Selaginella ludoviciana, S. apoda and Their Hybrids in the Southeastern United States

More subtle differences between the two species were revealed by anatomical studies of stems and leaves (Buck, 1974) and by examination of spores with the light microscope and the scanning electron microscope. For instance, in S. ludoviciana stomata are restricted to a few rows adjacent to the midribs of the leaves (Fig. 5), whereas in S. apoda they are scattered throughout the laminae (Fig. 2). Also, examination of leaf clearings and scanning electron micrographs showed a high percentage of papillate cells scattered throughout the lower epidermis of S. ludoviciana leaves and very few, if any, on those of S. apoda (Figs. 2 and 5). Comparisons of the spores of the two species reveal other differences. Knox (1950) described the microspores of both species as being warty with crenate margins. Hieronymus (1900), however, distinguished the species by their microspore morphology, describing those of S. ludoviciana as smooth and those of S. apoda as warty, verrucose, or irregularly rugose. In our study, we consistently found the microspores of S. ludoviciana to be smooth and those of S. apoda to be warty when viewed with the light microscope, thus confirming Hieronymus' findings. When viewed with the SEM, the microspores of S. apoda can be described as rugulo-reticulate (Figs. 8 and 9) to verrucate. Terminology for these spore wall descriptions is based primarily on that defined by Harris (1955). In some specimens (Fig. 7) the spores of S. apoda are covered with small spicules or papillae. In S. ludoviciana the so-called smooth spores are in fact covered with short papillae or spicules (Figs. 10-12). Measurements of microspores from 16 specimens of each species failed to show a significant size difference (x = 26.47ium for S. apoda and x = 27.24,m for S. ludoviciana).