Vessel contents in the petiole of a transpiring sunflower leaf. Vessels containing water shown in green, gas spaces (embolisms) in red. Some vessels are completely embolized, others contain varying amounts of water as well as gas. The proportion of embolized vessels was followed during a day's transpiration. Petioles were snap-frozen intact on the transpiring plant, and viewed, still frozen, in the cryo-scanning microscope.