P

P680 The reaction center for Photosystem II; a chlorophyll a molecule that is bound to a membrane protein and has a peak absorbance at 680 nm.

P700 The reaction center for Photosystem I; a chlorophyll a molecule that is bound to a membrane protein and has a peak absorbance at 700 nm.

Pacinian corpuscle A sensory receptor in skin, muscles, body joints, body organs, and tendons that is involved with the vibratory sense and firm pressure on the skin; also called a lamellated corpuscle.

paddy A flooded field used to cultivate lowland rice.

paedogenesis Reproduction by immature or larval animals caused by acceleration of maturation. Progenesis.

paedomorphosis Displacement of ancestral juvenile features to later stages of the ontogeny of descendants.

pain receptor A modified nerve ending that, when stimulated, gives rise to the sense of pain.

pair bond. An affiliation between an adult male and an adult female for reproduction. Characteristic of monogamous species.

Palatal palatum = the palate [the roof of the mouth].

palea One of two bracts around the grass flower.

Paleolithic Old Stone Age; a cultural period during which early humans obtained food solely by foraging; ending in some areas approximately 10,000 years ago.

paleontologist A person who studies fossils and all aspects of extinct life.

paleontology The study of early life-forms on earth.

Paleozoic The geological era beginning 590 million years ago and ending about 250 million years ago. During this time plants invaded, established, and diversified on the land

palisade The vertical photosynthetic cells below the upper epidermis in leaf tissue; these cells are a specialized parenchyma.

palisade parenchyma Parenchyma cells in the leaf mesophyll characterized by uniform rows of tightly packed cells with many chloroplasts beneath the upper epidermis.

pallium Mantle of a mollusc or brachiopod.

palmate An arrangement of leaflets (or lobes on a simple leaf), each originating from a common point, usually the axial end

palmately compound leaf Leaflets radiating from a common point.

Palpebrae Eyelid.

Pampiniform pampin = a tendril. A threadlike plexus.

pancreas A gland located in the abdomen that produces both digestive enzymes (exocrine pancreas) and hormones (endocrine pancreas). Key hormones produced by the pancreas are insulin and glucagon, which play roles in regulating blood glucose levels.

pancreatic islet An island of special tissue in the pancreas.

pandemic Very widely distributed epidemic.

Pangaea The single large landmass, or supercontinent, that broke up to form today's continents

pangenesis Darwin's hypothesis that hereditary characteristics are carried by individual body cells that produce particles that collect in the germ cells.

panicle A branched inflorescence with the branches bearing loose flower clusters.

pansporoblast Myxosporidean sporoblast that gives rise to more than one spore. Also called a sporoblast mother cell.

Panthalassa The large ocean that surrounded the supercontinent Pangaea and which was the ancestor of the modern Pacific Ocean.

Papatasi fever Virus disease transmitted by sand flies. Also called sand fly fever.

papilla A small nipplelike projection. A vascular process that nourishes the root of a hair, feather, or developing tooth.

papula Respiratory processes on skin of sea stars; also, pustules on skin.

Para- G. Beside or near.

parabasal body Golgi body located near the basal body (kinetosome) of some flagellate protozoa, from which the parabasal filament runs to the basal body.

parabasal filament Fibril, with periodicity visible in electron micrographs, that courses between the parabasal body and a kinetosome.

parabiosis The fusion of two individuals, resulting in mutual physiological intimacy.

parabolic mirrors Curved mirrors that focus light from a large area onto a single, central point, thereby concentrating solar energy and producing high temperatures.

parabronchi The tiny air tubes within the lung of a bird across which gas exchange occurs.

parallel venation A type of venation in which the main veins of a leaf are parallel or nearly so, but converge at the apex and base.

paralytic shellfish poisoning A condition caused when humans eat shellfish that have become contaminated with the

paramastigote Form of trypanosomatid in which the kinetosome and kinetoplast are beside the nucleus.

paramere Copulatory appendage in male cimicid bugs.

paramylon bodies Organelles containing the starch-like substance paramylon; in some algae and flagellates.

parapatric speciation Speciation that occurs in small, local populations, called demes.

paraphyletic groups Groups that do not include all descendants of a common ancestor.

paraphyly The condition that a taxon or other group of organisms contains the most recent common ancestor of all members of the group but excludes some descendants of that ancestor; contrasts with monophyly and polyphyly.

parapodia Paired lateral extensions on each segment of polychaetes (Annelida); may be used in swimming, crawling, and burrowing.

parapodium One of the paired lateral processes on each side of most segments in polychaete annelids; variously modified for locomotion, respiration, or feeding.

parapolar cells Cells making up the ciliated somatoderm immediately behind the calotte of a mesozoan.

parasite An organism that lives in or on another organism, deriving nourishment at the expense of its host, usually without killing it.

parasitic Denoting an association where one living organism benefits at the expense of another.

parasitic castration Condition in which a parasite causes retardation in development or atrophy of host gonads, often accompanied by failure of secondary sexual characteristics to develop.

parasitism The condition of an organism living in or on another organism (host) at whose expense the parasite is maintained; destructive symbiosis.

parasitoid Organism that is a typical parasite early in its development but that finally kills the host during or at the completion of development; often used in reference to many insect parasites of other insects.

parasitologist Quaint person who seeks truth in strange places; a person who sits on one stool, staring at another.

parasitophorous vacuole Vacuole within a host cell that contains a parasite.

parasympathetic One of the subdivisions of the autonomic nervous system, whose fibers originate in the brain and in anterior and posterior parts of the spinal cord.

parasympathetic nervous system Portion of the autonomic nervous system that arises from the brain and sacral region of the spinal cord.

paratenic host Host in which a parasite survives without undergoing further development. Also known as a transport host.

parathyroid gland One of the small glands located within a lobe of the thyroid gland.

paraxial (crystalline) rod Rod that runs alongside the axoneme in the flagellum of a kinetoplastid flagellate.

parenchyma A spongy mass of mesenchyme cells filling spaces around viscera, muscles, or epithelia in acoelomate animals. Depending on the species, parenchyma may function in providing skeletal support, nutrient storage, motility, reserves of regenerative cells, transport of materials, structural interactions with other tissues, modifiable tissue for morphogenesis, oxygen storage, and perhaps other functions that have yet to be determined.

parent material Undecomposed mineral particles and unweathered rock fragments beneath the subsoil; weathering of this layer produces new soil particles for the layers above.

parental investment Any investment in an offspring that increases its chances of survival and reproduction at the expense of the parents' ability to invest in other offspring.

parental manipulation The selective providing of care to some offspring at the expense of other offspring so as to maximize the parents' reproductive success.

parietal Bone of the cranium or coelomic membrane, as parietal peritoneum.

parietal cell Cell of a gastric gland that secretes hydrochloric acid and intrinsic factor.

parietal eye See median eye.

Parotid The parotid gland at the base of the ear.

pars prostatica Dilation of the ejaculatory duct of a flatworm, surrounded by unicellular prostate cells.

parthenocarpy Fruit development without fertilization.

parthenogenesis Unisexual reproduction involving the production of young by females not fertilized by males; common in rotifers, cladocerans, aphids, bees, ants, and wasps. A parthenogenetic egg may be diploid or haploid.

particulate material Atmospheric aerosols, such as dust, ash, soot, lint, smoke, pollen, spores, algal cells, and other suspended materials; originally applied only to solid particles but now extended to droplets of liquid.

parts per billion (ppb) Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid mixture.

parts per million (ppm) Number of parts of a chemical found in one million parts of a particular gas, liquid, or solid mixture.

parts per trillion (ppt) Number of parts of a chemical found in one trillion (1012) parts of a particular gas, liquid, or solid mixture.

parturition The process of childbirth.

paruterine organ Fibromuscular organ in some cestodes that replaces the uterus.

passive avoidance learning A form of operant conditioning, in which the animal does not make an overt response, but rather is trained to avoid making a particular response by being exposed to some type of noxious stimulus each time the response occurs.

passive continental margin A continental margin that is located at the "trailing edge" of a continent and as a result shows little geological activity . Compare active continental margin.

passive heat absorption The use of natural materials or absorptive structures without moving parts to gather and hold heat; the simplest and oldest use of solar energy.

passive immunization Immune state in an animal created by inoculation with serum (containing antibodies) or lymphocytes from an immune animal, rather than by exposure to the antigen.

pasture Enclosed domestic meadows or managed grazing lands.

patch model A type of optimal foraging model that assumes that prey occur in discrete clumps and that seeks to predict where and when organisms will forage for these prey.

patches Regions of localized concentrations of resources.

patchiness Within a larger ecosystem, the presence of smaller areas that differ in some physical conditions and thus support somewhat different communities; a diversity-promoting phenomenon.

patella Knee bone.

patent Stage in an infection at which infectious agents produce evidence of their presence, such as eggs or cysts. Contrast with prepatent.

pathogen An organism that produces disease in a host organism, disease being an alteration of one or more metabolic functions in response to the presence of the organism.

pathogenesis Production and development of disease.

pathogenic Producing or capable of producing disease.

pathogenicity Capability of an agent to produce disease.

paurometabolous metamorphosis A form of insect development in which immatures resemble parents, and molting is restricted to the immature stages.

