Bacillus (buh sil´ us) [L.: little rod] • Any of various rod-shaped bacteria.

Bacteria (bak teer´ ee ah) (singular: bacterium) [Gr. bakterion: little rod] • Prokaryote in the Domain Bacteria. The chromosomes of bacteria are not contained in nuclear envelopes.

Bacteriophage (bak teer´ ee o fayj) [Gr. bakterion: little rod + phagein: to eat] • One of a group of viruses that infect bacteria and ultimately cause their disintegration.

Bacteroids • Nitrogen-fixing organelles that develop from endosymbiotic bacteria.

Balanced polymorphism [Gr. polymorphos: having many forms] • The maintenance of more than one form, or the maintenance at a given locus of more than one allele, at frequencies of greater than 1 percent in a population. Often results when heterozygotes are more fit than either homozygote.

Bark • All tissues outside the vascular cambium of a plant.

Baroreceptor [Gr. baros: weight] • A pressure-sensing cell or organ.

Barr body • In mammals, an inactivated X chromosome.

Basal • Pertaining to one end -the base- of an axis.

Basal body • Centriole found at the base of a eukaryotic flagellum or cilium.

Basal metabolic rate (BMR) • The minimum rate of energy turnover in an awake (but resting) bird or mammal that is not expending energy for thermoregulation.

Base • (1) A substance which can accept a hydrogen ion in solution. (Contrast with acid.) (2) In nucleic acids, the purine or pyrimidine that is attached to each sugar in the backbone. (See purine; pyrimidine.)

Base pairing • See complementary base pairing.

Basic • Having a pH greater than 7.0 (i.e., having a hydrogen ion concentration lower than 10^-7 molar).

Basidium (bass id´ ee yum) • In basidiomycete fungi, the characteristic sporangium in which four spores are formed by meiosis and then borne externally before being shed.

Batesian mimicry • Mimicry by a relatively harmless kind of organism of a more dangerous one, by which the mimic enjoys protection from predators that mistake it for the dangerous model. (Contrast with Müllerian mimicry.)

Basophils • One type of phagocytic white blood cell that releases histamine and may promote T cell development.

B cell • A type of lymphocyte involved in the humoral immune response of vertebrates. Upon recognizing an antigenic determinant, a B cell develops into a plasma cell, which secretes an antibody. (Contrast with T cell.)

Benefit • An improvement in survival and reproductive success resulting from performing a behavior or having a trait. (Contrast with cost.)

Benign (be nine´) • A tumor that grows to a certain size and then stops, uaually with a fibrous capsule surrounding the mass of cells. Benign tumors do not spread (metastasize) to other organs.

Benthic zone [Gr. benthos: bottom of the sea] • The bottom of the ocean. (Contrast with pelagic zone.)

Beta-pleated sheet • Type of protein secondary structure; results from hydrogen bonding between polypeptide regions running antiparallel to each other.

Biennial • Referring to a plant whose life cycle includes vegetative growth in the first year and flowering and senescence in the second year. (Contrast with annual, perennial.)

Bilateral symmetry • The condition in which only the right and left sides of an organism, divided exactly down the back, are mirror images of each other. (Contrast with biradial symmetry.)

Bile • A secretion of the liver delivered to the small intestine via the common bile duct. In the intestine, bile emulsifies fats.

Binocular cells • Neurons in the visual cortex that respond to input from both retinas; involved in depth perception.

Binomial (bye nome´ ee al) • Consisting of two elements; for example, the binomial nomenclature of biology in which each species has two names (the genus name followed by the species name).

Biodiversity crisis • The current high rate of loss of species, caused primarily by human activities.

Biogeochemical cycles • Movement of elements through living organisms and the physical environment.

Biogeography • The scientific study of the geographic distribution of organisms.

Biogeographic region • A continental-scale part of Earth that has a biota distinct from that of other such regions.

Bioluminescence • The production of light by biochemical processes in an organism.

Biomass • The total weight of all the living organisms, or some designated group of living organisms, in a given area.

Biome (bye´ ome) • A major division of the ecological communities of Earth; characterized by distinctive vegetation.

Biota (bye oh´ tah) • All of the organisms, including animals, plants, fungi, and microorganisms, found in a given area.

Biradial symmetry • Radial symmetry modified so that only two planes can divide the animal into similar halves.

Blastocoel (blass´ toe seal) [Br. blastos: sprout + koilos: hollow] • The central, hollow cavity of a blastula.

Blastocyst (blass toe cist) • An early embryo formed by the first divisions of the fertilized egg (zygote). In mammals, a hollow ball of cells.

Blastodisc (blass´ toe disk) • A disk of cells forming on the surface of a large yolk mass, comparable to a blastula, but occurring in animals such as birds and reptiles, in which the massive yolk restricts cleavage to one side of the egg only.

Blastomere • A cell produced by the division of a fertilized egg.

Blastopore • The opening from the archenteron to the exterior of a gastrula.

Blastula (blass´ chu luh) [Gr. blastos: sprout] • An early stage in animal embryology; in many species, a hollow sphere of cells surrounding a central cavity, the blastocoel. (Contrast with blastodisc.)

Blood-brain barrier • A property of the blood vessels of the brain that prevents most chemicals from diffusing from the blood into the brain.

Blue light receptor • Molecule in plants that absorbs blue light (400-500 nm). Mediates many plant responses including phototropism, stomatal movements, and expression of some genes.

Body cavity • Membrane-lined, fluid-filled compartment that lies between the cell layers of many animals.

Body plan • A basic structural design that includes an entire animal, its organ systems, and the integrated functioning of its parts.

Boreal forests • Biome that is dominated by evergreen trees, have long, very cold winters and short summers and found equatorward from tundra and at lower elevations on temperate-zone mountains.

Bottleneck • Refers to stressful periods that only a few individuals of a once-large population survive, resulting in substantially reduced genetic variation in the population.

Bowman's capsule • An elaboration of kidney tubule cells that surrounds a know of capillaries (the glomerulus). Blood is filtered across the walls of these capillaries and the filtrate is collected into Bowman's capsule.

Brain stem • The portion of the vertebrate brain between the spinal cord and the forebrain.

Brassinosteroids • Plant steroid hormones that mediate light effects promoting the elongation of stems and pollen tubes.

Bronchus (plural: bronchi) • The major airway(s) branching off the trachea into the vertebrate lung.

Brown fat • Fat tissue in mammals that is specialized to produce heat. It has many mitochondria and capillaries, and a protein that uncouples oxidative phosphorylation.

Browser • An animal that feeds on the tissues of woody plants.

Bryophyte (bri´ uh fite´) [Gr. bruon: moss + phyton: plant] • A moss. This term was once frequently used to refer to all nontracheophyte plants.

Budding • Asexual reproduction in which a more or less complete new organism simply grows from the body of the parent organism and eventually detaches itself.

Buffer • A substance that can transiently accept or release hydrogen ions and thereby resist changes in pH.