Encyclopedia of Biology

perception The brain’s interpretation of a sensory
stimulus; conscious mental awareness.

perennial Aplant that lives through many seasons,
such as a tree.

pericycle Plant cells just inside the endodermis of the
root that remain meristematic, allowing new lateral
roots to grow (only the pericycle cells nearest the inter-
nal xylem poles perform this function). Located between
the endodermis and phloem. One of four outer layers of
the root (the others being epidermis, cortex, endodermis)
that surround the vascular tissue in the middle of a root.

periderm Secondary tissue produced by the cork

cambium that, when mature, is made up of dead cells

(called cork) that are composed of a waterproof sub-

stance called suberin. Acts as a protective shield against

water loss, extreme temperatures, and as a pathogen

barrier. Usually called bark.

peripheral nervous system Part of the nervous

system that consists of (a) sensory neurons composed

of stimulus receptors that notify the central nervous

system that stimuli have occurred and (b) motor

neurons, called effectors, that travel from the central

nervous system to make muscles and glands react.

The peripheral nervous system is subdivided into the

peripheral nervous system 263

mRNA

translation

ribosome

tRNA

codon

anti-codon

growing polypeptide chain of amino acids amino acid

peptide bond

Two or more amino acids joined by a peptide bond.(Courtesy Darryl Leja, NHGRI, National Institutes of Health)