Electron microprobe analysis of the distribution of silicon in the leaves of Selaginella emmeliana

Scanning electron microscopy and X-ray microanalysis of the ventral leaves of Selaginella emmeliana demonstrated high levels of silicon in the marginal sclereids and about one-half the abaxial epidermal cells. Although both of these cell types bear rows of knobs or warts on the outer tangential wall, the X-ray spectra indicate that silicon is distributed throughout the wall and that it is not concentrated in these projections. In S. emmeliana, stomata are confined to a band of low silicon cells overlying the midrib on the abaxial side of the leaf. A small proportion of these stomata also contain high levels of silicon and are characterized by a constricted stomatal aperture. A preliminary survey of three other species of Selaginella indicates that accumulation of silicon in the leaf epidermis is characteristic for each.