Studies of petiolar anatomy in ferns: structural diversity and systematic significance of the circumendodermal band

In ferns, leaf traces in the petioles are sometimes surrounded by a circumendodermal band (CB), a cell layer of varying structure and arrangement. We studied the CB of 89 fern species from 53 genera and 28 families, and its correlation with leaf trace configuration, leaf dissection and life form, to explore its anatomical diversity, possible origin and suitability for fern systematics. The cells of the CB always contain tannins in their lumina and their walls, but lack lignin and suberin. Cell wall thickness varies among species and may prove to be useful for fern systematics. The occurrence of the CB correlates with a larger number of vascular strands in the leaf trace and a lower degree of leaf dissection, but not with life form. Cystopteridaceae is the earliest branching family with a CB. The exclusive presence of the CB in nine of the most derived fern families supports the hypothesis of a single origin before the divergence of the two major clades of eupolypods. The discontinuity or absence of the CB in some species of derived families (e.g. Polypodiaceae, subfamily Polypodioideae) is interpreted as a secondary loss. Possible functions of the CB are discussed.