Silicification of developing internodes in the perennial scouring rush (Equisetum hyemale var. affine)

An electron microprobe (EMP) analysis of silica (SiO₂) deposition in the epidermis of developing internodes of the perennial scouring rush (Equisetum hyemale var. affine) indicates that SiO₂ is first detected in the stomatal apparatus beginning with internode 3, then the epidermal papillae (internode 8), and finally in radial cell walls of the long epidermal cells (internode 10). This process is initiated in the intercalary growth regions at the bases of the elongating internodes. The deposition of SiO₂ in long epidermal cell walls occurs after internodal extension has ceased and should therefore be considered as one of the final stages in internodal differentiation that involves strengthening the cellulosic framework of the cell wall. EMP measurements indicate that SiO₂ in stomata is equivalent to 30% of a pure SiO₂ standard and that SiO₂ in the radial walls of long epidermal cells averages twice that measured on the tangential walls of these same cells. This study supports the view that silicification plays a major role in strengthening the developing perennial scouring rush internodal system and that regulation of this process in this and other species of Equisetum, whose SiO₂ deposition patterns are markedly different, deserves further study.