L

labial palp 1. Chemosensory appendage found on the labium of insects (Arthropoda). 2. Flaplike lobe surrounding the mouth of bivalve molluscs that directs food toward the mouth.

Labialis labi = a lip.

labium The posterior mouthpart of insects. It is often referred to as the "lower lip," is chemosensory, and was derived evolutionarily from paired head appendages (Hexapoda, Arthropoda).

labium Mouthpart in insects composed of fused second maxillae; homologous to second maxillae of crustaceans.

labrum The upper lip of insects and crustaceans situated above or in front of the mandibles; also refers to the outer lip of a gastropod shell.

labyrinth organ One of a pair of equilibrium organs in vertebrates that contains three fluid-filled semicircular canals.

labyrinth Vertebrate internal ear, composed of a series of fluid-filled sacs and tubules (membranous labyrinth) suspended within bone cavities (osseous labyrinth).

labyrinthodont A group of Paleozoic amphibians containing the temnospondyls and the anthracosaurs.

Lacerum lacer = torn.

lachrymal Secreting or relating to tears.

lactation The production of milk by the mammary glands.

lacteal Noun, one of the lymph vessels in the villus of the intestine. Adj., relating to milk.

lactic acid The end product of anaerobic respiration in animals, causes muscle soreness.

lactose Milk sugar; a disaccharide composed of glucose and galactose.

lacuna pl. lacunae. A sinus; a space between cells; a cavity in cartilage or bone.

lacunae Channels making up the lacunar system in Acanthocephala. Also, in developing wings of insects, canals that contain nerves, tracheae, and hemolymph.

lacunar system System of canals in the body wall of an acanthocephalan, functioning as a circulatory system.

lagena Portion of the primitive ear in which sound is translated into nerve impulses; evolutionary beginning of cochlea.

lagoon A shallow, sheltered body of water separated from the open sea by coral reefs, sand bars, and/or barrier islands .

Lamarckism. Hypothesis, as expounded by Jean Baptiste de Lamarck, of evolution by the acquisition during an organism's lifetime of characteristics that are transmitted directly to offspring.

Lambdoidal lambda = like the Greek letter lambda.

lamella One of the two plates forming a gill in a bivalve mollusc. One of the thin layers of bone laid concentrically around an osteon (Haversian canal). Any thin, platelike structure.

lamp shells Invertebrates that have

a lophophore and a shell that consists of two valves.

lancelets Chordates with the three basic chordate characteristics but lacking a backbone.

land ethic A philosophy or attitude about land that provides for its protection or more careful management.

land races Traditional varieties of plant crops.

land reform Democratic redistribution of landownership to recognize the rights of those who actually work the land to a fair share of the products of their labor.

land rehabilitation A utilitarian program to repair damage and make land useful to humans.

landfills Land disposal sites for solid waste; operators compact refuse and cover it with a layer of dirt to minimize rodent and insect infestation, wind-blown debris, and leaching by rain.

landscape ecology The study of the reciprocal effects of spatial pattern on ecological processes. A study of the ways in which landscape history shapes the features of the land and the organisms that inhabit it as well as our reaction to, and interpretation of, the land.

landscape epidemiology Approach to epidemiology that employs all ecological aspects of a nidus. By recognizing certain physical conditions, the epidemiologist can anticipate whether a disease can be expected to exist.

landslide The sudden fall of rock and earth from a hill or cliff. Often triggered by an earthquake or heavy rain.

langmuir cells Parallel pairs of surface ocean convection cells driven by winds.

lappets. Lobes around the margin of scyphozoan medusae (phylum Cnidaria).

large intestine That part of the digestive system between the ileocecal valve of the small intestine and the anus; removes salt and water from undigested food and releases feces through the anus.

larva pl. larvae 1. The immature, feeding stage of an insect that undergoes holometabolous metamorphosis. 2. The immature stage of any animal species in which adults and immatures are different in body form and habitat.

Larvacea The class of urochordates whose members are planktonic and whose adults retain a tail and notochord. With a gelatinous covering of the body.

larvaceans Tunicates that retain the body of a tadpole larva throughout life.

larval instars Any of the different immature feeding stages of an insect that undergoes holometabolous metamorphosis.

larynx Modified upper portion of respiratory tract of air-breathing vertebrates, bounded by the glottis above and the trachea below; voice box; adj., laryngeal relating to the larynx.

last glacial maximum (LGM) the time that the last major continental glacial advance in the Northern Hemisphere reached its maximum extent, about 18,000 years ago

latent heat of fusion The heat that must be extracted from a liquid to freeze it to a solid at the same temperature. For water, it is 80 cal/g.

