L

Labor: A series of strong, rhythmic contractions of the uterus that expels a baby out of the uterus and vagina during childbirth.
Labyrinthulid: Member of a phylum of eukaryotes in the heterokont kingdom.
Lactation: The continued production of milk.
Lacteal: A tiny lymph vessel extending into the core of an intestinal villus and serving as a destination for absorbed chylomicrons.
Lactic acid fermentation: The conversion of pyruvate to lactate with no release of carbon dioxide.
Lagging strand: During DNA replication, the strand that is synthesized in the 3′ to 5′ direction by ligation of short DNA strands synthesized discontinuously in the 5′ to 3′ direction.
Lancelet: Member of the subphylum Cephalochordata, small blade-shaped marine chordates that lack backbone.
Landmark: A point of reference for orientation during navigation.
Landscape: Several different, primarily terrestrial ecosystems linked by exchanges of energy, material, and organisms.
Landscape ecology: The study of past, present, and future patterns of landscape uses, as well as ecosystem management and the biodiversity of interacting ecosystems.
Large intestine (colon): The tubular portion of the vertebrate alimentary tract between the small intestine and the anus; function in water absorption and the formation of the feces.
Large X effect: The observation that the X chromosome is involved in hybrid incompatibilities more often that would be expected from its size.
Larva: A free-living sexually immature form in some animal life cycles that may differ from the adult in morphology, nutrition, and habitat.
Larynx: The voice box, containing the vocal cords.
Last universal common ancestor (LUCA): The common ancestor of all modern life-forms.
Lateral gene transfer (LGT): The transmission of DNA form one evolutionary lineage to another. Also known as horizontal gene transfer.
Lateral geniculate nuclei: The destination in the thalamus for most of the ganglion cell axons that form the optic nerves.
Lateral inhibition: A process that sharpens the edges and enhances the contrast of a perceived image by inhibiting receptors lateral to those that have responded to light.
Lateralization: Segregation of functions in the cortex of the left and right hemispheres of the brain.
Lateral line system:  A mechanoreceptor system consisting of a series of pores and receptor units (neuromasts) along the sides of the body in fishes and aquatic amphibians; detects water movements made by the animal itself and by other moving objects
Lateral meristem: A meristem that thickens the roots and shoots of woody plants.
Lateral root: A root that arises from the outermost layer of the pericycle of an stablished root.
Law of independent assortment: Mendel’s second law, stating that each pair of alleles segregates independently during gamete formation; applies when genes for two characteristics are located on different pairs of homologous chromosomes.
Law of segregation: Mendel’s first law, stating that each allele in a pair separates into a different gamete during gamete formation.
Leading strand: During DNA replication, the strand that is synthesized in the 5′ to 3′ direction by continuous polymerization at the growing 3′ tip.
Leaf: The main photosynthetic organ of vascular plants.
Leaf primordia: Fingerlike projections along the flanks of a shoot apical meristem, from which leaves arise.
Leaf trace: A small vascular bundle that extends from the vascular tissue of the stem through the petiole and into a leaf.
Learning: A behavioral change resulting from experience.
Lek: An arena where males gather and are chosen as mates by females.
Lens: The structure in an eye that focuses light rays onto the retina.
Lenticels: Small raised areas in the bark of stems and roots that enable gas exchange between living cells and the outside air.
Lepidosaur: Member of the reptilian group that includes lizards, snakes, and two species of New Zealand animals called tuataras.
Lethal: A recessive lethal allele kills its bearer when homozygous, whereas a dominant lethal allele kills when present in a single copy.
Leucocyte: A white blood cell, typically functions in immunity, such as phagocytisis or anitbody production.
Leydig cell: A cell that produces testosterone and other androgens and is located between the semiiniferous tubules of the testes.
Lichens: Fungi that have evolved to house a population of unicellular or filamentous photosynthetic cells (photobionts) that provides the fungus host (mycobiont) with carbon compounds. Bacteria can also be part of the lichen as a third party.
Life cycle:  The generation-to-generation sequence of stages in the reproduction history of organisms.
Life expectancy at birth: The predicted average length of life at birth.
Life history: An organism’s pattern of survival and reproduction. The series of events from birth through reproduction and death.
Life-history trait: Traits, such as mortality rate, fertility, or age of reproduction, that are closely associated with fitness.
Life table: A table of data summarizing mortality in a population.
Ligament: A type of fibrous connective tissue that joins bones together at joints.
Ligand: A molecule that binds specifically to a receptor site of another molecule.
Ligand-gated ion channel: A protein pore in the plasma membrane that opens or closes in response to a chemical signal, allowing or blocking the flow of specific ions.
Light chain: One of the two types of polypeptide chains that make up an antibody molecule and B cell receptor; consisting of a variable region, which contributes to the antigen binding site, and a constant region.
Light-harvesting complex: Complex of proteins associated with pigment molecules that captures light energy and transfers it to reaction-center pigments in a photosystem.
Light microscope (LM): An optical instrument with lenses that refracts (bend) visible light to magnify images of specimens.
