The Spores and Taxonomy of Isoetes butleri and I. melanopoda

Abstract

Isoetes L. is a genus of lycopods consisting of at least 60 species, and is of worldwide distribution. Members of this genus are small, herbaceous, perennials and are aquatic, amphibious, or terrestrial. Isoetes is sometimes referred to by the common name of Quillwort, which refers to its elongate, quill-like leaves. Because its grass-like or rush-like habit often makes it difficult to discern from surrounding vegetation, it is often overlooked in the field. Due to its rather peculiar morphology, Isoetes has been the subject of some interesting statements. Linnaeus (1754, p. 486) described the staminate and pistillate flowers of Isoetes as incomplete and solitary in the axils of the leaf bases. Engelmann (1878) stated, species of Isoetes are the simplest vascular plants known. The tufted, quill-like leaves of Isoetes arise from a thick, flattened, corm-like, underground axis. A sporangium is located adaxially at the base of each fertile leaf; just above the sporangium is a small, flat, generally triangular ligule. The outer wall of the sporangium may be more or less covered by a flap of tissue called the velum. Isoetes is heterosporous, producing megaspores or microspores in sporangia on separate leaves. Monographers have used characters of the habitat, ligule, velum, and sporangium in the taxonomy of Isoetes. In addition, the size and ornamentation of the megaspores have often been used to separate the various taxa more precisely. The recognition of Isoetes melanopoda Gay & Durieu and 1. butleri Engelm. as distinct species has been questioned by a number of workers, from Pfeiffer in her monograph of the Isoetaceae (1922) to Evans in Manual of the Vascular Flora of the Carolinas (1968). Most recently, the validity of . butleri as a species has been questioned by Kral (1973, p. 367), who stated that leaf, ligule, velum, and corm characters all vary too much for accurate delimitation of the taxa. However, the spores of these two species normally do differ in size. The megaspores of I. butleri are usually greater than 480 ,/m in diameter, whereas those of 1. melanopoda are usually less than 450 ,tm across. It should be noted, however, that intermediate spore sizes do exist in both species. Since recently collected material from Arkansas and Illinois proved difficult to identify with certainty using the traditional morphological characters, the authors decided to examine both megaspores and microspores of these two taxa under the scanning electron microscope (SEM). Spores for examination were chosen from authentic or type material located in the Missouri Botanical Garden (MO) and from Arkansas, Illinois, and Missouri collections which, by the standards of previous monographers, fit either I. melanopoda or 1. butleri. The spores were prepared for examination by mounting them on double-sided plastic tape and