tinmg colonies of bacteria altenating with layers of sediments. Becoming more common during the Proterozoic, stromatolites persist today in marine environments where grazing by herbivorous organisms is limited. PICTURE 1 | PICTURE 2

style  Part of the female reproductive structure in the carpel of a þower; formed from the ovary wall. The tip of the style carries the stigma to which pollen grains attach. Part of the pistil that separates the stigma from the ovary. PICTURE 1 | PICTURE 2

subatomic particles  The three kinds of particles that make up atoms: protons, neutrons, and electrons.

suberin Waxy, waterproof chemical in some plant cells, notably cork (in stems) and endodermis cells (in roots).

subspecies  A taxonomic subdivision of a species; a population of a particular region genetically distinguishable from other such populations and capable of interbreeding with them.

substitution A type of mutation in which one base is substituted for another.

substrate feeders  Animals such as earthworms or termites that eat the soil or wood through which they burrow.

sudden infant death syndrome (SIDS)  A disorder resulting in the unexpected death during sleep of infants, usually between the ages of two weeks and one year. The causes are not fully understood, but are believed to involve failure of automatic respiratory control.

superior vena cava Blood from the head returns to the heart through this main vein.

suppressor T cells  T cells that slow down and stop the immune response of B cells and other T cells. Immune system cells that shut off the antibody production when an infection is under control.

suprachiasmic nucleus (SCN)  A region of the hypothalamus that controls internal cycles of endocrine secretion.

symbiosis  An interactive association between two or more species living together; may be parasitic, commensal, or mutualistic. The relationship between two organisms.

sympathetic system  The subdivision of the autonomic nervous system that dominates in stressful or emergency situations and prepares the body for strenuous physical activity, e.g., causing the heart to beat faster.

synapse  The junction between an axon and an adjacent neuron. PICTURE

synapsis  The alignment of chromosomes during meiosis I so that each chromosome is beside its homologue.

synaptic cleft  The space between the end of a neuron and an adjac