A further comparison of Isoetes roots and stigmarian appendages

Abstract

Four additional structural similarities between the roots of Isoetes and Stigmaria are reported. Proximal swellings and connectives are reported for the first time in Isoetes roots. The size ranges of the roots of Isoetes and Stigmaria are shown to overlap, and a definite epidermis is described for the first time on the roots of Stigmaria. The structural diversity present among the roots of various lycopods is reviewed. It is concluded that the root is of polyphyletic origin within the Lycopsida. The roots of Lycopodium and Selaginella show many similarities with stems and are considered to be derived from the stems of some rootless ancestor, whereas the roots of Isoetes and the lepidodendrids show many similarities with their own leaves and are considered to be derived from leaves or from some less specialized appendages from which both the leaves and roots of these plants were derived.