On-Line Biology Book: GLOSSARY

S

S phase That period of interphase when new DNA is synthesized as part of replication of the chromatin. PICTURE

salivary amylase  An enzyme secreted by the salivary glands that begins the breakdown of complex sugars and starches.

salivary glands  Glands that secrete salvia into the mouth.

saprophytes  Organisms that obtain their nutrients from decaying plants and animals. Saprophytes are important in recycling organic Material.

sapwood  Layers of secondary xylem that are still functional in older woody plants; visible as the outer lighter areas in the cross section of a tree trunk.

sarcomeres  The functional units of skeletal muscle; consist of Þlaments of myosin and actin. PICTURE

saturated fat A fat with single covalent bonds between the carbons of its fatty acids. PICTURE

Schwann cells  Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axons of some neurons, thus forming the myelin sheath.

scientific method Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluation that forms the basis for modern science.

sclereids Plant cells with thick secondary walls that provide the gritty textures in pears. PICTURE

sclerenchyma  One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardened with lignin; provide support for the plant. Sclerenchyma cells include Þbers and sclereids. Plant tissue type consisting of elongated cells with thickened secondary walls for support of the plant. PICTURE 1 | PICTURE 2

scrotum  In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides proper temperature for the testes. PICTURE

secondary cell wall  In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulose and lignin.

secondary compounds  Plant products that are not important in metabolism but serve other purposes, such as attracting animals for pollination or killing parasites.

secondary extinction  The death of one population due to the extinction of another, often a food species.

secondary growth Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth is secondary x