-for garnish Note: Save shells from the shrimp, including heads, if available. This is the quintessential Thai soup aromatic, flavorful, citrus-scented, delicious. If this doesn't hook you on Thai cooking and gardening, nothing will. Serves four. Adapted from Southeast Asian Cookery by Sallie Morris (Grafton Books). Shell and devein shrimp, reserving heads and shells. Place shells and heads in a large saucepan. Add water, lemongrass and Siamese ginger. Using a mortar and pestle, pound two of the chilies with the coriander roots and add to pan. Add the salt and five of the lime leaves, shredded into little pieces. Bring the soup to a boil and simmer uncovered 20 to 25 minutes. Strain stock into a clean pan. Don't panic if it doesn't taste wonderful at this point because the seasoning adjustments will make a big difference. A few minutes before serving, bring the stock to a boil, add the shrimp and cook 2 to 3 minutes. Add the nam pla, lemon and lime juices and the straw mushrooms. Adjust seasoning. The flavor should be tangy. Serve in bowls garnished with strips of remaining chili, shredded lime leaf and coriander leaves. Judith M. Fertig in Flower and Garden, 2-3/93.