Periderm Production in the Mississippian CladoxylopsidCladoxylon taeniatumand a Review of Periderm Occurrence in Paleozoic Plants

Abstract

Premise of research. Periderm is a protective secondary tissue that replaces the epidermis in stems and roots having secondary growth. The first periderm most commonly originates immediately below the epidermis, but in some species, it arises deeper in the stem, usually in the primary phloem. Periderm is a common feature of extant plants and has been reported in numerous fossil taxa. Here we investigate the presence of this tissue in a Carboniferous representative of the cladoxylopsids, an extinct group related to the ferns s.l.Methodology. We describe anatomically preserved stems of Cladoxylon taeniatum with a well-preserved periderm from Tournaisian deposits in France and Germany. We also provide a short review of the current state of our knowledge on periderm production in Paleozoic plants, especially in relation to secondary vascular growth.Pivotal results. The new specimens are 7–15 mm in diameter. They have a well-developed secondary xylem and a thin periderm that has a deep-seated origin in the innermost part of the cortex. This shows that Cladoxylon had the ability to accommodate secondary vascular growth with the production of a periderm that replaced the cortex and epidermis. Cladoxylon taeniatum is, to date, the only cladoxylopsid in which this feature is described, and it increases the diversity of anatomical strategies for secondary growth documented in the group. Changes in the nature and geometrical distribution of tissue between typical stems with a cortex and stems with a periderm suggest that C. taeniatum was non-self-supporting, a growth habit that also differs from that of most other older cladoxylopsids.Conclusions. The new specimens increase the diversity of Paleozoic plants that produced both secondary vascular tissues and a periderm. Our review highlights the fact that anatomical homologies between the structures called “periderm” in these various taxa remain unclear; numerous questions about the origins of the periderm and the possible relation between secondary vascular growth and periderm production remain.