Pauropoda A class of arthropods whose bodies are small, soft, 11 segmented, and have 9 pairs of legs.

Pavlovian conditioning See classical conditioning.

PCBs See polychlorinated biphenyls.

PCR .See polymerase chain reaction.

peanut worms Burrowing invertebrates with an unsegmented body and an anterior end that can be pulled into the body.

peat moss A relatively sterile, inert medium composed of partially decomposed plants of the genus Sphagnum. Exceptionally high water-holding capacity.

peat Deposits of moist, acidic, semidecayed organic matter.

pebrine An infectious disease of silkworms caused by the protozoan Nosema bombicis.

peck order. A hierarchy of social privilege in a flock of birds.

pecten Any of several types of comblike structures on various organisms, for example, a pigmented, vascular, and comblike process that projects into the vitreous humor from the retina at a point of entrance of the optic nerve in the eyes of all birds and many reptiles.

pectin A complex polysaccharide in the middle lamella and primary walls of plant cells.

pectines Sensory appendage on abdomens of scorpions.

pectoral fin Each of the pair of fins just behind the head of fishes.

pectoral Of or pertaining to the breast or chest; to the pectoral girdle; or to a pair of horny shields of the plastron of certain turtles.

pedal locomotion The type of locomotion exhibited by flatworms, some cnidarians, and the gastropod molluscs. This locomotion involves waves of activity in the muscular system, which is applied to the substratum.

pedalium Flattened blade at the base of the tentacles in cubozoan medusae (Cnidaria).

pedicel An individual stalk of a flower that is part of an inflorescence.

pedicellariae Pincerlike structures found on the body wall of many echinoderms. They are used in cleaning and defense.

pedipalps Second pair of appendages in chelicerate arthropods, modified variously in different groups.

pedogenesis See paedogenesis.

peduncle A stalk. Also, a band of white matter joining different parts of the brain.

pelage Hairy covering of mammals.

pelagic organism An organism that lives above the bottom in the open sea.

pelagic Pertaining to the open ocean.

pellagra Lassitude, torpor, dermatitis, diarrhea, dementia, and death brought about by a diet deficient in tryptophan and niacin.

pellicle A thin, frequently noncellular covering of an animal (e.g., the protective and supportive pellicle of protists occurs just below the plasma membrane); may be composed of a cell membrane, cytoskeleton, and other organelles.

pelvic fin Each of the second pair of ventral fins of fishes.

pelvic Situated at or near the pelvis, as applied to girdle, cavity, fins, and limbs.

pelycosaur Any of a group of carnivorous Permian synapsids distinguished by powerful jaws, stabbing teeth, and a large skin-covered sail on the back.

pen in squids, a thin, chitinous structure extending the length of the mantle tissue

penicillin An antibiotic produced by certain species of Penicillium. An ascomycete fungus.

Pennsylvanian A geological period of the Carboniferous beginning about 320 million years ago.

pentadactyl With five digits, or five fingerlike parts, to the hand or foot.

pentamerous symmetry A radial symmetry based on five or multiples thereof.

pentaradial symmetry A form of radial symmetry found in the echinoderms in which body parts are arranged in fives around an oral-aboral axis.

Pentastomida A phylum of worms that are all endoparasites in the lungs or nasal passageways of carnivorous vertebrates. Tongue worms.

pentose A 5-carbon sugar.

PEP carboxylase The enzyme responsible for CO2 fixation in the primary fixation of C4 metabolism.

pepo A berry with the outer wall formed from receptacle tissue fused to the exocarp; the fleshy interior is mesocarp and endocarp.

peptidase An enzyme that breaks down simple peptides, releasing amino acids.

peptide bond A chemical bond formed between the amino group of one amino acid and the carboxyl (acidic) group of an adjacent amino acid.

peptide bond. A bond that binds amino acids together into a polypeptide chain, formed by removing an OH from the carboxyl group of one amino acid and an H from the amino group of another to form an amide group _CO_NH_.

peptides Two or more amino acids linked by a peptide bond.

perception The analysis and interpretation of sensory information.

perceptual world The manner in which an organism's brain analyzes and interprets all incoming stimuli. Each species has a different view of the world, its Umwelt.

percolation The movement of water by gravitational pull down through soil particles.

pereiopod Thoracic appendage of a crustacean.

perennial species Plants that grow for more than two years.

perennial A plant that overwinters and continues to grow for many years. It may reproduce every year, or only on rare occasions.

perennibranchiate Having permanent gills, relating especially to certain paedomorphic salamanders.

perfect flower A flower having both stamens and carpels.

perfect stage The phase during the life cycle of a fungus when sexual fusion occurs, producing characteristic sexual spores.

perianth The petals and sepals together.

pericardium Area around heart; membrane around heart.

pericarp The fruit wall that develops from the ovary wall.

pericycle Root tissue sandwiched between the endodermis and the phloem; it is the outermost layer of the stele; meristematic region that gives rise to branch roots.

periderm Protective tissue that replaces the epidermis after secondary growth begins; it includes the cork, the cork cambium, and sometimes other cells.

perigynous A flower in which the base of the sepals, petals, and stamens form a cup around the ovary.

period In ocean waves, the time required for two successive waves to pass a reference point

period The duration of one cycle of a biological rhythm.

periostracum Outer horny layer of a mollusc shell.

peripheral nervous system The nerves and ganglia of the nervous system that lie outside of the brain and spinal cord.

peripheral protein A protein on the surface of a biological membrane.

peripheral Structure or location distant from center, near outer boundaries.

periproct Region of aboral plates around the anus of echinoids.

perisarc Sheath covering the stalk and branches of a hydroid.

perissodactyl Pertaining to an order of ungulate mammals with an odd number of digits.

Peristalsis Movement provided by the smooth muscles to the contents of the digestive tract, ureters, urethra, uterine duct, and vas deferens.

peristomium The segment of the body of an annelid that surrounds the mouth.

perithecium A flask-shaped ascocarp.

peritoneum The membrane that lines the coelom and covers the coelomic viscera.

peritreme Elongated sclerite extending forward from the stigma of certain mites, mainly in the suborder Mesostigmata.

peritrophic membrane Noncellular, delicate membrane lining an insect's midgut.

permafrost The permanently frozen soil in polar regions.

permanent parasite Parasite that lives its entire adult life within or on a host.

permanent retrievable storage Placing waste storage containers in a secure building, salt mine, or bedrock cavern where they can be inspected periodically and retrieved, if necessary, for repacking or for transfer if a better means of disposal or reuse is developed.

permanent wilting point The soil moisture content at the point when a given plant's root system can no longer absorb water.

permeability A property of membranes allowing all substances to pass freely.

permease. A transporter molecule; a molecule in the cell membrane that makes it possible for another molecule (to which the membrane is not otherwise permeable) to be transported across the membrane, that is, mediated transport.

Permian The last geological period of the Paleozoic era beginning about 286 million years ago and ending 251 million years ago. The end of the Permian was a time of abrupt extinction in the diversity of flora and fauna, especially of sea life.

peroxisome A cellular microbody containing enzymes involved with photorespiration and photosynthesis.

peroxisome A microbody found in leaves and often associated with chloroplasts.

peroxisomes Small organelles containing enzymes of the glyoxylate cycle, catalase, and peroxidases.

