leaf Any flattened, green outgrowth from the stem of a vascular plant. Leaves manufacture oxygen and glucose, which nourishes and sustains both plants and animals. Leaves and stem tissue grow from the same api¬ cal bud. A typical leaf has a broad, expanded blade (lamina), attached to the stem by a stalklike petiole. The leaf may be simple (a single blade), compound (separate leaflets), or reduced to a spine or scale. The edge (margin) may be smooth or jagged. Veins transport materials to and from the leaf tissues, radiating from the petiole through the blade. They are arranged in a netlike pattern in dicot leaves and are parallel in monocot leaves (see cotyledon). The leafs outer layer (epidermis) protects the interior (mesophyll), whose soft-walled, unspecialized green cells (paren¬ chyma) produce carbohydrate food by photosynthesis. In autumn the green chlorophyll pigments of deciduous leaves break down, revealing other pigment colors (yellow to red), and the leaves drop off the tree. Leaf scars that form during wound healing after the leaves drop are useful for iden¬ tifying winter twigs. In conifers, evergreen needles, which are a type of leaf, persist for two or three years.
Structures of a leaf. The epidermis is often covered with a waxy protective cuticle that helps prevent water loss from inside the leaf. Oxygen, carbon dioxide, and water enter and exit the leaf through pores (stomata) scattered mostly along the lower epidermis. The stomata are opened and closed by the contraction and expansion of surrounding guard cells. The vascular, or conducting, tissues are known as xylem and phloem; water and minerals travel up to the leaves from the roots through the xylem, and sugars made by photosynthesis are transported to other parts of the plant through the phloem. Photosynthesis occurs within the chloroplast-containing mesophyll layer.
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leaf-footed bug See squash bug
leaf insect or walking leaf Any of about 25 species of flat green insects (family Phylliidae) with a leaflike appearance. Leaf insects, which range from India to the Fiji Islands, are about 2.3 in. (60 mm) long. The female has large leathery forewings (tegmina) that lie edge to edge on the abdomen and resemble, in their vein pattern, the midrib and veins in a leaf. The hind wings have no function. The male has small tegmina and ample, non-leaflike, functional hind wings. The newly hatched young are reddish, but become green after feeding on leaves.
Leacock, photograph by Yousuf Karsh
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© 2006 Encyclopaedia Britannica, Inc.
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leaf miner Any of various insect larvae that live and feed within a leaf, including caterpillars, sawfly larvae, beetle and weevil grubs, and dipteran maggots. Most leaf-miner burrows or tunnels are either thin, winding, whitish trails or broad, whitish or brownish blotches. Though leaf miners do not usually cause injury, they mar the appearance of ornamental trees and shrubs. One method of control is to remove and bum infested leaves; spraying with nicotine solutions or dusting with insecticides is effective only when the adults are emerging.
leafhopper Any of the small, slender, often beautifully coloured and marked sap-sucking insects of the large family Cicadellidae. There is a leafhopper species for almost every type of plant. Most are less than 0.5 in. (12 mm) long. Leafhoppers can be serious economic pests. Their feeding may remove sap, destroy chlorophyll, transmit disease, or curl leaves; they also puncture the host plant while laying eggs. Hopperburn is a diseased condition caused by their injection of a toxin into the plant as they feed.
League of Arab States See
Arab League
League of Nations Organization for international cooperation estab¬ lished by the Allied Powers at the end of World War I. A league covenant, embodying the principles of collective security and providing for an assembly, a council, and a secretariat, was formulated at the Paris Peace Conference (1919) and contained in the Treaty of Versailles. The covenant also set up a system of colonial mandates. Headquartered at Geneva, the League was weakened by the failure of the U.S., which had not ratified the Treaty of Versailles, to join the organization. Discredited by its failure to prevent Japanese expansion in Manchuria and China, Italy’s conquest of Ethiopia, and Germany’s seizure of Austria, the League ceased its activities during World War II. It was replaced in 1946 by the United Nations.
Leahy \'la-he\, William D(aniel) (b. May 6, 1875, Hampton, Iowa, U.S.—d. July 20, 1959, Bethesda, Md.) U.S. naval officer. After gradu¬ ating from the U.S. Naval Academy, he served in the Spanish-American War, the Philippine insurrection, and the Boxer Rebellion. He commanded a navy transport during World War I, when he began a friendship with Franklin D. Roosevelt, then assistant secretary of the navy. He served as chief of naval operations (1937-39), as governor of Puerto Rico (1939), and as U.S. ambassador to France (1940). He was Roosevelt’s chief of staff during World War II and continued in that post under Harry S. Tru¬ man. He was made a fleet admiral in 1944.
Leakey family Family of archaeologists and paleoanthropologists known for their discoveries of hominin and other fossil remains in eastern Africa. Louis S.B. Leakey (b. 1903—d. 1972), born of British missionary parents, grew up in Kenya, was educated at Cambridge University, and eventually (1931) began field research at Olduvai Gorge in Tanzania. He was joined there by his wife, Mary D. Leakey (b. 1913—d. 1996), who in 1959 uncovered remains of a form of Australopithecus. The couple later uncovered the first known remains of Homo habilis, as well as those of Ken- yapithecus, a possible common ancestor of humans and apes that lived c. 14 million years ago. L.S.B. Leakey commissioned Jane Goodall, Birute Galdikas, and Dian Fossey to undertake pioneering studies of chimpanzees, orangutans, and gorillas, respectively. Mary Leakey continued to make important discoveries, including the Laetoli footprints, after her husband’s death. Their son, Richard Leakey (b. 1944), is known for his work at the Koobi Fora site on the shores of Lake Turkana in Kenya, where he uncovered evidence of H. habilis dated as early as 2.5 million years ago. His wife, zoologist Meave Leakey (b. 1942), discovered two new hominin species.
Leal, Juan de Valdes See Juan de Valdes Leal
Lean, Sir David (b. March 25, 1908, Croydon, Surrey, Eng.—d. April 16, 1991, London) British film director. He worked at Gaumont Studios from 1928, becoming head film editor. He codirected In Which We Serve (1942) with Noel Coward and was sole director of Coward’s Blithe Spirit (1945) and Brief Encounter (1945). He directed film adaptations of Great Expectations (1946) and Oliver Twist (1948). Lean won wide acclaim for The Bridge on the River Kwai (1957, Academy Award) and later for
Lawrence of Arabia (1962, Academy Award), Dr. Zhivago (1965), and A Passage to India (1984). His literate, epic productions featured spectacu¬ lar cinematography and stunning locales.
Leaning Tower of Pisa White marble campanile in Pisa, Italy, famous for the uneven settling of its foundation, which caused it to lean 5.5 degrees (about 15 ft [4.5 m]) from the perpendicular. Begun in 1173 as the third and final structure of the city’s cathedral complex, it was designed to stand 185 ft (56 m) high. Work was suspended several times as engineers sought solutions; the tower, still leaning, was completed in the 14th century. Subsiding at the rate of 0.03 in (1.2 mm) a year, the structure was in danger of collapse, and in 1990 it was closed as engi¬ neers undertook a strengthening project that decreased the lean by 17 in (44 cm) to about 13.5 ft (4.1 m). The work was completed in May 2001.
Lear, Edward (b. May 12, 1812, Highgate, near London, Eng.—d. Jan. 29, 1888, San Remo, Italy) English painter and comic poet. From age 15 he earned his living by drawing.