Positron emission tomography (PET) scans of a poplar tree (Populus sp.) sapling. The scan at right was taken after the plant was put under stress with a simulated insect attack. The scan at left was taken before this attack. One hour prior to both scans an individual leaf was exposed to carbon dioxide gas that contained a radioactive carbon tracer. The carbon is converted into sugar by the plant and is used in biochemical pathwaus. The PET scanner images the distribution of the sugar in the plant. High levels are shown in red, low levels are blue. The plant that is under attack has sent more of its sugar to the roots for storage.