Sleep. A readily reversible state of reduced awareness and metabolic activity that occurs periodically in many animals. Usually accompanied by physical relaxation, the onset of sleep in humans and other mammals is marked by a change in the electrical activity of the brain, which is recorded by an electroencephalogram as waves of low frequency and high amplitude (slow-wave sleep). This is interspersed by short bouts of high-frequency low-amplitude waves (similar to wave patterns produced when awake) associated with restlessness, dreaming, and rapid eye movement (REM); this is called REM (or paradoxical) sleep. Several regions of the brain are involved in sleep, especially the reticular formation of the brainstem.

Slide.
(1) Oblong of glass, usually 3 in. by 1 in. and 1-2 mm thick, on which whole mounts, sections, etc., are placed for microscopical inspection.}
(2) A complete preparation made on such a piece of glass.

Solute. A substance that is dissolved in a solvent.

Somatic cells (Soma). The cells of an organism, other than the germ-cells; (see Germ-plasm) Somatic tissues. Tissues other than those of viscera or blood vessels; or tissues which surrounded body cavity. S. motor nerves supply striped muscle; s.sensory nerves supply receptors of somatic tissues.

Somite. A serial segment of the animal body.

Sparging. The introduction of air into a microbial fermenter under pressure.

Spathe. Bract enclosing inflorescence of some monocotyledons, e.g. cuckoo pint.

Speciation. Origin of species.

Species. The smallest unit of classification commonly used; i.e. the group whose members have the greatest mutual resemblance. The common names of familiar animals and plants often denote species e.g. man, fox, beech.