Recipes for stock often specify the use of parsley stalks because they have more flavor than the florets, which will turn bitter if cooked for several hours. This is also economical as the parsley leaves can then be used for garnishing. Other herbs suitable for adding to stocks are the woody ones that will stand up to prolonged cooking: sprigs of marjoram, tarragon and thyme, as well as bay leaves. A slice of lemon or pared lemon rind is a good addition to chicken stock and onion or tomato skins can also be added to give a strong golden color and extra flavor.
** Asian Recipes