Anatomy of the palm Rhapis excelsa, IX. Xylem structure of the leaf insertion

STEMS OF PERENNIAL PLANTS, particularly trees, represent a considerable investment in biomass. Trees can survive even under the most adverse conditions, but only if the hydraulic integrity of the stem is preserved. A very important and vulnerable part of the stem is the xylem. As water is pulled into the top of the tree, a period of excessive drought can drop xylem pressures to such low negative values that water columns break (cavitation). The tracheary elements in which this happens are permanently lost as functional parts unless positive pressures refill them within a very short time (perhaps hours?).