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.

Fig. 2. Worn off piece of Stigmaria from the Piesberg. Width of the picture 15 cm. Westphalian D.

Fig. 3. Stigmaria with clear scars and some appendices attached. Ibbenbüren, Westfalian C. Height of the photo 8 cm.

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.

Habitat
The club moss trees were adapted to moist to very wet soils. In places with clayey soils the water cannot seep away and such areas were ideal for these trees. The forests they formed were often very uniform without significant undergrowth. Only in drier places in the often very extensive marsh forests cordaits and herbaceous plants grew, such as sphenophylls and perhaps ferns,

Many generations of club moss trees formed thick layers of peat, which were compressed by the pressure of newly formed layers. The formation of peat required that the subsidence of the soil kept pace with the formation of new peat. As the subsidence accelerated, it drowned the peat including living vegetation and covered it with a layer of sediment of mud and/or sand. Further carbonisation of the peat into lignite, coal and anthracite occurred as a result of temperature increase. The anthracite stage was only reached when the layers ended up at great depths or when magma rose, which made the temperature very high. The latter may have been the case at the 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.

Fig. 4. Large block from the root layer with Stigmaria with attached appendices. Piesberg, Westphalian D	Fig. 5. Stigmaria with attached appendices from a root layer  of the Piesberg. Westphalian D. Height of the photo 35 cm
Fig. 6. Stigmaria rugulosa of Duisburg-Hamborn (D), Westfalian A. Width of the photo 7 cm. Collection and photo A. Reinink	Fig. 7. Structure of Stigmaria. After Stewart 1947

Figures 4 and 5 are of large pieces of Stigmaria ficoides from the root layer with the appendices lying in the split plane.
Stigmaria ficoides can be found all over the world. In addition, some other species have been described, such as Stigmaria rugulosa which is shown in picture 6. Most probably, however, these are preservational forms of S. ficoides.

Structure
The internal structure of the root carriers can be seen in figure 7. There is a central cavity surrounded by secondary wood, formed by a one-sided cambium. This is a layer of cells that forms wood to one side, the inside. It is surrounded by a three-layered bark. The anatomical structure has not been preserved in compression fossils. In some silicified Stigmaria's the structure has been fossilized.

Fig. 8. Rootlet of Sigillaria: the vascular bundle  is connected to the bark. Photo: H. Kerp.

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.

Fig. 9. Secondary wood of a young Stigmaria with some sections of appendices lying around it.  Width of the photo 2,8 cm. Coal ball from Yorkshire, Upper Carboniferous.

Fig. 10. Sections of appendices. The vascular bundle lies in the central cavity. Width of the photo 4,7 cm. Coal ball from Yorkshire, Upper Carboniferous