Pes A foot.

pest resurgence Rebound of pest populations due to acquired resistance to chemicals and nonspecific destruction of natural predators and competitors by broadscale pesticides.

pest Any organism that reduces the availability, quality, or value of a useful resource.

pesticide Any chemical that kills, controls, drives away, or modifies the behavior of a pest.

pesticide rain Long-range transport of pesticides by air currents and deposition through precipitation in sites far from its origin; analogous to acid rain.

pesticide treadmill A need for constantly increasing doses or new pesticides to prevent pest resurgence.

petal The often showy flower component attached just inside the sepals; petals are usually colorful to attract pollinators.

petaloid A modified, flattened filament of a stamen that may resemble a petal.

petaloids Describes flowerlike arrangement of respiratory podia in irregular sea urchins.

petiole A stalklike portion of a leaf connecting the blade to the stem or branch.

petiole The stalk of a leaf.

petrifaction A fossil formed when plant parts are infiltrated or replaced by mineral substances such that the structure is preserved but the fossil is actually rock.

petroleum products Oil and the materials made from crude natural petroleum.

Petrous petro = a rock.

Peyer's patches Lymphoid tissue in the wall of the intestine; not circumscribed by a tissue capsule.

PGA 3-phosphoglyceric acid, the first compound produced in the Calvin cycle.

pH scale The numerical scale that measures acidity and alkalinity, ranging from 0 (most acidic) to 14 (most basic); pH stands for potential hydrogen and refers to the concentration of hydrogen ions (H+).

pH (potential of hydrogen). A symbol referring to the relative concentration of hydrogen ions in a solution; pH values are from 0 to 14, and the lower the value, the more acid or hydrogen ions in the solution. Equal to the negative logarithm of the hydrogen ion concentration.

Phaeophyta The group of organisms commonly termed the brown algae, consisting of about 1500 species, including the kelps.

phagocyte Any cell that engulfs and devours microorganisms or other particles.

phagocytosis The engulfment of a particle by a phagocyte or a protozoan.

phagolysosome Vacuole in a cell in which a phagocytosed particle is digested.

phagolysosome The organelle that is formed when a lysosome combines with a vesicle.

phagosome Membrane-bound vesicle in cytoplasm containing food material engulfed by phagocytosis.

phagotroph A heterotrophic organism that ingests solid particles for food.

pharmaceuticals Medicinal drugs.

pharyngeal gill slits See gill slit.

pharynx The part of the digestive tract between the mouth cavity and the esophagus that, in vertebrates, is common to both digestive and respiratory tracts. In cephalochordates the gill slits open from it.

phase A specified, recognizable portion of the activity cycle.

phasmid One of a pair of glands or sensory structures found in the posterior end of certain nematodes.

phellem Also termed cork; the outer, protective tissue of stems and roots composed of nonliving cells with suberized cell walls, formed centrifugally by the cork cambium or phellogen.

phelloderm A tissue laid down to the inside of the phellogen; the inner part of the periderm.

phellogen The cork cambium.

phenetic Refers to the use of a criterion of overall similarity to classify organisms into taxa; contrasts with classifications based explicitly on a reconstruction of phylogeny.

phenotype The expression that results from an interaction of one or more gene pairs and the environment.

phenotype matching A mechanism by which kin may recognize one another; individuals use kin as a referent whose phenotypes are learned by association.

phenotypic gradualism. The hypothesis that new traits, even those that are strikingly different from ancestral ones, evolve by a long series of small, incremental steps.

phenotypic ratio The relative numbers of progeny in each phenotypic category produced by a genetic cross.

pheromone Chemical substance released by one organism that influences the behavior or physiological processes of another organism.

philopatric Remaining near the place of birth after sexual maturation.

phloem rays The part of the vascular ray which is located in the secondary phloem.

phloem The food-conducting tissue of vascular plants, composed of sieve elements, various kinds of parenchyma cells, fibers, and sclereids.

phonoreceptor A specialized nerve ending that responds to sound.

phonotaxis Orientation with respect to sound.

phoresis Form of symbiosis when the symbiont, the phoront, is mechanically carried about by its host. Neither is physiologically dependent on the other.

Phoronida A phylum of marine animals whose members live in permanent chitinous tubes in muddy, sandy, or solid substrates. Feed via an anterior lophophore with two parallel rings of long tentacles.

phoronids Tube-dwelling, unsegmented invertebrates that possess a horseshoe-shaped or circular lophophore.

phosphagen A term for creatine phosphate and arginine phosphate, which store and may be sources of high-energy phosphate bonds.

phosphate An important nutrient in the ocean.

phosphatide A lipid with phosphorus, such as lecithin. A complex phosphoric ester lipid, such as lecithin, found in all cells. Phospholipid.

phospholipid A type of lipid molecule occurring in a bilayer in biological membranes; a lipid with two fatty acids and a phosphate group attached to glycerol.

phosphorus cycle The movement of phosphorus atoms from rocks through the biosphere and hydrosphere and back to rocks.

phosphorylation The addition of a phosphate group to a molecule.

photic zone The surface layer where there is enough light for photosynthesis to occur. Also see epipelagic zone.

photoautotroph Organism that synthesizes organic matter using the energy of light.

photochemical oxidants Products of secondary atmospheric reactions. See smog.

photodegradable plastics Plastics that break down when exposed to sunlight or to a specific wavelength of light.

photoinhibition reduction of photsynthetic rates due to too much light

photolysis The splitting of a molecule of water during the light reactions of photosynthesis; the Hill reaction.

photon A unit of light energy.

photooxidation The change in the structure of a molecule due to exposure to light; sometimes referred to as bleaching.

photoperiodism The system within organisms that causes certain events, including the onset of reproduction, to be related to the length of day.

photophore An organ that produces bioluminescence.

photophosphorylation The formation of ATP utilizing light energy in photosynthesis.

photopigment A molecule in visual receptor cells (e.g., rhodopsin) that responds to light energy.

photoreceptors Sensory cells that contain photopigments and respond specifically to light energy.

photorespiration In C3 plants, respiration rates increase

photosynthesis The biochemical process by which green plants and some bacteria capture light energy and use it to produce chemical bonds. Carbon dioxide and water are consumed while oxygen and simple sugars are produced.

photosynthetic efficiency The percentage of available light captured by plants and used to make useful products.

photosynthetic pigment A molecule such as chlorophyll that is responsible for capturing solar energy in photosynthesis.

photosynthetic unit A group of associated chlorophyll molecules, including antenna molecules and a central chlorophyll a collector molecule.

photosystem I The second part of the Z-scheme in which a chlorophyll a molecule absorbs most effectively at 700 nm.

photosystem II The first part of the Z-scheme in which the chlorophyll a molecule absorbs most effectively at 680 nm.

phototaxis A taxis in which light is the orienting stimulus. An involuntary tendency for an organism to turn toward (positive) or away from (negative) light.

phototrophs Organisms capable of using CO2 in the presence of light as a source of metabolic energy.

phototropism The bending of a plant toward a unidirectional light source.

photovoltaic cell An energy-conversion device that captures solar energy and directly converts it to electrical current.

phragmoplast A system of microtubules and vesicles that arises between two daughter nuclei at telophase and forms the cell plate.

phycobilins A group of photosynthetic pigments that includes phycocyanin, a bluish pigment in cyanobacteria (blue-green algae), and phycoerythrin, a red pigment in red algae.

phycobilosomes Groups of pigmented cells containing phycocyanin and phycoerythrin.

phycocolloid One of several starch-like chemicals found in some seaweeds. They are of significant commercial importance. Also see agar, algin, and carrageenan.

phycocyanin A blue photosynthetic pigment found in cyanobacteria and red algae.

phycoerythrin A red photosynthetic pigment found in cyanobacteria and red algae.

phyletic gradualism. A model of evolution in which morphological evolutionary change is continuous and incremental and occurs mainly within unbranched species or lineages over long periods of geological time; contrasts with punctuated equilibrium.

phyletics Phylogenetic systematics, cladistics.

phyllopodium Leaflike swimming appendage of branchiopod crustaceans.

phylogenetic species concept. An irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent.

phylogenetic systematics The study of the phylogenetic relationships among organisms in which true and false similarities are differentiated; cladistics.

phylogenetic tree a branching diagram depicting the evolutionary relationships between major groups of organisms

phylogeny The origin and diversification of any taxon, or the evolutionary history of its origin and diversification, usually presented in the form of a dendrogram.

phylum A chief category, between kingdom and class, of taxonomic classifications into which are grouped organisms of common descent that share a fundamental pattern of organization.

physical or abiotic factors Nonliving factors, such as temperature, light, water, minerals, and climate, that influence an organism.

physiological dependence The condition in which there is a physical need for a drug to avoid withdrawal symptoms.

physiological limits The tolerance range of given species for temperature induced rates of metabolic activity.

physiology A branch of biology dealing with the organic processes and phenomena of an organism or any of its parts or of a particular bodily process.

physostomous swim bladder In fish, a swim bladder with an air passage or duct to the esophagus.

phytochrome The protein pigment responsible for the phenomenon of photoperiodism.

phytoflagellates Members of the class Phytomastigophorea, plantlike flagellates.

