Anatomy of the palm Rhapis excelsa, VIII. Vessel network and vessel-length distribution in the stem

THE CONCEPT of the vessel, a conducting unit consisting of a series of vessel elements lined up end to end, has been known for well over a century (e.g., Hartig, 1878). The fact that vessels are of limited length is of considerable functional importance. If a vessel is damaged (for example, by an insect), air is drawn into its lumen as water withdraws into neighboring tissue, because xylem water is normally under less than atmospheric pressure. The damaged vessel is thus permanently lost as a functioning unit. Numerous small vessels therefore represent a conservative and safe water-conducting system, while wide and long vessels are much more vulnerable, although a great deal more efficient (Zimmermann, 1978).