Scanning electron micrograph of a petiole of a water lily, Nymphaea alba. The picture shows a cross section of a petiole that has also been sliced longitudinally. The petiole is the stalk that joins a floating leaf to the submerged stem (rhizome) of the plant. It contains tunnels, called air canals. Three are visible in section, below and to the right of the picture, with two, sliced open, above to left. The canals bring photosynthetic oxygen from the leaves to sustain the rhizome, which is buried in anoxic mud. Each canal is an open tube; an easy route for pests or pathogens to reach the rhizome, once they have entered the floating leaves. To deter them, the canals are lined with sclereids (stiffened hairs, yellow), 1mm from tip to tip, that are covered with small crystals of poisonous calcium oxalate. Aggregations of detached crystals are visible inside the canals to picture right.