Light reactions: The steps in photosynthesis that occur in the thylakoid membranes of the chloroplast and that convert solar energy to chemical energy of ATP and NADPH, evolving oxygen in the process.
Lignin: A hard material embedded in the cellulose matrix of vascular plant cell walls that functions as an important adaption for support in terrestrial species.
Likelihood: Given a hypothesis, the probability of observing certain data.
Limbic system: A group of nuclei (cluster of nerve cell bodies) in the lower part of the mammalian forebrain that interact with the cerebral cortex in determining emotions, includes the hippocampus and the amygdala.
Limiting nutrient: An element that must be added for production to increase in a particular area.
Limmetic: Occurring in the deeper open water or ponds.
Lineage: A group of related organisms descended from a common ancestor. Lineages make up parts of a phylogenetic tree and lineages can include multiple clades.
Linkage:  Genes that are carried on the same chromosome are said to be linked.
Linkage disequilibrium: Nonrandom association between alleles at two or more genetic locus.
Linkage equilibrium: Absence of linkage disequilibrium, so that haplotype frequencies are equal to the product of allele frequencies.
Linkage map: A genetic map based on the frequencies of recombination between markers during crossover of homologous chromosomes.
Linked genes: Genes located close enough together on a chromosome to be usually inherited together.
Lipid: One of a family of compounds including fats, phospholipids, and steroids that are insoluble in water.
Liposome: A spherical lipid bilayer.
Littoral zone: In a lake, the shallow, well-lit waters close to the shore.
Liver: The largest organ in the vertebrate body. its functions include the production of bile, preparation of nitrogenous waste for disposal, and detoxifying poisonous chemical in the blood.
Liverwort: A small herbaceous nonvascular plant that is a member of  the phylum Hepatophyta.
Loam: The most fertile of all soils, made up of roughly equal amounts of sand, silt, and clay.
Lobe-fin: Member of the vertebrate subgroup Sarcopterii, osteihthyans with rod-shaped muscular fins, including coelacanths and lungfishes, as well as the lineage that gave rise  to tetrapods.
Lobopod: The fossil member of a phylum of caterpillar-like animals. The living form are terrestrial and commonly refer as velvet worms. The fossils are marine and were diverse in the Cambrian (phylum Onychophora).
Local mate competition: Competition for mates within a local group (e.g. between fig wasps within a single fig).
Local regulator: A chemical messenger that influences cells in the vicinity.
Locomotion: Active movement from place to place.
Locus: A location on the genome. It may refer to a single nucleotide site or to a substantial stretch of DNA sequence.
Long branch attraction: A phenomenon in phylogenetic analysis when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationship.
Long-day plant: A plant that flowers (usually in late spring or early summer) only when the light period is longer than a critical length.
Long interspersed nucleotide element (LINE): A class of transposable element.
Long-term memory: The ability to hold, associate, and recall information over one’s life.
Long-term potentiation (LTP): An enhance responsiveness to an action potential (nerve signal) by a receiving neuron.
Loop of Henle: The long hairpin turn, with an ascending and descending limb, of the renal tubule in the vertebrate kidney; functions in water and salt reabsorption.
Loose connective tissue: The most widespread connective tissue in the vertebrate body. It binds epithelia to underlying tissues and functions as packing material, holding organs in place.
Lophophore: A horse-shoe shaped or circular fold of the body wall bearing ciliated tentacles that surrounds the mouth.
Lophotrochozoan: Member of a major subdivision within the protostomes that includes annelids, mollusks, bryozoans, brachiopods, and several additional small phyla.
Low-density lipoprotein (LDL): A cholesterol carrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded. High levels of LDL correlates with a tendency to develop blocked blood vessels and heart disease.
Lung: An invaginated respiratory surface of terrestrial vertebrates, land snails, and spiders that connect to the atmosphere by narrow tubes.
Luteal phase: That portion of the ovarian cycle during which endocrine cells of the corpus luteum secrete female hormones.
Luteinizing hormone (LH): A tropic hormone produced and secreted by the anterior pituitary that stimulates ovulation in females and androgen production in males.
Lycophyte: Member of a diverse group of early plants including giant trees in the Carboniferous coal swamp forest and the living club mosses.
Lymph: A colorless fluid, derived from the interstitial fluid, in the lymphatic system of vertebrate animals.
Lymph node: Organ located along a lymph vessel. Lymph nodes filter lymph and help attack viruses and bacteria.
Lymphatic system:  A system of vessels and lymph nodes, separated from the circulatory system, that returns fluids, proteins, and cells to the blood.
Lymphocyte: A type of white blood cell that mediates acquired immunity. Lymphocytes that complete their development in the bone marrow are called B cells, and those that mature in the thymus are called T cells.
Lysogenic: Describes bacteria and archaea that have bacteriophage integrated into their genomes. The bacteriophage may be activated, leading to lysis of the cell.
Lysosome: A membrane-enclosed sac of hydrolytic enzymes found in the cytoplasm of eukaryotic cells.
Lysozyme: An enzyme in sweat, tears, and saliva that attacks bacterial cell walls.
Lytic cycle: A type of viral (phage) replication cycle resulting in the release of new phages by lysis (and death) of the host cell.