walnut - The nuts of all the species are edible, but the walnuts commonly available in stores are from the Persian Walnut, the only species which has a large nut and thin shell. A horticultural form selected for thin nut shells and hardiness in temperate zones is sometimes known as the 'Carpathian' walnut. The nuts are rich in oil, and are widely eaten both fresh and in cookery. Oil paint often employs walnut oil as an effective binding medium, known for its clear, glossy consistency and non-toxicity. Walnuts are also an excellent source of omega-3 fatty acids, and have been shown as helpful in lowering cholesterol. They need to be kept dry and refrigerated to store well; in warm conditions they become rancid in a few weeks, particularly after shelling.