Science / Biology Glossary

Shoot: The plant stem; provides support for the leaves and fiowers; one of the three major plant organs; also referred to as the shoot system.

Short-Day Plants: Plants that fiower during early spring or fall when nights are relatively long and days are short; e.g., poinsettia and dandelions.

Sickle Cell Anemia: Human autosomal recessive disease that causes production of abnormal red blood cells that collapse (or sickle) and cause circulatory problems.

Sieve Cells: Conducting cells in the phloem of vascular plants. See sieve elements

Sieve Elements: Tubular, thin-walled cells that form a system of tubes extending from the roots to the leaves in the phloem of plants; lose their nuclei and organelles at maturity, but retain a functional p . . . View Full Definition

Sieve Plates: Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of sieve tube cells that are perforated (sieves).

Sieve Tube Members: Phloem cells that form long sieve tubes. See sieve elements.

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.

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 . . . View Full Definition

Sinoatrial (SA) Node: A region of modified muscle cells in the right atrium that sends timed impulses to the hearts other muscle cells, causing them to contract; the hearts pacemaker.

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

Skeletal Muscle: Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle fibers. Voluntary muscle cells that have a striated appearance. These muscles control skel . . . View Full Definition

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.

Skin: One of eleven major body organ systems in animals; the outermost layer protecting multicellular animals from the loss or exchange of internal fiuids and from invasion by foreign microorganis . . . View Full Definition

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.

Sliding Filament Model: Model of muscular contraction in which the actin filaments in the sarcomere slide past the myosin filaments, shortening the sarcomere and therefore the muscle.

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.

Smog: A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often due to pollutants produced by fuel combustion.

Smooth Muscle: Muscle that lacks striations; found around circulatory system vessels and in the walls of such organs as the stomach, intestines, and bladder. Involuntary, not striated cells that control au . . . View Full Definition

Social Behavior: Behavior that takes place in a social context and results from the interaction between and among individuals.

Societies: The most highly organized type of social organization; consist of individuals that show varying degrees of cooperation and communication with one another; often have a rigid division of labor.

Sodium-Potassium Pump: The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a nerve impulse.

Soil: Weathered rocks and minerals combined with air, water and organic matter that can support plants.

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