Phytomastigophorea The protozoan class where members usually have chloroplasts; mainly autotrophic, some heterotrophic. Examples: Euglena, Volvox, Chlamydomonas.

phytophagous Organisms that feed on plants.

phytoplankton Microscopic, free-floating, autotrophic organisms that function as producers in aquatic ecosystems.

pia mater The innermost meninx that is in direct contact with the brain and spinal cord.

picoplankton the small-sized groups of phytoplankton with cells less than 2 mm in width

picoplankton The component of the plankton that consists of extremely small organisms, mostly bacterioplankton, 0.2 to 2 mm (0.0002 to 0.002 mm) in size(fig. 14.3); they are too small to catch in a standard plankton net.

pigments Molecules that reflect and absorb light at particular wavelengths.

pilidium larva Free-swimming, hat-shaped larva of nemertean worms characterized by an apical tuft of cilia.

pilidium Free-swimming, hat-shaped larva of nemertine worms.

piloting The use of familiar landmarks to find a direction or area.

pinacocyte Thin, flat cell covering the outer surface, and some of the inner surface, of poriferans.

pinacoderm The layer of pinacocytes in sponges.

pineal gland An endocrine gland located near the midline of the brain that produces melatonin, a hormone involved in biological rhythms, particularly in annual cycles.

Pineal pinea = a pine cone.

pinfeather See filoplume feather.

pinkeye Bacterial conjunctivitis, sometimes transmitted by flies of the genus Hippolates.

pinna The external ear. Also a feather, wing, or fin or similar part.

pinnate Denoting an arrangement of leaflets (or lobes on a simple leaf) along a main central unit.

pinnately compound leaf Leaflets attached on both sides of a common axis.

pinnipeds Mammals with paddle-shaped flippers: seals, eared seals (sea lions and fur seals), and the walrus.

pinocytosis Taking up of fluid by endocytosis; cell drinking.

pioneer community The first community to become established in an area.

pioneer organism An organism that first colonizes soil or pioneer species.In primary succession on a terrestrial site, the plants, lichens, and microbes that first colonize the site.

pipestem fibrosis Thickening of the walls of a bile duct as the result of the irritating presence of a parasite.

piroplasm Any of the class Piroplasmea, while in a circulating erythrocyte.

pistil The female reproductive part of a flower, consisting of stigma, style, and ovary. Also termed the carpel.

pistillate flower A flower having carpels but no stamens.

pistillate Denoting a unisexual flower having a pistil but no stamens (=carpellate).

pit A pore in a secondary cell wall.

pit organ Receptor of infrared radiation (heat) on the head of some snakes (pit vipers).

pith The central tissue of a dicot stem, consisting of parenchyma cells.

pith ray The region or tissue located between vascular bundles in a stem and connecting the pith and cortex; also termed interfascicular region or medullary ray.

pith Parenchymal tissue in the center of a stem located interior to the vascular bundles.

Pituitary pituitar = mucous secretion. An endocrine gland at the base of the brain.

pituitary gland The master gland of the endocrine system of vertebrate animals. Located directly below the hypothalamus of the brain, the pituitary produces or releases a variety of hormones that target other endocrine glands of the body.

placenta Structure by which an unborn child or animal is attached to its mother's uterine wall and through which it is nourished.

placid Plates on Kinorhyncha.

placode Localized, plate-like thickening of vertebrate head ectoderm from which a specialized structure develops; such structures include eye lens, special sense organs, and certain neurons.

placoderms A group of heavily armored jawed fishes of the Lower Devonian to Lower Carboniferous.

placoid scale Type of scale found in cartilaginous fishes, with basal plate of dentin embedded in the skin and a backward-pointing spine tipped with enamel.

Placozoa A phylum of small, flattened, marine animals that feed by forming a temporary digestive cavity. Tricoplax adherans.

planidium First instar of hypermetamorphic, parasitic Diptera and Hymenoptera, which is apodous but moves actively by means of thoracic and caudal setae.

planktivore An animal that feeds on plankton.

plankton Animals (zooplankton) and unicellular algae (phytoplankton) that have only limited locomotory capabilities and are therefore distributed by water movements.

planktonic Small organisms that passively float or drift in a body of water.

plant growth regulator Any molecule that exhibits hormone-like effects in a plant, whether synthesized or naturally occurring.

Plantae One of the five kingdoms of life; characterized by being eukaryotic and multicellular, and having rigid cell walls and chloroplasts.

Plantar The sole of the foot.

plantigrade Pertaining to animals that walk on the whole surface of the foot (for example, humans and bears); compares with digitigrade.

plants Members of the kingdom Plantae, which consists of photosynthetic, eukaryotic, and mostly multicellular organisms.

planula Free-swimming, ciliated larval type of cnidarians; usually flattened and ovoid, with an outer layer of ectodermal cells and an inner mass of endodermal cells.

planuloid ancestor Hypothetical form representing ancestor of Cnidaria and Platyhelminthes.

Plasma plasma = form. An undifferentiated tissue from which another tissue or tissues may form.

plasma cell A mature, differentiated B lymphocyte chiefly occupied with antibody synthesis and secretion; a plasma cell lives for only five to seven days.

plasma membrane Outermost membrane of a cell; its surface has molecular regions that detect changes in external conditions and act as a selective barrier to the passage of ions and molecules between the cell and its environment. The external covering of a protozoan.

plasmalemma The cell membrane.

plasmid A small circle of DNA that may be carried by a bacterium in addition to its genomic DNA.

plasmodesma A cytoplasmic strand that connects adjacent plant cells through pores in the cell wall.

plasmodium Multinucleate ameboid mass, syncytial.

plasmolysis The osmotic removal of water from the cytoplasm and vacuole, causing the cytoplasm to pull away from the cell wall and clump in the center.

plasmotomy Division of a multinucleate cell into multinucleate daughter cells, without accompanying mitosis.

plastic song One of the stages of song development in birds during which the bird has begun to sing using the species-typical pattern, but with a certain degree of variation.

plasticity Cell wall "stretchability" induced by auxin.

plastid A membranous organelle in plant cells functioning in photosynthesis and/or nutrient storage, for example, chloroplast.

plastron Ventral body shield of turtles; structure in corresponding position in certain arthropods; thin film of gas retained by epicuticle hairs of aquatic insects.

plate tectonics the collective geologic processes that move the crustal plates of the earth and cause continental drifting and seafloor spreading

platelet A tiny, incomplete cell in the blood that releases substances initiating blood clotting.

Platyhelminthes The phylum of flatworms; bilateral acoelomates.

play behavior Certain locomotor, social, and manipulative behavior patterns exhibited by young and some adult mammals and birds.

pleiotropic Pertaining to a gene producing more than one effect; affecting multiple phenotypic characteristics.

