Equisetum xylem: SEM studies and their implications

Scanning electron microscope (SEM) studies of xylem of three species of Equisetum reveal numerous details not previously reported on the basis of light microscopy. SEM images of thick (ca. 1 mm) sections reveal pit shapes, cell contexts, and microstructure of pit membranes. Pit shapes are remarkably diverse in comparison to those of ferns or conifers. Nodal tracheary elements are isodiametric to fusiform in shape, and have crowded circular (mostly) to elongate prominently bordered pits and uniformly thick secondary walls. Internodal tracheary elements, by contrast, have relatively large circular pits with inconspicuous borders. Secondary walls of internodal tracheids are thin, with thickenings that are annular, looplike, or of some intermediate form. Metaxylem internodal tracheary elements line the inner surface of carinal canals (= proxylem lacunae), and many of the large circular (often crateriform) pits facing the canals may lack pit membranes, especially in E. giganteum and E. myriochaetum. Because dye experiments show that carinal canals can conduct water in stems of Equisetum, the fact that portions of tracheary elements facing the canals may have perforations (many of the pits in E. myriochaetum lack pit membranes despite careful handling techniques) is significant. This opens the possibility that internodal tracheary elements may, in some species, be vessel elements that permit conduction from the carinal canals of one internode to those of the next internode (carinal canals are not intercontinuous between internodes), aided by metaxylem tracheids of the nodal plates. Such vessel elements would not be the same as those reported by Bierhorst, who found vessels only a few vessel elements long in rhizomes.