Although a model of air seeding through pit membranes embedded in hydrogel is recently focused on and whether the capillary failure is an appropriate physical model comes to be a question, the consensus is that air seeding through the pit membranes is the principle mechanism of embolism spread in xylem. Shen et al. (2002) analyzed the expansion of a preexistent bubble in xylem and one formed by air seeding. After that the article (Shen et al. 2003) made a further analysis of bubble expansion by the equilibrium criterion of the Helmholtz function. It proved that when xylem pressure decreases to a special value from a value higher than, or equal to, or lower than ( is atmospheric pressure), an air bubble in xylem can grow up steadily, corresponding to minimums of the Helmholtz function. As soon as is lower than , since will be a decreasing function when , resulting in non-equilibrium of the bubble, it will break inducing a cavitation event. Given , if an air bubble could enter a conduit, it would be in a stable equilibrium. When an air bubble entering a conduit will be in an unstable equilibrium. As the water further vaporizes, it will break at once. Considering the shape of a conduit, we presented that there are three types of cavitation caused by air-seeding(Shen et al. 2012). After an air seed enters a conduit at high xylem pressure, along with the drop of the water potential, it will expand gradually to a long-shaped bubble and extend continually. This is the type of expanding gradually. When the xylem pressure is moderate, right after an air seed enters a conduit, it will expand first. Then, as soon as the pressure reaches a threshold the bubble will blow up to form a bubble in long shape, accompanied by acoustic (or ultra acoustic) emission. It will extend further as xylem pressure decreases continually. This is the type of expanding——exploding， becoming a long shaped bubble——lengthening by degrees. In the range of , soon after an air seed is sucked into a conduit it will explode immediately and the conduit will be full of the gas of the bubble instantly. This is the type of sudden exploding and filling conduit instantly. The third type is the frequent event in daily life of plant. Under the theoretical guidance on the cavitation types, three phenomena have been captured in the xylem sections of P. orientalis using a light microscope(Shen et al. 2015). Firstly, an air bubble appearing in a conduit expanded and elongated gradually to fill it; Secondly, a bubble emerging in a conduit expanded gradually, suddenly became a long shaped one, then extended continually. And thirdly, a bubble fully filled a conduit instantly. Considering practical factors, the three phenomena captured in our experiment were explained by our theory. The phenomenon of bubble's development in partly intact corn leaf by X-ray phase contrast microscopy could also be interpreted by the theory.