Glossary terms starting with "S"
Secondary side
The secondary side of a valve is the side that is formed following on the side of joining the raphe sternum, in the ontogeny of raphid diatoms. If terminal raphe fissures are turned towards the same side of the valve (unilaterally deflected), they are usually turned to the secondary side. Voight discontinuities are always found on the secondary side of a valve, although they are not always present.
SEM
Abbreviation for "scanning electron microscope". The SEM is used extensively to view the silica structures of diatoms. A number of species require SEM to view the taxonomically important features. This type of microscopy provides an image of the surface structures, while light microscopes (LM) provide a view that penetrates the silica structure.
Separation spines
In certain species within Aulacoseira, separation spines are formed by some valves, causing the frustules within a chain to break apart. Separation spines are long, in contrast to short, linking spines that join frustules together.
Septum
The inward expanded plate from the a copulum, or girdle band. Septae are present in a number of genera, including Tabellaria. Note that the septum differs from a pseudoseptem by its site of origin: septae are attached to copulae, pseudoseptae are attached to the valve.
Setae
Setae are simple or robust extensions of the diatom valve. They are more elongate than spines and found in the genera Chaetoceros and Bacteriastrum. Setae function to join frustules together to form chains and maintain buoyancy of colonies in a planktonic environment.
Sigmoid
Having the shape of the letter S
Silicification
The process by which a structure is formed from the deposition of silica
Spine
Diatoms have many different types of spines. "Spine" is a general term for a pointed silica extending from the valve. Spines may be solid or hollow, very long or tiny, single or many and they may arise from different points of the valve in different taxa.
Stauros
An unornamented, thickened area in the central portion of a valve in the genus Stauroneis. Some references consider fascia and stauros as equivalent terms, although recent work (Cox 1999, 2001, 2012) has shown that the stauros has a distinct pattern of development that differs from that of other genera. During valve formation in Stauroneis, a transverse strip of silica develops from each side of the central nodule.
Stellate
Starlike; having similar parts radiating from a common center; as, stellate flowers.
Sternum
The sternum is the thickened silica structure that extends, usually, on the apical axis of pennate diatoms. The sternum may be centrally (i.e., Navicula) or marginally (i.e. Eunotia) positioned. The sternum is the ontogenic center of pennate valves, that is, it is the first silica deposited in the initial valve formation following cell division. In many raphid diatoms, the sternum also contains the raphe.
Stigma - isolated punctum
Opening, or perforation, through the valve that is distinct in structure from the areolae. The stigma is positioned in the central area. The external opening is typically round, or close to round, while the internal opening is slit-like, or complex. The plural of stigma is stigmata. A stigma is present in many genera, including Cymbella, Didymosphenia, Geissleria, Gomphoneis, Gomphonema, Luticola and others, although it is not clear that the structure is homologous across these taxa.
Striae
Rows of areolae are arranged to form striae. Striae are usually oriented along the transapical axis. Features of the striae are important in diatom taxonomy and phylogeny, from the stria density (number of striae that occur in 10 µm), to the orientation of the striae (radiate, parallel, or convergent), to the shape of the areolae (lineolate, punctate, C-shaped) that form the striae.