steinkerns Internal casts of a fossil. Steinkerns may reveal internal anatomy of an organism, such as muscle attachment, and other details of soft tissue structure.

stem cells  Cells in bone marrow that produce lymphocytes by mitotic division.

sternum  The breastbone.

steroids  Compounds with a skeleton of four rings of carbon to which various side groups are attached; one of the three main classes of hormones. PICTURE

sticky ends Term applied to DNA sequences cut with restriction enzymes where the cuts will bond with each other or with another sequence cut with the same enzyme.

stigma  Part of the female reproductive structure of the carpel of a þower; the sticky surface at the tip of the style to which pollen grains attach. The receptive surface of the pistil (of the flower) on which pollen is placed by a pollinator. PICTURE 1 | PICTURE 2

stimulus  A physical or chemical change in the environment that leads to a response controlled by the nervous system.

stolons Stems that grow along the surface of the ground; a method of plant vegetaive propagation.

stomach  The muscular organ between the esophagus and small intestine that stores, mixes, and digests food and controls the passage of food into the small intestine. PICTURE

stomata  Pores on the underside of leaves that can be opened or closed to control gas exchange and water loss. Openings in the epidermis (usually of the leaf) that allow gas exchange. PICTURE 1 | PICTURE 2

stomatal apparatus The stomata and guard cells that control the size of the stoma. PICTURE 1 | PICTURE 2

stop codon  The codon on a messenger RNA molecule where protein synthesis stops.

stratification  The division of water in lakes and ponds into layers with different temperatures and oxygen content. Oxygen content declines with depth, while the uppermost layer is warmest in summer and coolest in winter.

stressed community  A community that is disturbed by human activity, such as road building or pollution, and is inadvertently simpliÞed. Some species become superabundant while others disappear.

stroma  The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thylakoids, grana) inside the chloroplast. PICTURE

stromatolite A sedimentological and biological "fossil" represen