Pteridophyte Vascular Anatomy

species identification and phylogeny
Anatomie der Farnpflanzen: Artbestimmung und Evolution
by Florian MENZEL
Jh. Ges. Naturkde Württ. 155(1999): 107-133

Abstract

No other plant taxon shows such a diversity in its vascular systems as ferns. The rachis steles of different species form one C-,X- or V-shaped bundle, two elliptical bundles, different patterns of several small bundles, etc.

This diversity was used in the above-mentioned report to work out a key for the middle European pteridophytes which is based only on anatomic (i.e. histologic) characters. Plants can easily be identified to species (or, at least, to genus) by a cross section through the rachis (ferns) or the stem (lycopods and sphenophytes) respectively. This is especially useful when identifying young and sterile plants or hybrids.

Due to its diversity, the stelar system can also be important for taxonomic observations. To support that an attempt was made to work out a proposal for the stelar evolution of the Polypodiales rachises.

According to this hypothesis the steles of all leptosporangiate fern species can be derived from one primitive form by processes of fusion and reduction. There is also one stele that may represent the "basic type" of the derived ferns.

Small and large ferns are exposed to different selection pressures on mechanical rigidity. Different steles among closely related species can therefore often be explained by differences in leaf size (e.g. in Bolbitis). This correlation is especially obvious in the Aspleniaceae.
All Aspleniaceae species bear two small bundles in the petiole which fuse to a single one in the upper part of the leaf. Apart from the Polypodiaceae (which have side bundles, see below), the Aspleniaceae are the only family where the bundles and their xylems fuse in their middle, thus forming an elliptic bundle with an X-shaped xylem (instead of a U-shaped one, which occurs in other taxa). In smaller species (up to 20cm leaf size), the single elliptic bundle can be found all over the petiole and rachis. This bundle provides much less rigidity than a U-shaped one (which occurs in other families). This observation correlates to the fact that most of the Aspleniaceae species do not exceed approximately 40cm in leaf size and do not need to very rigid steles.
The only very large species is Asplenium nidus (up to 140cm leaf size). In comparison to other Asplenium species, its stele is modified, presumably so that it provides enough rigidity: The upper arms of the X-shaped xylem and bundle are strongly eked out, probably by concaulescence of the pinna strands. Besides, the rachis of this species has a significant keel which is supposed to stabilize the leaf additionally.

Asplenium nidus, petiolar bundle	Dicksonia antarctica, upper right part of petiolar bundle

A special case is represented by the so-called "side bundles". These are small bundles arranged in a circle or semi-circle. They occur only among the derived ferns. Several characteristics give evidence that they evolved separate from the other bundles. One possible explanation for their evolution is that they originated from adventitious roots that fused with the rachis. These roots have only one vascular bundle and are formed multitudinously at the base of the leaf stalks among many fern species. The presumed fusion with the rachis may have provided the opportunity for leaves growing larger. This hypothesis also explains why side bundles only occur among rather large species. Side bundles seem to have evolved in at least three families independently.

bundle and xylem arrangement in Cibotium menziesii (Dicksoniaceae), Pteridium aquilinum (bracken,
Dennstaedtiaceae), and Gymnocarpium robertianum (Aspidiaceae) (from left to right)

bundle and xylem arrangement in Asplenium nidus (Aspleniaceae), Thelypteris limbosperma (Thelypteridaceae)
and Phlebodium aureum (Polypodiaceae, with side bundles)