526 Glossary Plantigrade Walking on the soles of the feet An organism that walks in this manner; e.g., humans, bears, mice Compare DIGITIGRADE, UNGULIGRADE Planula (plural planulae) The ciliated planktonic larval stage of a coral Plasmid A piece of DNA (usually circular) which can replicate independently of the chromosome of a cell and which codes for supplementary (noncore) functions Plasticity The ability of a plant to respond to temporal changes or spatial variation in environmental conditions by altering the size or the distribution of its parts These are phenotypic rather than genetic changes Plastron In aquatic insects, a set of body structures that trap a thin layer of air against the body; oxygen from the surrounding water is added by means of diffusion to the same extent that oxygen is withdrawn by breathing The flat lower portion of the shell of a turtle Plate tectonics A modern geological theory of tectonic activity, according to which the earth’s crust is divided into a small number of large, rigid plates whose independent movements relative to one another cause deformation, volcanism, and seismic activity along their margins The study of the movement of these plates and of how this process has been involved in the evolution of Earth’s crust Pleiotropic Describing a gene that has more than one independent phenotypic effect Pleiotropy The fact of one gene affecting more than one trait Pleistocene The geologic period from million to 10,000 years before the present, which was characterized by a series of glacial advancements (ice ages) separated by warmer interglacial periods Pleopod An abdominal appendage in crustaceans Plesiomorphic Describing an ancestral or primitive character, relative to sister group Plesiomorphy An apomorphic character or character state that specifies a more inclusive group than that under consideration Pliocene An epoch within the Cenozoic, the most recent prior to the Quaternary, from about 5.3 to 2.6 million years ago The Pliocene is characterized by largely modern flora and fauna, and is the most recent epoch to have experienced extended periods globally warmer than the present Plow horizon The homogenous soil layer of an agricultural field, typically at 0–20 cm depth, that results from the field being plowed; designated as the ‘‘Ap’’ horizon in soil morphology See also SOIL HORIZON Plume prospecting The process of detecting chemical and physical anomalies in hydrothermal plumes to locate hydrothermal vents Plurivoltine Describing species with several generations of offspring per year Also, MULTIVOLTINE Pneumatophores Roots growing vertically from underground roots that spread laterally from the main stem, occurring in some genera of mangrove, such as Avicennia and Sonneratia They typically stand 10–20 cm above the soil surface, enabling gas exchange to take place with the underground roots Podzol A soil type highly characteristic of boreal forests and developed as a consequence of podzolization (see next) Podzolization The process of acid leaching in soil, in which clay, organic particles, and mineral ions (primarily iron and aluminum) are carried downwards and deposited in the B soil horizon, leaving an impoverished and leached A horizon above This occurs as a consequence of low temperatures and precipitation in excess of the needs of evapotranspiration See also SOIL HORIZON Poikilohydry A condition in certain organisms, such as some aquatic algae, in which their internal water balance varies with changes in ambient humidity; i.e., they are not able to compensate for fluctuating humidity Contrasted with HOMOIOHYDRY Point source A specific, readily identifiable input of pollution, such as when treated wastes are discharged from a municipal, industrial, or agricultural facility to receiving waters through a pipe or a drain Contrasted with NONPOINT SOURCE Poisson distribution A discrete probability distribution used to model the number of events of a given type occurring in a specific time and/or space In this context, the probability of distribution of numbers of organisms per area if the organisms are distributed completely at random in the space; e.g., the number of prairie dogs in a square mile of prairie [Introduced by the French mathematician Sime´on Poisson.] Polarity The relative occurrence of derived versus ancestral character states Pollard A tree repeatedly cut at about 6–10 ft above ground, to produce successive crops of wood Pollution The introduction into air, water, or soil of contaminant substances in quantities that are threatening to living resources, biological processes, and/or human health and activities Polyandrous Describing a female that has more than one male mate Polyandry A mating system in which a female mates with more than one male Polyculture The agricultural practice of growing two or more crops either simultaneously or in sequence on the same piece of land The presence of a diversity of species, or genotypes, of crop plants or other organisms Polygamous Reproducing with several different mates Polygamy A reproductive pattern in which males and females both have several different mates Polygynous Describing a male that has more than one female mate Polygyny A mating system in which a male mates with more than one female Polymerase chain reaction (PCR) An enzymatic technique for selectively copying (amplifying) a specific segment of DNA across several orders of magnitude A common variation of this is quantitative PCR (qPCR), wherein the progression of the amplification reaction is recorded optically in real time, providing a means for rapidly estimating the abundance of a particular sequence of DNA in a given sample Polymorphic Having to with or showing a genetic difference between two or more individuals of the same species Polymorphism The existence at intermediate frequencies of two or more variants at a given gene locus within a population Polyp The basic unit of the coral animal, consisting of a simple tubular structure with a mouth and ring of tentacles at one end The polyp sits in a skeletal cup called the corallite Polypeptide A single chain of amino acids