Stigmaria

Stigmaria Temporal range: Carboniferous-Permian, Possible Devonian record
Stigmaria, a fossil lycopsid rhizome
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Lycophytes
Class:	Lycopodiopsida
Order:	†Lepidodendrales
Genus:	†Stigmaria Brongn.

Stigmaria is a form taxon for common fossils found in Carboniferous rocks. They represent the underground rooting structures of arborescent lycophytes such as Sigillaria and Lepidodendron under the order Lepidodendrales.

Description & Morphology

In situ lycopsid with attached stigmarian system. Joggins Formation (Pennsylvanian), Cumberland Basin, Nova Scotia.

Bedding plane view of a flattened Stigmaria preserved atop a shallow-water carbonaceous limestone in the Joggins Formation (Pennsylvanian), Cumberland Basin, Nova Scotia.

Overview

The Paleozoic swamps had tree-like plants that grew to 50 meters in height and were anchored by an extensive network of branching underground structures with rootlets attached to them. Analysis of the morphology and anatomy of stigmarian systems suggests they were shoot-like and so they are called rhizomes or rhizomorphs. Most of the Stigmaria fossil specimen described derive from the common species of Stigmaria ficoides.

Stigmaria ficoides

Rootlets

The stigmarian systems had rhizomorph axes that typically had circular scars around the rhizomorphic apices, where the root hairs were attached to the rootlet scars. These rootlets were branched dichotomously, meaning that there were multiple orders of branching to comprise areas of the stigmarian system. Since the stigmarian systems were shoot-like, these rootlets may be modified leaves, adapted to serve the function of roots. Stigmaria had a complex branching structure and root hairs; therefore, it is comparable to the rhizomes of the extant (living) relative, the quillworts (genus Isoetes). The stigmarian system extends leaning from the foundation of the stem as four proximal axes, leading to a hollow rooting structure.

Along the rhizomorph axes, the appendages are linked to each axis in a circular pattern and would shed during the growth stage, forming the circular scars of Stigmaria. The appendages have a circular layout, and the growth abscission is more distinctive of leaves than present lateral roots. The four proximal axes dichotomize, creating a long underground system ranging up to 15 m (49 ft) in radius, while being up to 40 cm (16 in) long and 0.5–1 cm (0.20–0.39 in) wide. The stigmarian rootlets vary in size and do not dichotomize as they distance away from the central point of the rootlets. In terms of the position, each rootlet has a vascular strand monarch which is crowded by a condensed inner cortex. Externally, this inner cortex consists of a thin outer cortex and hollow middle cortex, and a network enlarges from the inner cortex to the outer cortex. Endarch is defined as the primary xylem of Stigmaria, and organized and arranged in forked vascular strands encompassed by vascular cambium. Tracheids of the secondary xylem are formed in spiral lines and consist of scalariform wall thickenings, while the fimbrils are similar to those in the aerial branches. The radially aligned tracheids within stigmarian rhizomorph axes did not produce vascular cambium, but by a thickening meristem.

Stigmaria was primarily the main root structure of lycophyte trees.

Development

The development of Stigmaria is linked to the changes in aerial stems found in present roots. Stigmaria's features are void in their functionality when relating to present functions of roots. The spiral system of the rootlet attachment is incomparable to the unsymmetrical adjustment of present roots. While there were root hairs in Stigmaria, none were found in present roots. However, fungi has mycorrhizae, which are functioned from cortical parenchyma cells. Though vascular bundles in leaves are bilaterally symmetrical including the rootlets of Stigmaria and the monarch vascular bundle, present roots have a radial point of symmetric vascular tissues. Furthermore, the rootlets' active shedding from the primary axes is apparent within a certain growth stage, where the foliar abscission or shedding is in uniform style. Nonetheless, root abscission is relatively unperceived in modern plants. The stigmarian rootlets indicate that there is a similarity in the Lepidodendrales aerial structure system, and carry support to its stability features including absorbent organs, branching, and forking of proximal axes. The linkage to the Lepidodendrales vascular plants proposes that the development and changes of the aerial shoot system including the stems existed among stigmarian systems.

While some of the lycopsids had a height of up to 40 m (130 ft), their stigmarian system was commonly limited to the surface water levels, and therefore it is debatable to how the underground system could handle the plants. Since many lycopsids from the Paleozoic grew in unsteady engulfed and saturated soil. Evidence to support their height was compared to the extensive root system. Thus, it may be that the progression of the rhizomorph axes appeared to have moderate advancement. They may have been preferred to stand upright since arborescent lycopyhtes had bushy branches and only a few secondary xylem. The branches of neighboring lycopsids could interweave and deliver conjunct support. The branch density and development of the wood in present trees typically have a large support to their tree uprooting.