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Stomata of a Horsetail or Scouring Rush (Equisetum palustre). Stomata are important in regulating gas exchange in plants. The oval cells on either side of the opening are the guard cells.  The small rounded structures that cover the surface of the plant are deposits of silica which gives the plant a rough feel. Horsetails are living fossils and are the oldest plants on earth. SEM X2500 (based on 4" x 5")