Pleistocene A geological period, which began about 2 million years ago, characterized by a series of ice ages.

pleopod One of the swimming appendages on the abdomen of a crustacean.

plerocercoid Metacestode that develops from a procercoid. It usually shows little differentiation.

plerocercoid larva Metacestode that develops from a procercoid larva; it usually shows little differentiation.

plerocercus Tapeworm metacestode in the order Trypanorhyncha in which the posterior forms a bladder, the blastocyst, into which the rest of the body withdraws.

plesiomorphic Ancestral characters; characters possessed by members of both ingroup and outgroup.

pleura The membrane that lines each half of the thorax and covers the lungs.

pleurite Lateral sclerite of a somite in an arthropod.

plexus A network, especially of nerves or blood vessels.

plica polonica Develops in untreated head louse (Pediculus humanus capitis) infection, hair matted together with exudate, fungal growth, fetid odor.

plumule The shoot apex of a seedling, including embryonic leaves.

pluteus Echinoid or ophiuroid larva with elongated processes like the supports of a desk; originally called "painter's easel larva."

plywood A building material consisting of two or more thin sheets of wood bonded together.

pneumatic duct in fish, the connection between the esophagus and swim bladder

pneumatic sacs Gas-filled sacs that arise from the esophagus, or another part of the digestive tract, of fishes. Pneumatic sacs are used in buoyancy regulation (swim bladders) or gas exchange (lungs).

pneumatocyst A gas-filled bladder in seaweeds.

pneumatophore A specialized root that grows upward into the air from roots growing in the mud; aids in gaseous exchange.

pneumostome The opening of the mantle cavity (lung) of pulmonate gastropods to the outside.

poachers Those who hunt wildlife illegally.

pod 1. A dry dehiscent fruit that splits along two seams; a legume.2. A school of cetaceans.

podium A footlike structure, for example, the tube foot of echinoderms.

podomere More or less cylindrical segment of a limb of an arthropod, generally articulated at both ends.

podosoma Portion of the body of a tick or mite that bears the legs.

poecilostome Describes mouthparts borne by members of the copepod order Poecilostomata (buccal cavity large, somewhat slitlike, with sickle-shaped mandibles); also, a member of the order Poecilostomata.

Pogonophora A phylum of protostomate, marine animals that are distributed throughout the world's oceans. Live in secreted, chitinous tubes in cold water at depths exceeding 100 m. Lack a mouth and digestive tract. Nutrients absorbed across the body wall and from endosymbiotic bacteria that they harbor. Beard worms.

pogonophorans See beard worms.

poikilotherm an organism whose body temperature varies with and is largely controlled by environmental temperatures

poikilothermic Pertaining to animals whose body temperature is variable and fluctuates with that of the environment; cold blooded; compares with ectothermic.

point mutations A change in the structure of a gene that usually arises from the addition, deletion, or substitution of one or more nitrogenous bases.

point sources Specific locations of highly concentrated pollution discharge, such as factories, power plants, sewage treatment plants, underground coal mines, and oil wells.

polar capsule Compartment bearing the polar filaments in myxozoans.

polar covalent bond The type of bond that is formed by asymmetrical moving electrons.

polar easterlies Variable winds that blow at high latitudes.

polar filament Threadlike organelles in Myxozoa and Microspora.

polar granule Refractile granule within a coccidian oocyst.

polar ice cap The portions of the globe close to the poles that are permanently covered with ice.

polar nuclei Two of the eight haploid nuclei of a megagametophyte that migrate, one from each end, to the middle of the embryo sac. These two nuclei fuse with a sperm nucleus during double fertilization.

polar ring Electron-dense organelles of unknown function, located under the cell membrane at the anterior tip of sporozoites and merozoites.

polarity In systematics, the ordering of alternative states of a taxonomic character from ancestral to successively derived conditions in an evolutionary transformation series. In developmental biology, the tendency for the axis of an ovum to orient corresponding to the axis of the mother. Also, condition of having opposite poles; differential distribution of gradation along an axis.

polarization The arrangement of positive electrical charges on one side of a surface membrane and negative electrical charges on the other side (in nerves and muscles).

polarized A description for a membrane that has a potential difference due to an unequal distribution of ions across the membrane.

polaroplast Organelle, apparently a vacuole, near the polar filament of a microsporidean.

Polian vesicles Vesicles opening into ring canal in most asteroids and holothuroids.

pollen grain The structure into which a haploid microspore develops; contains a halpoid tube nucleus and two haploid sperm nuclei at maturity.

pollen The collective term for pollen grains, the male gametophytes.

pollen tube A tube that develops from the pollen grain and carries the sperm to the ovule.

Pollex The thumb.

pollination The transfer of pollen to a receptive surface; the stigma in angiosperms or the pollination droplet in most gymnosperms.

pollinator An organism that effects pollination.

pollution charges Fees assessed per unit of pollution based on the "polluter pays" principle.

pollution The introduction of harmful substances or heat energy into

the environment by humans.

polyandrous Having more than one male mate. Polyandry is advantageous when food is plentiful but, because of predation or other factors, the chances of successfully rearing young are low.

polyandry Condition of having more than one male mate at one time.

polycarpic Denoting a plant that flowers more than once during its lifetime.

polycentric complex Cities with several urban cores surrounding a once dominant central core.

Polychaeta The class of annelids whose members are mostly marine and are characterized by a head with eyes and tentacles and a body with parapodia. Parapodia bear numerous setae. Examples: Nereis, Arenicola.

polychaetes Segmented worms that have parapodia.

polychlorinated biphenyls (PCBs) A group of nonbiodegradable pollutants.

polyembryony Development of a single zygote into more than one offspring.

polygamy Condition of having more than one mate at a time.

polygenes Genes at multiple loci that influence a trait in a quantitative fashion.

polygenic Multiple genetic influences controlling a single trait, causes the trait to display continuous variation.

polygenic inheritance. Inheritance of traits influenced by multiple alleles; traits show continuous variation between extremes; offspring are usually intermediate between the two parents; also known as blending and quantitative inheritance.

polygenic trait Traits that are influenced by many genes and that are usually continuously distributed within a population.

polygyny Condition of having more than one female mate at one time.

polygynous Having more than one female mate. Polygyny tends to occur in species whose young are relatively independent at birth or hatching.

polygyny threshold The point at which a female will benefit more by joining an already mated male possessing a good territory rather than an unmated male on a poor territory.

polykinetid Rows or fields of kinetids in ciliates linked by fibrous networks.

polymer A chemical compound composed of repeated structural units called monomers.

polymerase chain reaction (PCR). A technique for preparing large quantities of DNA from tiny samples, making it easy to clone a specific gene as long as part of the sequence of the gene is known.

polymerization The process of forming a polymer or polymeric compound.

polymorphic A locus that contains two or more alleles within a population.

polymorphism The presence in a species of more than one structural type of individual.

polynomial A scientific name for an organism composed of more than two words.

polynucleotide A nucleotide of many mononucleotides combined.

polynucleotide chains Attachment of one nucleotide to another in a linear fashion.

polyp Individual of the phylum Cnidaria, generally adapted for attachment to the substratum at the aboral end, often form colonies.

polypeptide A molecule consisting of many joined amino acids, not as complex as a protein.

polyphyletic group An assemblage of organisms that includes multiple evolutionary lineages. Polyphyletic assemblages usually reflect insufficient knowledge regarding the phylogeny of a group of organisms.

polyphyly The condition that a taxon or other group of organisms does not contain the most recent common ancestor of all members of the group, implying that it has multiple evolutionary origins; such groups are not valid as formal taxa and are recognized as such only through error. Contrasts with monophyly and paraphyly.

polyphyodont Having several sets of teeth in succession.

polypide An individual or zooid in a colony, specifically in ectoprocts, which has a lophophore, digestive tract, muscles, and nerve centers.

Polyplacophora The class of molluscs whose members are elongate, dorsoventrally flattened, and have a shell consisting of eight dorsal plates.

polyploid An organism possessing more than two full homologous sets of chromosomes.

polyploidy Having more than two sets of chromosomes.

polypod larva Caterpillar type of larva found in Lepidoptera and some Hymenoptera. It has thoracic appendages and abdominal locomotory processes (prolegs). Also called cruciform.

polysaccharide A carbohydrate composed of many monosaccharide units, for example, glycogen, starch, and cellulose.

polysome Two or more ribosomes connected by a molecule of messenger RNA.

polytene chromosomes Chromosomes in the somatic cells of some insects in which the chromatin replicates repeatedly without undergoing mitosis.

polyunsaturated fat A fat having several to many double bonds between carbon atoms.

polyzoic A tapeworm forming a strobila of several to many proglottids; also, a colony of many zooids.

pome A fleshy fruit derived from a compound inferior ovary; the fleshy edible part is the ripened tissue surrounding the ovary (derived from receptacle and perianth tissue); the ovary matures into the core and contains the seed.example: apple or pear.

pongid Of or relating to the primate family Pongidae, comprising the anthropoid apes (gorillas, chimpanzees, gibbons, orangutans).

pons A portion of the brain stem above the medulla oblongata and below the midbrain.

