In many plants, the highest resistance to water flow occurs at the leaf trace or in the base of the petiole. This means that as water stress builds during drought, cavitation (formation of air cavities in xylem elements) occurs first in the leaves, so they may wilt and die and the water system in the trunk remains relatively intact. It may be possible to produce new leaves but not a new trunk, specially in palm trees, for example, in which there is no secondary growth of xylem in the trunk. Thus, leaves wither away earlier than stems.