latent heat of melting The amount of heat energy needed to melt a substance, that is, to change it from a solid to a liquid.

latent heat of evaporation The amount of heat energy that is needed to evaporate a substance, that is, to change it from a liquid to a gas.For water, it is 540 cal/g.

latent heat of vaporization the heat energy required to convert a liquid to a gas at the same temperature.

latent learning Associations made with neither immediate reinforcement or reward nor with particular behavior evident at the time of learning. The animal may store and use such information in appropriate situations at a later time.

lateral Of or pertaining to the side of an animal; a bilateral animal has two sides.

lateral bud Bud found in the axil of a leaf, also called an axillary bud.

lateral line A system of canals and sensory cells on the sides of fishes that helps them detect vibrations in the water.

lateral meristem Meristems that give rise to secondary tissues, typically by the formation of radial files of cells and thereby increasing the diameter and circumference of an axis over time; the vascular cambium and cork cambium.

lateral root A root that arises from another, older root. Often termed secondary or branch roots.

lateral-line system 1. A line of sensory receptors along the side of some fishes and amphibians used to detect water movement (phylum Chordata). 2. The external manifestation of a lateral excretory canal of nematodes (phylum Nematoda).

laterite Group of hard, red soils from topical areas that show intense weathering and leaching of bases and silica, leaving aluminum hydroxides and iron oxides; adj. lateritic.

latewood The small, thick-walled xylem cells produced at the end of the growing season that appear as a dense ring of wood adjacent to the thin-walled earlywood.

latex Milky juice exuded from some plants.

Latissimus later = side.

latitude the angular distance north or south of the equator of a position on the earth's surface; measured in degrees ( )

lattice A crystalline-like structure caused by the precise orientation of molecules in a solid or liquid.

Laurasia One of the two large continents, the northern one, that formed when the supercontinent Pangaea broke up about 180 million years ago. Also see Gondwana.

Laurer's canal Usually blind canal extending from the base of the seminal receptacle of a digenetic trematode. It probably represents a vestigial vagina.

lay botanist A nonscientist who understands botanical principles and appreciates the value of the plant world.

layering A method of asexual propagation in which portions of the stem are wounded and covered with a medium, usually soil, to stimulate the production of adventitious roots.

LD50 A chemical dose lethal to 50 percent of a test population.

LDL (low density lipoprotein) Transports fats and cholesterol to the body cells, including the cells lining the bloodstream.

leaching The process of removal of ions or molecules by flushing with water.

leaf margin Refers to the edge of a leaf, the area between the apex and base.

leaf primordium The first stage of leaf development; a small lateral protuburance formed by an apical shoot meristem that will expand to form a leaf.

leaf scars A scar left on a stem or twig when the leaf abscises.

leaf sheath The lower part of a blade or petiole that invests the stem more or less completely.

leaf-area index A numerical index of the ratio of leaf area to ground area in a plant community.

leaflet An individual blade unit of a compound leaf.

learning The relatively permanent change in behavior or potential for behavior that results from experience.

learning curve A graphical presentation of the response measures in a learning situation. This may be depicted as correct or incorrect responses, or as a proportion of the trials given to the animal on which it gave correct or incorrect responses.

learning set The acquisition of a learning strategy by the animal; the ability to transfer learning between problems of the same type.

leeches Segmented worms that are specialized predators and parasites.

leeward The side away from the direction of a prevailing wind.

legume Simple, dry, dehiscent fruit that splits along two seams, a pod. Member of the Fabaceae; a type of bean or pea.

Leishman-Donovan (L-D) body Amastigote in the Trypanosomatidae.

leishmaniasis Infection by a species of Leishmania.

lek An area where animals assemble for communal courtship display and mating.

lemma Bract in a grass flower.

lemniscus One of a pair of internal projections of the epidermis from the neck region of Acanthocephala, which functions in fluid control in the protrusion and invagination of the proboscis.

lentic ecosystem A freshwater ecosystem that has standing water (e.g., lakes or ponds).

lentic Of or relating to standing water such as swamp, pond, or lake.

lenticel Isolated areas of loosely arranged cells in the cork surfaces of stems, roots, and fruit that allow interchange of gases between internal tissues and the atmosphere through the periderm.

lepidosaurs A lineage of diapsid reptiles that appeared in the Permian and that includes the modern snakes, lizards, amphisbaenids, and tuataras, and the extinct ichthyosaurs.

lepospondyls A group of Paleozoic amphibians distinguished by the possession of spool-shaped vertebral centra.

leptocephalus pl. leptocephali. Transparent, ribbonlike migratory larva of the European or American eel.

leptocephalous larva The leaf-shaped larva of freshwater eels and other fishes.