Popliteal poples = the back of the knee.

population A group of organisms of the same species inhabiting a specific geographical locality.

population crash A sudden population decline caused by predation, waste accumulation, or resource depletion; also called a dieback.

population explosion Growth of a population at exponential rates to a size that exceeds environmental carrying capacity; usually followed by a population crash.

population genetics The study of events occurring in gene pools.

population hurdle The need for investment in infrastructure and social services in a rapidly growing population that prevents the capital investment necessary for real economic development.

population momentum A potential for increased population growth as young members reach reproductive age.

populational gradualism. The observation that new genetic variants become established in a population by increasing their frequencies across generations incrementally, initially from one or a few individuals and eventually characterizing a majority of the population.

pore cell The tube-like cell of sponges that forms a pore.

pore space Space between soil particles.

Porifera The animal phylum whose members are sessile and either asymmetrical or radially symmetrical. Body organized around a system of water canals and chambers. Cells are not organized into tissues or organs. Sponges.

porocyte Type of cell found in asconoid sponges through which water enters the spongocoel.

porose area Sunken areas on the basis capituli of certain mites and ticks.

porosity The relative amount of pore space versus soil particulate space in soil.

portal porta = gate. A pathway or entrance.

portal system System of large veins beginning and ending with a bed of capillaries; for example, hepatic portal and renal portal system in vertebrates.

post-kala-azar dermal leishmanoid Disfiguring dermal condition developing about one to two years after inadequate treatment of kala-azar.

postanal tail A tail that extends posterior to the anus; one of the four unique characteristics of chordates.

posterior Situated at or toward the rear of the body; situated toward the back; in human anatomy the upright posture makes posterior and dorsal identical.

posterior station Development of a protozoan in the hindgut or posterior midgut of its insect host, such as in the section Stercoraria of the Trypanosomatidae.

postharvest physiology The study of the storage and perishability of fruits following harvest.

postmaterialist values A philosophy that emphasizes quality of life over acquisition of material goods.

postmating isolation Isolation that occurs when fertilization is prevented even though mating has occurred.

potential energy Stored energy that is latent but available for use. A rock poised at the top of a hill or water stored behind a dam are examples of potential energy.

potocytosis Endocytosis of certain small molecules and ions bound to specific receptors limited to small areas on the cell surface. The areas of the receptors are invaginated and pinch off to form tiny vesicles. See caveolae.

power The rate of energy delivery; measured in horsepower or watts.

prairie An area of land dominated by grasses with occasional shrubby plants and small trees occurring where the grass cover is broken and with herbaceous perennials during certain seasons.

praniza Parasitic larva of the isopod suborder Gnathiidea. It parasitizes fishes and feeds on blood.

preadaptation Occurs when a structure or a process present in members of a species proves useful in promoting reproductive success when an individual encounters new environmental situations.

prebiotic synthesis. The chemical synthesis that occurred before the emergence of life.

Precambrian The geological era beginning with the formation of the earth, about 4700 million years ago and extending until about 590 million years ago. During this time the origin of life, the origin eukaryotes, and multicellular life-forms appeared.

precious corals Gorgonians that secrete a red or pink skeleton consisting of fused calcareous spicules.

precipitation Moisture falling from the atmosphere in the form of rain, sleet, snow, or hail.

precocial Having developed to a high degree of independence at the time of hatching or birth.

precocial young Young that are born or hatched at a relatively advanced state of development and are capable of a more independent existence beginning at birth or hatching. Compare to altricial.

precycling Making environmentally sound decisions at the store and reducing waste before we buy.

predaceous, predacious Living by killing and consuming other animals; predatory.

predation The derivation of an organism of elements essential for its existence from organisms of other species that it consumes and destroys. The ingestion of prey by a predator for energy and nutrients.

predator An organism that preys on other organisms for its food.

predictive model a somewhat simplified representation of a physical process that yields information with predictive value

preformation The erroneous idea that gametes contain miniaturized versions of all of the elements present in an adult.

prehensile Adapted for grasping.

premating isolation When behaviors or other factors prevent animals from mating.

premunition Resistance to reinfection or superinfection, conferred by a still existing infection, that does not destroy the organisms of the infection already present.

prenymph Nonfeeding, quiescent stage in the life cycle of a chigger mite.

preoptic nuclei Neurons in the hypothalamus that produce various neuropeptides.

preparedness The genetically based predisposition to learn. Depending upon the learning situation, animals of a particular species may be prepared, unprepared, or contraprepared to learn.

prepatent Developmental stage in an infection before agents produce evidence of their presence.

presoma The proboscis, neck, and attached muscles and organs of an acanthocephalan.

pressure The weight exerted over a unit area of surface. This is equal to 1 atmosphere (14.7 lbs per square inch) at the sea surface and 1 atmosphere plus the pressure exerted by the water column, which is 1 atmosphere per 10 m (33 ft) of depth.

pressure potential The water force created by a real pressure against a membrane.

Pressure Flow Hypothesis The theory that organic solutes move along a concentration gradient from source to sink through the phloem.

prevalance In epidemiology, the number of cases of a disease at a given time; that is, a static measurement. Contrast with incidence.

prevention of significant deterioration A clause of the Clean Air Act that prevents degradation of existing clean air; opposed by industry as an unnecessary barrier to development.

prey model A type of optimal foraging model that addresses the types of organisms an individual should try to catch and eat (i.e., its diet).

Priapulida A phylum of aschelminths commonly called priapulids.

price elasticity A situation in which supply and demand of a commodity respond to price.

primary bilateral symmetry. Usually applied to a radially symmetrical organism descended from a bilateral ancestor and developing from a bilaterally symmetrical larva.

primary cell wall The cellulosic wall of all plant cells laid down at the time of mitosis and cytokinesis.

primary consumer An organism that consumes a producer organism as a food source; a herbivore.

primary endosperm nucleus The product of the fusion of a sperm and two polar nuclei in the embryo sac of angiosperms; double fertilization.

primary germ layers Blocks or layers of embryonic cells that give rise to tissues and organs of animals. See ectoderm, mesoderm, and endoderm.

primary growth Growth originating in the apical meristems of shoots and roots resulting in an increase in length of the axis.

primary immune response The initial immune response following antigen exposure.

primary pit fields Regions within the primary cell wall in which plasmodesmata traverse the cell wall.

primary pollutants Chemicals released directly into the air in a harmful form.

primary producer An autotrophic organism; able to build its own complex organic molecules from simple inorganic substances in the environment.

primary production The conversion of the inorganic carbon in carbon dioxide into organic carbon by autotrophs.

primary productivity Synthesis of organic materials (biomass) by green plants using the energy captured in photosynthesis.

primary radial symmetry .Usually applied to a radially symmetrical organism that did not have a bilateral ancestor or larva, in contrast to a secondarily radial organism.

primary standards Regulations of the 1970 Clean Air Act; intended to protect human health.

primary succession Plant successional events occurring in a pristine or newly forming habitat.

primary tissue Any tissue derived from the apical meristem, either shoot or root.

primary treatment A process that removes solids from sewage before it is discharged or treated further.

primary wall The wall layer of a plant cell deposited during cell expansion, generally thin and elastic.

primate Any mammal of the order Primates, which includes the tarsiers, lemurs, marmosets, monkeys, apes, and humans.

priming pheromone A chemical signal that alters the physiology of another organism, eventually causing a change in its behavior. Contrast with releasing pheromone.

primite Anterior member of a pair of gregarines in syzygy.

primitive streak A medial thickening along the dorsal margin of an amniote embryo that forms during the migration of endodermal and mesodermal cells into the interior of the embryo.

primitive Primordial; ancient; little evolved; said of characteristics closely approximating those possessed by early ancestral types.

