless, one remains in doubt whether that was sufficient to withstand the great forces of weather and wind.
To these axes root-like appendages were attached in a spiral. They are called appendices. These are not real rootlets because the so-called root cap, a cap-shaped group of cells that protects the growth tip, is missing. Over time, these appendices were rejected leaving a characteristic round scar (stigma) on the axis (see Figures 2 and 3). Therefore the fossils of this underground system have been given the genus name Stigmaria. The most common species is called Stigmaria ficoides.
It is remarkable that the many, often very different, club moss trees all have the same root system. For example, Lepidodendron and Lepidophloios have trunks and/or side branches that are forked many times, while the trunks of Sigillaria only branched twice at the most. And Lepidodendron had a real crown, while the leaves of Sigillaria formed only a kind of giant shaving brush.
The appendices were 5 - 10 mm thick and they branched sporadically. Whether the subterranean axes were real roots similar to those of today's trees is debatable. This is because the appendices are spirally attached to the axes, just like leaves. This indicates that those splitting axes were in fact a kind of adapted side branches and that the roots were adapted leaves. The fact that they were shed over time is also a characteristic they have in common with leaves.
generations of club moss trees formed thick
layers of peat, which were compressed by the pressure of newly formed
The formation of peat required that the subsidence of the soil kept
the formation of new peat. As the subsidence accelerated, it drowned
including living vegetation and covered it with a layer of sediment of
and/or sand. Further carbonisation of the peat into lignite, coal and
anthracite occurred as a result of temperature increase. The anthracite
was only reached when the layers ended up at great depths or when magma
which made the temperature very high. The latter may have been the case
Piesberg coal beds.
The stiffness of the club moss trunks came mainly from the outer layer, which consisted of thick-walled cells. The proportion of wood in the trunks was low, in contrast to the trunks of today's deciduous and coniferous trees. It is therefore assumed that club moss trees had a short lifespan (10 to 15 years) and that they grew (and decayed) rapidly.
Stigmaria is mainly found in the root layer: that is the layer below the coal layer, wheras the plant layer is above the coal layer.
4 and 5 are of large pieces of Stigmaria ficoides from the root layer with the
appendices lying in the split plane.
That is also the case in coal balls. These are concretions of magnesium or calcium carbonate formed in the peat stage in which the plant residues are preserved up to cellular level. They occur in South Limburg, Belgium, Germany and England (Lancashire/Yorkshire) and a lot of information has been extracted using various techniques. Figures 9 and 10 are from slides purchased by the University of Groningen in England in 1922. On the left is a picture of the secondary wood of a very thin axis of Stigmaria. The wood is divided into sectors by a number of wood rays, of which the cells have disappeared here. Compare also picture 7.
Figure 9 is a cross-section of seven appendices. Their narrow vascular bundles are in the central cavity.
Although all club moss trees have the same root system, there is a small difference between trees from the Sigillaria-group and the non-Sigillaria-group. In Sigillaria the central vascular bundle is connected to the bark, while this is not the case with Lepidodendron and its relatives. Compare figures 8 and 10. This difference can only be seen in material in coal balls and silifications.