According to Professor Julian W. Sauls on the Texas A&M Extension website, limes are second only to lemons in terms of their importance in food and industrial uses. Lime trees grow in the wild and for agricultural purposes and are becoming increasingly popular as potted trees. Although known for their fruit, their nondescript leaves support their life and indicate to their caretakers when something is amiss.
Lime tree leaves are broad, slightly heart-shaped and medium green, and the halves are not symmetrical. The edges of the leaf are jagged with many tiny "teeth." The underside of the leaf is lighter than the top, and the leaf's veins are displayed prominently. Some species of lime trees have tufts of hair where the leaf veins fork. Other species, such as the kaffir lime tree, have leaves shaped in unexpected and distinct ways.
Lime tree leaves produce energy for the tree through photosynthesis. When light strikes the leaf, small structures called chloroplasts absorb the energy, becoming excited and causing a series of chemical reactions that produce sugar from water and carbon dioxide. This sugar powers the plant's life processes and collects in its fruits, giving limes their sweet flavor. Plants can feed themselves using energy from the sun. Lime trees use this energy to produce fruit and leaves--both used in cooking--to feed organisms higher on the food chain.
Lime trees are subtropical evergreens, so they do not normally lose all of their leaves at any single time. This feature confines lime trees to warmer climates, where their leaf tissues will not become damaged by low temperatures and frosts. When grown indoors or in containers, leaf loss generally indicates the plant is stressed, receiving too much or too little water or is in need of fertilizer. Diseases and pest infestations may also cause leaf loss.
Citrus trees, including lime trees, are prone to diseases and pest infestations. Leaves that develop spots or discoloration are often affected with a fungal disease, such as greasy spot or sooty mold. Diseases may also cause lime trees to shed their leaves. Insect pests that affect lime tree leaves include caterpillars, aphids, whiteflies, scales and snails. These pests may chew holes in leaves or drain the sap, causing the leaves to become dry, curled and deformed.
Thai recipes often call for kaffir lime leaves, which are either sliced into thin strips for use in curries and stir fry or used whole to flavor soups, similar to bay leaves. Kaffir lime leaves have a distinctive appearance, resembling two leaves stuck together, end to end. To prepare them, leaves should be tightly rolled and sliced as thinly as possible to distribute the strong flavor throughout the dish.