principle of competitive exclusion A result of natural selection whereby two similar species in a community occupy different ecological niches, thereby reducing competition for food.

principle of independent assortment One of Mendel's observations on the behavior of hereditary units during gamete formation. A modern interpretation of this principle is that genes carried in one chromosome are distributed to gametes without regard to the distribution of genes in nonhomologous chromosomes.

principle of segregation One of Mendel's observations on the behavior of hereditary units during gamete formation. A modern interpretation of the principle of segregation is that genes exist in pairs, and during gamete formation, members of a pair of genes are distributed into separate gametes.

pristine Denoting a natural and undisturbed state.

proboscis A snout or trunk. Also, tubular sucking or feeding organ with the mouth at the end as in planarians, leeches, and insects. Also, the sensory and defensive organ at the anterior end of certain invertebrates.

procaryotes Bacteria and cyanobacteria that lack the structural complexity and defined nucleus found in eucaryotes.

procercoid Cestode metacestode developing from a coracidium in some orders. It usually has a posterior cercomer.

procercoid larva Cestode developing from a coracidium in some orders; it usually has a posterior cercomer. Developmental stage between oncosphere and plerocercoid.300

process To project from.

procuticle Thicker layer beneath the epicuticle of arthropods that lends mass and strength to the cuticle. It contains chitin, sclerotin, and also inorganic salts in Crustacea. The layers within the procuticle vary in structure and composition.

producer An organism that synthesizes food molecules from inorganic compounds by using an external energy source; most producers are photosynthetic.

producers Organisms, such as plants, able to produce their own food from inorganic substances.

production frontier The maximum output of two competing commodities at different levels of production.

production. In ecology, the energy accumulated by an organism that becomes incorporated into new biomass.

productivity The amount of plant product resulting from a combination of water, nutrients and environmental factors.

profile A graph that shows changes in temperature, salinity, or any other parameter with depth

profundal zone The deepest portion of a lake.

progametangia Swollen hyphae of fungi that fuse at the point of contact and eventually form gametangia.

progesterone Hormone secreted by the corpus luteum and the placenta; prepares the uterus for the fertilized egg and maintains the capacity of the uterus to hold the embryo and fetus.

proglottid One set of reproductive organs in a tapeworm strobila; usually corresponds to a segment. One of the linearly arranged segmentlike sections that make up the strobila of a tapeworm.

progymnosperm An extinct group of plants, the Progymnospermophyta, which included woody plants bearing large plannated braching systems; the prototypes of the gymnospems (e.g., Archaeopteris).

prohaptor Collective adhesive and feeding organs at the anterior end of a monogenetic trematode.

prohormone A precursor of a hormone, especially a peptide hormone.

prokaryote Organism in which the chromosomes are not contained within membrane-bound nuclei.

prokaryotic cell A type of cell lacking a nucleus and membrane-bound organelles; found in the Kingdom Monera.

prokaryotic, procaryotic Not having a membrane-bound nucleus or nuclei. Prokaryotic cells characterize the bacteria and cyanobacteria.

prolegs Unjointed abdominal appendage in the larva of Lepidoptera and some other insects.

promastigote Form of Trypanosomatidae with the free flagellum and the kinetoplast anterior to the nucleus, as in Leptomonas.

Promethean environmentalism A form of technological optimism that predicts that human ingenuity and enterprise will find cures for all our problems.

promiscuity A mating system in which there is no prolonged association between the sexes and multiple mating by at least one sex.

promoter. A region of DNA to which the RNA polymerase must have access for transcription of a structural gene to begin.

pronatalist pressures Influences that encourage people to have children.

pronephros Most anterior of three pairs of embryonic renal organs of vertebrates, functional only in adult hagfishes and larval fishes and amphibians, and vestigial in mammalian embryos. Adj., pronephric.

prop roots Adventitious roots arising on a stem above the ground and imparting some mechanical support to plants. The angled roots may provide for absorption of water and nutrients.

propagation The process of increasing in number.

prophase The stage of mitosis during which the chromosomes become visible under a light microscope. The first stage of mitosis during which the chromosomes are condensed but not yet attached to a mitotic spindle.

proplastids Membrane-bound particles that develop some internal structure; may subsequently develop into chloroplasts, chromoplasts, or leucoplasts.

propodeum First abdominal segment of hymenopterans, fused to the thorax.

propodosoma Portion of the podosoma that bears the first and second pairs of legs of a tick or mite.

propolar cells Anterior tier of cells in the calotte of a dicyemid mesozoan.

proprioceptor Sensory receptor located deep within the tissues, especially muscles, tendons, and joints, that is responsive to changes in muscle stretch, body position, and movement.

prosimian Any member of a group of arboreal primates including lemurs, tarsiers, and lorises, but excluding monkeys, apes, and humans.

prosoma Anterior part of an invertebrate in which primitive segmentation is not visible; fused head and thorax of arthropod; cephalothorax.

prosopyle Connections between the incurrent and radial canals in some sponges.

prostaglandins A family of fatty-acid hormones, originally discovered in semen, known to have powerful effects on smooth muscle, nerves, circulation, and reproductive organs.

prostate gland Gland located around the male urethra below the urinary bladder that adds its secretions to seminal fluid during ejaculation.

Prostate The prostate gland is "before" the bladder.

prosthetic group Nonprotein groups that are attached to an enzyme or other protein and necessary for its function.

prostomium Anterior closure of a metameric animal, anterior to the mouth.

protandric hermaphroditism A pattern of sexuality in which a single individual functions as male and then female in sequence.

protandrous Condition of hermaphroditic animals and plants in which male organs and their products appear before the corresponding female organs and products, thus preventing self-fertilization.

protandry The condition in a monoecious organism in which male gonads mature before female gametes; prevents self-fertilization.

protease An enzyme that digests proteins; includes proteinases and peptidases.

protective coloration Coloration that benefits the individual by providing concealment from predators.

protein synthesis The assemblage of protein as determined by the nucleotide sequence of a messenger RNA and the assistance of transfer RNA aligning amino acids in the proper arrangement.

protein A macromolecule of carbon, hydrogen, oxygen, and nitrogen and sometimes sulfur and phosphorus; composed of chains of amino acids joined by peptide bonds; present in all cells.

protelean parasite Organism parasitic during its larval or juvenile stages and free living as an adult, usually changing form with each stage.

proterosoma Combination of the gnathosoma and propodosoma of the body of a tick or mite.

prothallial cell Sterile cells present in the pollen grain of gymnosperms, believed to represent the last remnant of the vegetative male gametophyte thallus, which was, ancestrally,

prothoracic gland An invertebrate endocrine gland located in the prothorax region directly behind the head, which secretes ecdysone, a steroid hormone that promotes molting.

prothoracic glands. Glands in the prothorax of insects that secrete the hormone ecdysone.

prothoracicotropic hormone See ecdysiotropin.

prothorax The first of the three thoracic segments of an insect; usually contains the first pair of walking appendages.

prothrombin A constituent of blood plasma that is changed to thrombin by a catalytic sequence that includes thromboplastin, calcium, and plasma globulins; involved in blood clotting.

protist A member of the kingdom Protista, generally considered to include the protozoa and eukaryotic algae.

Protista The kingdom whose members are characterized by being eukaryotic and unicellular or colonial.

proto-oncogene. See oncogene.

protochordates Chordates that lack a backbone.

protocoel The anterior coelomic compartment in some deuterostomes, corresponds to the axocoel in echinoderms.

protocooperation. A mutually beneficial interaction between organisms in which the interaction is not physiologically necessary to the survival of either.

protoderm The dermal or outer tissue of an apical meristem that gives rise to the epidermis.

protogynous Hermaphroditism in which the female gonads mature before the male gonads.

protogyny A sequentially hermaphroditic species in which individuals change from females to males.

protomerite Anterior half of a cephaline gregarine protozoan.

proton A positively charged particle found in the atomic nucleus. The mass of a proton is about 1840 times that of an electron.

protonema The early, filamentous growth of the gametophyte in bryophytes and ferns.

protonephridium Primitive osmoregulatory or excretory organ composed of a tubule terminating internally with a flame bulb or solenocyte; the unit of a flame bulb system. Protonephridia are specialized for ultrafiltration.

protonymph Early, bloodsucking stage in the life cycle of some mesostigmatid mites.

protoplasm Organized living substance; cytoplasm and nucleoplasm of the cell.

protoplast fusion The technique of enzymatically digesting the cell wall of two distinctly different cells, then treating the plasma membrane so that protoplasts of the two cells fuse. The resulting hybrid may be difficult or impossible to achieve with traditional plant breeding.

protoplast All of a plant cell excluding the wall.

protopod (protopodite)Coxa and basis together.

protopod larva Larva found in some parasitic Hymenoptera and Diptera; limbs are rudimentary or absent; internal organs are incompletely differentiated; requires highly nutritive and sheltered environment for further development.

protopod, Basal portion of crustacean appendage, containing coxa and basis.

protopodite The basal segment of a biramous appendage of a crustacean.

protoscolex Juvenile scolex budded within a coenurus or a hydatid metacestode of a taeniid cestode.

protostome Animal in which the embryonic blastopore becomes the mouth; often possesses a trochophore larva, schizocoelous coelom formation, and spiral embryonic cleavage.

