sieve tube members Phloem cells that form long sieve tubes. See sieve elements.

silica Silicon dioxide.

Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land. PICTURE

simple leaf A leaf in which the blade does not form leaflets.

sink  A body or process that acts as a storage device or disposal mechanism; e.g., plants and the oceans act as sinks absorbing atmospheric carbon dioxide. Also, a location in a plant where sugar is being consumed, either in metabolism or by conversion to starch.

sinoatrial (SA) node  A region of modiÞed muscle cells in the right atrium that sends timed impulses to the heart's other muscle cells, causing them to contract; the heart's pacemaker. PICTURE

sister chromatids  Chromatids joined by a common centromere and carrying identical genetic information (unless crossing-over has occurred). PICTURE

sleep movement  In legumes, the movement of the leaves in response to daily rhythms of dark and light. The leaves are horizontal in daylight and folded vertically at night.

skeletal muscle  Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle Þbers. Voluntary muscle cells that have a striated appearance. These muscles control skeletal movements and are normally under conscious control. PICTURE

skeletal system  One of eleven major body organ systems in animals; supports the body, protects internal organs, and, with the muscular system, allows movement and locomotion. PICTURE

skin  One of eleven major body organ systems in animals; the outermost layer protecting multicellular animals from the loss or exchange of internal þuids and from invasion by foreign microorganisms; composed of two layers: the epidermis and dermis. PICTURE

sliding filament model  Model of muscular contraction in which the actin Þlaments in the sarcomere slide past the myosin Þlaments, shortening the sarcomere and therefore the muscle. PICTURE 1 | PICTURE 2 | PICTURE 3 | PICTURE 4

slime molds Protistans that may represent a transition between protistans and fungi.

small intestine  A coiled tube in the abdominal cavity that is the major site of chemical digestion and absorption of nutrients; composed of the duodenum, jejunum, and ileum. PICTURE