secondary immunity  Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells produced during the Þrst exposure to the antigen, the second response is faster and more massive and lasts longer than the primary immune response.

secondary macronutrients  Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.

secondary (lateral) meristems Plant meristems that produce secondary growth from a cambium.

secondary phloem Phloem produced by the vascular cambium in a woody plant stem or root.

secondary structure  The structure of a protein created by the formation of hydrogen bonds between different amino acids; can be a pleated sheet, alpha helix, or random coil. Shape of a protein caused by attraction between R-groups of amino acids. PICTURE

secondary xylem Xylem produced by the vascular cambium in a woody plant stem or root; wood. PICTURE

second law of thermodynamics (entropy) The energy available after a chemical reaction is less than that at the beginning of a reaction; energy conversions are not 100% efficient.

second messenger  The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the cell's plasma membrane activating a molecule&emdash;the second messenger&emdash;that activates other intracellular molecules that elicit a response. The second messenger can be cyclic AMP, cyclic GMP, inositol triphosphate, diacrylglycerol, or calcium. PICTURE 1 | PICTURE 2 | PICTURE 3

secretin  A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits gastric emptying.

secretion  The release of a substance in response to the presence of food or speciÞc neural or hormonal stimulation.

sediment Loose aggregate of solids derived from preexisting rocks, or solids precipitated from solution by inorganic chemical processes or extracted from solution by organisms.

sedimentary rock  Any rock composed of sediment, i.e., solid particles and dissolved minerals. Examples include rocks that form from sand or mud in riverbeds or on the sea bottom.

seed Structure produced by some plants in which the next generation sporophyte is surrounded by gametophyte nutritive tissues. An immature sporophyte in an arrested state of development, surrounded by a protective seed coat