Protostomia A group of phyla in which cleavage is determinate, the coelom (in coelomate forms) is formed by proliferation of mesodermal bands (schizocoelic formation), the mesoderm is formed from a particular blastomere (called 4d), and the mouth is derived from or near the blastopore. Includes the Annelida, Arthropoda, Mollusca, and a number of minor phyla. Compares with Deuterostomia.

protostyle A rotating mucoid mass into which food is incorporated in the gut of a gastropod (phylum Mollusca).

protoxylem The first xylem cells formed in the primary xylem.

protozoa A subkingdom (formerly a phylum) comprising the simplest organisms called protista; divided into seven phyla.

protozoan Unicellular eukaryotic heterotrophs.

protozoans Animal-like protists; the various groups of unicellular and eukaryotic organisms that are mostly heterotrophic.

protozoologist A person who studies protozoa.

Protract A muscle action that draws an element anteriorly.

proventriculus In birds the glandular stomach between the crop and gizzard. In insects, a muscular dilation of foregut armed internally with chitinous teeth.

proviral state The condition of a host cell after having been transformed.

proximal Situated toward or near the point of attachment; opposite of distal, distant.

proximate causation (factors)Mechanistic explanations for how behavior occurs, including, in particular, hormones, the nervous system, and behavior development.

proximate cause The factors that underlie the functioning of a biological system at a particular place and time, including those responsible for metabolic, physiological, and behavioral functions at the molecular, cellular, organs, and population levels.

pruning The selective removal of parts of a plant, usually woody shrubs or trees.

pseudocoel A body cavity not lined with peritoneum and not a part of the blood or digestive systems, embryonically derived from the blastocoel.

pseudocoelom A body cavity between the mesoderm and endoderm; a persistent blastocoele that is not lined with peritoneum. Also pseudocoel.

pseudocoelomate Animals having a pseudocoelom, as the aschelminths.

pseudoconditioning The strengthening of a response to a previously neutral stimulus by repeatedly eliciting the response with another stimulus without pairing the presentation of the two stimuli.

pseudocyst Pocket of protozoa within a host cell but not surrounded by a cyst wall of parasite origin.

pseudolabia Bilateral lips around the mouth of many nematodes of the order Spirurata; they are not homologous to the lips of most other nematodes but develop from the inner wall of the buccal cavity.

pseudomyiasis Presence within a host of a fly not normally parasitic.

pseudopodia Temporary cytoplasmic extensions of amoebas that are used in feeding and locomotion.

pseudopodium A temporary cytoplasmic protrusion extended out from a protozoan or ameboid cell, and serving for locomotion or for taking up food.

pseudoreplication Results from either repeated testing of the same animal without proper statistical correction using a repeated measures design, or counting members of an interacting social group as separate data points. In both cases, problems with independence of data points arises with pseudoreplication.

Psilophyta A division of leafless, rootless, vascular, homosporous plants, as exemplified by the genus Psilotum.

Psoas The loin.

psychoactive drug A drug that affects the central nervous system by influencing the release of neurotransmitters or mimicking their actions.

psychoactive plants Plants that contain compounds that

psychobiology The study of the mechanism and function of the central nervous system from both psychological and biological perspectives.

psychological dependence A condition marked by the strong desire to repeat the use of a drug to reexperience the feelings of well-being induced by the drug.

psychopharmacology The study of the brain's behavior in terms of chemical, physiological, and psychological parameters.

Pterobranchia The class of hemichordates whose members lack gill slits and have two or more arms. Colonial, living in externally secreted encasements.

Pterophyta The division of vascular plants commonly referred to as the ferns.

pteropods Pelagic gastropods in which the foot is modified for swimming and the shell is reduced or absent.

Pterygoid pterygion = wing; oid = like.

ptilinum Balloonlike organ in the head of teneral dipterans that pushes off the operculum of the puparium.

puberty The age at which an organism can first reproduce.

pubescence Having hairs or trichomes on the surface.

Pubis The hair that appears at puberty.

public trust A doctrine obligating the government to maintain public lands in a natural state as guardians of the public interest.

Pudendo- pudenda = to be ashamed. External female genitalia.

puff. Strands of DNA spread apart at certain locations on giant chromosomes of some flies where that DNA is being transcribed.

pull factors (in urbanization) Conditions that draw people from the country into the city.

pulmonary circuit The system of blood vessels from the right ventricle of the heart to the lungs, transporting deoxygenated blood and returning oxygenated blood from the lungs to the left atrium of the heart.

pulmonary Relating to or associated with lungs.

pulmonate lung The gas exchange structure in the Pulmonata_the land snails and slugs.

pulping The process of partially digesting and breaking up wood fibers to make paper.

pumice A lightweight white, yellow, or gray stone formed from volcanic glass; used in polishing and cleaning.

punctuated equilibria The hypothesis that evolution occurs in relatively rapid bursts, interspersed with long periods of stasis.

punctuated equilibrium. A model of evolution in which morphological evolutionary change is discontinuous, being associated primarily with discrete, geologically instantaneous events of speciation leading to phylogenetic branching; morphological evolutionary stasis characterizes species between episodes of speciation; contrasts with phyletic gradualism.

punctuated equilibrium model The idea that evolutionary change can occur rapidly over periods of thousands of years and that these periods of rapid change are interrupted by periods of constancy (stasis).

Punnett square A tool used by geneticists to help predict the results of a genetic cross. Different kinds of gametes produced by each parent are placed on each axis of the square. Combining gametes in the interior of the square gives the results of random fertilization.

pupa Inactive quiescent stage of the holometabolous insects. It follows the larval stages and precedes the adult stage.

pupariation Formation of a puparium by the third-stage larvae of certain families of Diptera.

puparium A pupal case formed from the last larval exoskeleton. See pupa.

purine A nitrogen-containing organic compound that contributes to the structure of a DNA or RNA nucleotide; uric acid is also derived from purines.

push factors (in urbanization)Conditions that force people out of the country and into the city.

pycnocline the ocean layer, usually near the bottom of the photic zone, marked by a sharp change in density

Pycnogonida The class of chelicerate arthropods whose members have a reduced abdomen and four to six pairs of walking legs. Without special respiratory or excretory structures. Sea spiders.

pygidium Posterior closure of a metameric animal, bearing the anus.

pygostyle The fused posterior caudal vertebrae of a bird; helps support tail feathers that are important in steering.

pyloric caecum (Each of the slender blind tubes found in the intestine of many bony fishes.

pyogenic Pus producing.

pyramid of biomass A representation showing the total mass of living matter in each trophic level of an ecosystem.

pyramid of energy A representation showing the total energy in each trophic level of an ecosystem.

pyramid of numbers The decrease in the number of individuals that is observed in each higher level of a food chain.

pyrenoid Part of the chloroplast that synthesizes and stores polysaccharides.

pyrimidine An organic base composed of a single ring of carbon and nitrogen atoms; parent substance of several bases found in nucleic acids.

pyrogenic Substance that causes a rise in body temperature; causes fever.

Pyrrhophyta A division of protists commonly termed the dinoflagellates.

pyruvic acid The end product 3-carbon compound resulting from glycolysis of glucose which enters the Krebs cycle in aerobic respiration.