Sympatric. (Of geographical relationship of different species or subspecies) occurring together, i.e with areas of distribution that coincide or overlap.

Sympetalae. Sub-class of Angiospermae in which the petals are unite into a sympetalous (gamopetalous) corolla.

Symplasm. (Bot.) Protoplasmic continuum via plasmodesmata between cells e.g., from root hair cells to those of the stele.

Sympodium. Composite axis produced and increasing in length, by successive development of lateral buds just behind the apex.

Symptom. Any change in normal function or activity associated with a particular diseases.

Synangium. A compound structure formed by lateral union of sporangia, present in certain ferns.

Synapse. A junction between two adjacent neurons (nerve cells), i.e. between the axon ending of one and the dentrites of the next. At a synapse, the membranes of the two cells are in close contact, with only a minute gap between them. A nerve impulse is transmitted across the synapse by the release from the to of the axon of a neurotransmitter substance which diffuses across the synaptic space to the dendrites of the second neurone.

Synchronous Culture. Culture (of micro-organisms or of tissue cells) in which, by the result of suitable treatment, all cells are at any one time in approximately the same stage of development or of the mitotic cycle. Contrasts with untreated culture in which cells are in all stages.
By vastly increasing amount of material available at any desired stage it provides opportunity to study changes, e.g. biochemical that occur in development or in mitotic cycle that would be impossible by using only a few cells.

Syncytium. (Zool.). Mass of cytoplasm, enclosed in a single continuous plasma membrane, containing many nuclei. Cytoplasm may be in the form of a sheet (forming an epitheliums, e.g. trophoblast of many mammalian placentas); of a cylinder, e.g. striped muscle ; of network of almost discrete cells.
Each with a nucleus, but with Cytoplasmic continuity through intercellular bridges, though it is difficult to establish this with certainty. This term is not used in botany; but where protoplasmic continuum exists via plasmaodesmata between cells, the term is Symplasm.