lesion An area of tissue that has been destroyed by an agent such as electric current or a chemical.

less-developed countries (LDC) Nonindustrialized nations characterized by low per capita income, high birth and death rates, high population growth rates, and low levels of technological development.

leucon The sponge body form that has an extensively branched canal system; the canals lead to chambers lined by choanocytes.

leucoplast Colorless plastid typically associated with starch formation and storage.

leukemism Presence of white pelage or plumage in animals with normally pigmented eyes and skin.

leukocyte Any of several kinds of white blood cells (for example, granulocytes, lymphocytes, monocytes), so-called because they bear no hemoglobin, as do red blood cells.

leukorrhea White, puslike discharge resulting from infection.

liana A large, woody vine common to the tropical forests; it climbs the tall trees and often trails from the canopy.

library. In molecular biology, a set of clones containing recombinant DNA. Obtained from and representing the genome of the organism.

lichen An organism composed of a symbiotic association of an ascomycete fungus with algal or cyanobacterial cells.

lien The spleen.

life expectancy The average number of years lived by a group of individuals after reaching a given age; the probable number of years of survival for an individual of a given age.

life range The larger geographic area that an animal utilizes over the course of its life.

life span The longest period of life reached by a type of organism.

life-cycle analysis Evaluation of material and energy inputs and outputs at each stage of manufacture, use, and disposal of a product.

ligament A tough, dense band of connective tissue connecting one bone to another.

ligand A molecule that specifically binds to a receptor; for example, a hormone (ligand) binds specifically to its receptor on the cell surface.

light The electromagnetic radiation produced by the sun that heats and illuminates the earth.

light harvesting antenna A complex of several hundred chlorophyll and carotenoid molecules that form a part of each photosystem.

light intensity The strength of light rays; the degree of brightness dependent on the number of photons striking a given area at a point in time.

light microscope The type of microscope in which the specimen is viewed under ordinary illumination.

light reaction that part of the photosynthetic process that, in the presence of light, captures energy to form ATP and NADPH2 to be used to synthesize complex organic molecules in the dark reaction

lignification Impregnated with lignin.

lignified Impregnated with lignin, such as the secondary cell walls of woody plants.

lignin A complicated organic molecule found as an important constituent of many secondary cell walls; imparts strength and rigidity to the cellulose microfibrils.

lignite Soft, brown to black coal of low BTU value with original plant components still discernible.

limax form Form of pseudopodial movement in which entire organism moves without extending a discrete pseudopodium.

limbic system See rhinencephalon.

limiting factors Chemical or physical factors that limit the existence, growth, abundance, or distribution of an organism. The principle of limiting factors states that for each physical factor in the environment, there are both minimum and maximum limits (tolerance limits) beyond which a particular species cannot survive. The single factor closest to a tolerance limit for a given species at a given time is the critical factor.

limiting resource An essential factor whose short supply prevents the growth of a population.

limnetic zone The open-water zone of a lake or pond; extends to the depth of effective light penetration.

limnologist A scientist who studies freshwater biology.

lingual Pertaining to the tongue.

linkage group Genes linked to the same chromosome that tend to be inherited together.

linkage Tendency of genes located on the same chromosome to be inherited together.

linked gene Gene located on the same chromosome as another gene.

lip cell Thin-walled cells that interrupt the annulus in fern sporangia.

lipase An enzyme that accelerates the hydrolysis or synthesis of fats.

lipid A fat, oil, or fatlike compound that usually has fatty acids in its molecular structure. An organic compound consisting mainly of carbon and hydrogen atoms linked by nonpolar covalent bonds. Examples include fats, waxes, phospholipids, and steroids that are insoluble in water.

liposome Artificial lipoid particle used to deliver antiparasitic drugs directly to macrophages (which eat the particles).

liquid metal fast breeder A nuclear power plant that converts uranium 238 to plutonium 239; thus, it creates more nuclear fuel than it consumes. Because of the extreme heat and density of its core, the breeder uses liquid sodium as a coolant.

lithogenous sediment A marine sediment that is derived from the breakdown, or weathering, of rocks. Also see red clay.

lithosphere The crust and the top part of the mantle that covers the earth's surface. It is broken into separate lithospheric plates.

litter Dead organic material such as branches, tree trunks, and dry grass that accumulates on the floor of a forest. Litter acts as additional fuel in a forest fire, producing increased destruction.

littoral Adj., pertaining to the shore. Noun, that portion of the sea floor between the extent of high and low tides, intertidal; in lakes, the shallow part from the shore to the lakeward limit of aquatic plants.

littoral zone The shallow-water zone of a lake or pond; light penetrates to the bottom, and the area is occupied by rooted plants such as water lilies, rushes, and sedges.

