portal system  An arrangement in which capillaries drain into a vein that opens into another capillary network.

positive feedback Biochemical control where the accumulation of the product stimulates production of an enzyme responsible for that product's production.

positive feedback control  Occurs when information produced by the feedback increases and accelerates the response.

precambrian Informal term describing 7/8 of geologic time from the beginning of the earth to the beginning of the Cambrian Period of the Paleozoic Era. During this time the atmosphere and oceans formed, life originated (or possibly "colonized" Earth), eukaryotes and simple animals evolved and by the end of the precambrian they began to accumulate hard preservable parts, the common occurrence of which marks the beginning of the Cambrian. PICTURE

precipitation  The part of the hydrologic cycle in which the water vapor in the atmosphere falls to Earth as rain or snow.

predation  One of the biological interactions that can limit population growth; occurs when organisms kill and consume other living organisms.

predatory release  Occurs when a predator species is removed from a prey species such as by great reduction in the predator's population size or by the migration of the prey species to an area without major predators. The removal of the predator releases the prey from one of the factors limiting its population size.

prehensile movement  The ability to seize or grasp.

prenatal testing  Testing to detect the presence of a genetic disorder in an embryo or fetus; commonly done by amniocentesis or chorionic villi sampling.

presymptomatic screening  Testing to detect genetic disorders that only become apparent later in life. The tests are done before the condition actually appears, such as with Huntington disease.

prey switching  The tendency of predators to switch to a more readily available prey when one prey species becomes rare; allows the Þrst prey population to rebound and helps prevent its extinction.

primary cell wall  The cell wall outside the plasma membrane that surrounds plant cells; composed of the polysaccharide cellulose.

primary body Those parts of a plant produced by the shoot and root apical meristems.

primary compounds Chemicals made by plants and needed fo