Apical regions of developing aerial shoots of Psilotum nudum (L.) Beauv. were studied using both light and scanning electron microscopy (SEM) with the aim of improving our understanding of early stages in stomatal and epidermal ontogenesis. Samples for SEM were fixed in 3% gluteraldehyde in phosphate buffer (0.065M, pH 7), dehydrated in a graded ethanol series, critical point dried in CO2 and coated with ca. 30 nm gold-palladium alloy. Samples for light microscopy were fixed in FAA, dehydrated in a graded ethanol series and embedded in paraffin with a melting point of 51-53 C. Sections were stained with 0.05% toluidine blue, a general protein stain. Developing stomata in P. nudum grow from less than 20 µm to about 70 µm long. SEM observations of specimens show 20 µm-long domed meristemoid cells as the first detectable stage of differentiation of the guard cell mother cells (GCMCs). First detection of a furrow dividing the guard cells is at ca. 30 µm long. Deposition of wax that will eventually cover the entire stomatal aperture begins In 70 µm long stomata. Longitudinal sections of the shoot show GCMCs with an unusual hourglass-like appearance and the presence of a cytoplasmic protein net surrounding the nucleus and occupying all of the developing guard cell. The cytoplasmic net appears more compact when the developing furrow becomes deeper, and is probably involved as an organizer of cell morphology. This staining pattern was detected only in the cytoplasm of developing guard cells and not in surrounding epidermal cells. This information will be used as a basis for ongoing studies comparing Psilopsida, whose systematics are still equivocal, to other taxa.

Key words: ontogeny, Psilotum, stomata