liver A large gland of a dark-red color. It carries out many vital functions such as the formation of urea, manufacture of plasma proteins, synthesis of amino acids, synthesis and storage of glycogen, and many others.

liverwort A group of the division Bryophyta charcterized by a small, inconspicuous, liver-shaped thallus and that lives in moist environments.

loam A mixture of sand-, silt-, and clay-sized soil particles.

lobopodium A blunt, lobelike pseudopodium that is commonly tubular, and is composed of both ectoplasm and endoplasm.

local chemical messenger A chemical that acts on nearby cells.

local mate competition Competition for mates that may arise among siblings depending on the population size and operational sex ratio and that may cause the evolution of skewed offspring sex ratios.

local resource competition model In female philopatric primates, females giving birth to a disproportionate number of males that will disperse and not compete with each other, or high-ranking females giving birth to a disproportionate number of daughters that are able to compete for resources effectively.

locomotion-fear dichotomy A process during the imprinting process in young birds whereby they initially are more likely to locomote and follow a stimulus object and later develop a fear toward objects; thus, imprinting occurs when the locomotor tendency is high and fear of objects is low.

loculi Shallow, suckerlike depressions in an adhesive organ of a flatworm.

locus A section of a chromosome (a gene) that codes for a particular protein.

logistic equation. A mathematical expression describing an idealized sigmoid curve of population growth.

logistic growth Growth rates regulated by internal and external factors that establish an equilibrium with environmental resources; species may grow exponentially when resources are unlimited but slowly as the carrying capacity is reached. See S curve.

long-day plant A plant that flowers when the length of day exceeds some critical value.

longevity The length or duration of life; compare to survivorship.

longitude the angular distance of a position on the earth's surface east or west of the Greenwich Prime Meridian; measured in degrees.

looping movement The type of locomotion exhibited by leeches and some insect larvae whereby they alternate temporary points of attachment to move forward.

loose connective tissue The type of tissue in which the matrix contains strong, flexible fibers of the protein collagen that are interwoven with fine, elastic, and reticular fibers.

lophocyte Type of sponge amebocyte that secretes bundles of fibrils.

lophophore Tentacle-bearing ridge or arm within which is an extension of the coelomic cavity in lophophorate animals (ectoprocts, brachiopods, and phoronids).

lorica Protective external case found in some protozoa, rotifers, and others.

Loricifera A phylum of aschelminths. The most recent animal phylum to be described; members are commonly called loriciferans.

loriciferans Tiny invertebrates that live among sediment particles and have a body enclosed by six plates.

lotic Of or pertaining to running water, such as a brook or river.

lotic ecosystems Flowing water ecosystems. They include brooks, streams, and rivers.

lottery hypothesis The hypothesis that unpredictability plays a key role in the development of communities.

low tide the lowest level reached by the falling tide

low-head hydropower Small-scale hydro technology that can extract energy from small headwater dams; causes much less ecological damage.

low-quality energy Diffuse, dispersed energy at a low temperature that is difficult to gather and use for productive purposes.

LSD Lysergic acid diethylamide; a synthetic hallucinogenic derivative of lysergic acid.

LULUs Locally unwanted land uses such as toxic waste dumps, incinerators, smelters, airports, freeways, and other sources of environmental, economic, or social degradation.

lumbar Relating to or near the loins or lower back.

Lumbosacral lumb = the loin; sacrum = sacrum.

lumen The cavity of a tube or organ.

lung An organ of the respiratory system in which gas exchange occurs between body fluids (e.g., blood) and air.

lunules Small, suckerlike discs on the anterior margin of some copepods in the family Caligidae, functioning as organs of adhesion.

lycophora Ten-hooked larva that hatches from the egg of a cestodarian tapeworm. Also called a decacanth.

Lycophyta A division of "lower" (i.e., non-seed-producing) vascular plants, commonly referred to as club and spike mosses.

lymph varices Dilated lymph ducts.

lymph The interstitial (intercellular) fluid in the body, also the fluid in the lymphatic system.

lymphadenitis Inflamed lymph node.

lymphatic system The one-way system of lymphatic vessels.

Lympho- lymph = water.

lymphocytes A type of white blood cell; a component of the immune system produced by stem cells in the bone marrow.

lymphokine A molecule secreted by an activated or stimulated lymphocyte that causes physiological changes in certain other cells.

lysergic acid diethylamide (LSD) A derivative of lysergic acid alkaloids, first isolated from ergot; strongly hallucinogenic.

lysosome Cytoplasmic, membrane-bounded organelle that contains digestive and hydrolytic enzymes, which are typically most active at the acid pH found in the lumen of lysosomes.