Longitudinal section of a Zea (corn) stem. Helical thickening of secondary wall on a xylem tracheary element is clearly visible. Sieve tubes visible to left of xylem.
Micrograph by John Tiftickjian
Spirally-thickened xylem in castor oil plant
Xylem; SEM of a pit membrane from a bordered pit-pair (white pine). The thickening at center, the torus, is surrounded by the membranous margo. Pit pairs, found in the cell walls or tracheids, permit H2O movement to other plant tissues. Mag 2000x at 35mm.
Circular bordered pits of pine tracheids seen in face view
Micrograph by John Tiftickjian
Bordered pits of pine tracheids seen in cross sectional view
Micrograph by John Tiftickjian
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Exarch xylem in dicot root :[/h]
Ranunculus young root x.s. showing Casparian strips.
Micrograph by John Tiftickjian
A simple pit seen in cross-sectional view.
A bordered pit seen in cross-sectional view.
Drawing by John Tiftickjian
Drawing of bordered pit with torus, common in conifers. The middle of the pit membrane is modified to form the torus.
Drawing by John Tiftickjian
The valve-like action of the torus in a bordered pit prevents the spread of cavitation.
Drawing by John Tiftickjian