A careful physical analysis of gas bubble dynamics in xylem.

Many studies have confirmed that cavitation in xylem is caused by air bubbles. Recently Shen et al. (Tree Physiol. 22 (2002) R655), analysed the expansion of a pre-existent bubble in xylem and one formed by air seeding. The present paper makes a further analysis of bubble expansion by the equilibrium criterion of the Helmholtz function. It has been proved that when xylem pressure Pl′ decreases to a special value Pl′∗ from a value higher than, or equal to, or lower than −Po (Po is atmospheric pressure), an air bubble in xylem can grow up steadily, corresponding to minimums of the Helmholtz function F(r). As soon as Pl′ is lower than Pl′∗, since F(r) will be a decreasing function when Pl′More