Synthesis
The formation of a more complex substance from simpler ones.
Synthesis phase
In the cell cycle, the phase in which the DNA of the chromosomes is replicated and DNA-
associated proteins, such as histones, are synthesized.
Systematics
The branch of biology that studies the diversity of life; encompasses taxonomy and is
involved in reconstructing phylogenetic history.
Systemic acquired resistance (SAR)
A defensive response in infected plants that helps protect healthy tissue from pathogenic
invasion.
Systole
The stage of the heart cycle in which the heart muscle contracts and the chambers pump
blood.
Systolic pressure
The pressure in an artery during the ventricular contraction phase of the heart cycle.
T
T cell
A type of lymphocyte responsible for cell-mediated immunity that differentiates under the
influence of the thymus.
Tadpole
The limbless aquatic larva of a frog or toad, having gills and a long flat tail. As the tadpole
approaches the adult stage, legs and lungs develop, and the tail gradually disappears. Also
called polliwog.
Taiga
The coniferous or boreal forest biome, characterized by considerable snow, harsh winters,
short summers, and evergreen trees.