raceme An inflorescence, a flower structure, in
which stalked flowers are borne in succession along an
elongate axis, with the youngest at the top and oldest
at the base.
racemic Pertaining to a racemate, an equimolar mix-
tureof a pair of ENANTIOMERs. It does not exhibit opti-
cal activity.
radial cleavage Aform of embryonic development
wherecleavage planes are either parallel or perpendicu-
lar to the vertical axis of the embryo. Found in
deuterostomes, which include echinoderms and chor-
dates.
radial symmetry A body shape characterized by
equal parts that radiate outward from the center like a
pie. Found in cnidarians and echinoderms.
Radiata The animal phylum that includes cnidarians
and ctenophores; radially symmetric.
radiation Released energy that travels through
space or substances as particles or electromagnetic
waves and includes visible and ultraviolet light, heat,
Xrays, and cosmic rays. Radiation can be nonionizing
(infrared, visible light, ultraviolet, electromagnetic) or
ionizing (alpha, beta, gamma, and X rays). Ionizing
radiation can have severe effects on human health, but
it is also used in medical diagnostic equipment and
can be used to provide a host of other economic
benefits from electrical power generation to smoke
detectors.radical Amolecular entity possessing one or more
unpaired electrons, formerly often called “free radical.”
Aradical can be charged positively (radical cation) or
negatively (radical anion). Paramagnetic metal ions are
not normally regarded as radicals.radicle An embryonic plant root.radioactive isotope An isotope is an element with a
different amount of neutrons than protons. Isotopes
are unstable and spin off energy and particles. There
are radioactive and nonradioactive isotopes, and some
elements have both, such as carbon. Each radioactive
isotope has its own unique half-life, which is the time it
takes for half of the parent radioactive element to
decay to a daughter product. Some examples of
radioactive elements, their stable daughters, and half-
lives are: potassium 40–argon 40 (1.25 billion years);
rubidium 87–strontium 87 (48.8 billion years); thorium
232–lead 208 (14 billion years); uranium 235–lead 207283