pepsinogen  An inactive form of pepsin; synthesized and stored in cells lining the gastric pits of the stomach.

peptic ulcer  Damage to the epithelial layer of the stomach lining; generally caused by bacterial infection.

peptide bond  A covalent bond that links two amino acids together to form a polypeptide chain. A covalent bond between the amine end of one amino acid and the acid end of another amino acid. PICTURE

peptides  Short chains of amino acids.

perichondrium  A layer of connective tissue that forms around the cartilage during bone formation. Cells in the perichondrium lay down a peripheral layer that develops into compact bone.

perennials Plants that persist in the environment for more than one year (as in the case of annuals).

period The fundamental unit in the hierarchy of time units; a part of geologic time during which a particular sequence of rocks designated as a system was deposited. Units of geological time that are the major subdivisions of Eras.

periosteum  A Þbrous membrane that covers bones and serves as the site of attachment for skeletal muscles; contains nerves, blood vessels, and lymphatic vessels.

peripheral nervous system  The division of the nervous system that connects the central nervous system to other parts of the body. Components of the nervous system that transmit messages to the central nervous system.

peristalsis  Involuntary contractions of the smooth muscles in the walls of the esophagus, stomach, and intestines that propel food along the digestive tract. Waves of muscle contraction in the esophagus that propel food from the oral cavity to the stomach. PICTURE

Permian Period The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth history, when nearly 96% of species died out. PICTURE

peroxisomes  Membrane-bound vesicles in eukaryotic cells that contain oxidative enzymes.

pesticides Chemicals that are applied to agricultural crops or domesticated plants and which kill or inhibit growth of insects.

petals  Usually brightly colored elements of a þower that may produce fragrant oils; nonreproductive structures that attract polli