The structure of diatom frustules has been investigated in great detail with light and electron microscopy. Concepts and terminology relative to the structure of valve and girdle features have changed since keys (and their accompanying glossaries) were written for north American taxa. Guides to the structure of diatom frustules have included Anonymous (1975) and Ross et al. (1979), both of which have been extensively cited in the literature. Two excellent treatments (including excellent illustrations) of diatom frustular morphology have recently been published; Round et al. (1990) and the suite of books by Krammer and Lange-Bertalot (1986, 1988, 1991). The glossary below is presented as a guide to terminology present in the keys and descriptions of genera.
Planes of symmetry Apical (longitudinal) Transapical Valvar
Axes of symmetry Apical (longitudinal) Pervalvar Transapical
Apical pore field. Group of porelli at one or both poles in freshwater cymbelloid and gomphonemoid diatoms, through which mucilaginous stalks are secreted.
Areolae. Perforation through valve with internal or external sieve membrane.
Axial area. Unornamented area along the apical axis. In the older literature this area is referred to as the pseudoraphe when applied to the araphid diatoms.
Canal raphe. Raphe type where raphe opens into a channel or canal. Internal openings of canal are called portules.
Central area. Unornamented are in the central or middle part of the valve face.
Central nodule. Area between internal proximal raphe ends, usually thickened. Central nodule expanded to valve mantle is termed a stauros.
Central sternum. Apically-oriented costa which may or may not be perforated by raphe slit. Central sternum not perforated by raphe slit has previously been termed a pseudoraphe.
Costae. Thickened bands or ribs. May be oriented apically or transapically.
Distal raphe ends. External terminus of raphe at poles/ends.
Epicingulum. All girdle elements associated with epivalve.
Epivalve. Larger (and thus older) of two valves of a frustule.
Fascicle. Series or groups of rows of areolae, oriented radially in centric diatoms.
Fibulae. Struts along canal raphe, extending transapically from valve face to valve mantle (in Denticula fibulae may extend entirely across valve face). Densities of these, termed keel puncta in the older literature, are used to help identify taxa.
Hypocingulum. Girdle elements associated with hypovalve.
Hypovalve. Smaller ( and thus newer) of two valves of a frustule.
Isolated puncta. Puncta set off from others in a striae.
Keel puncta. See fibulae.
Labiate process. A tube through the valve (on the face or mantle) with internally thickened sides (giving the appearance of lips, hence the name) that may be flat or elevated.
Locules. Older term used for the chambers/components associated with the girdle bands in Mastogloia. See partecta.
Longitudinal lines. Lines running along apical axis, on either side of axial area.
Ocelli. Group of areolae or porelli that are physically separated from areolae by unornamented rim.
Partecta. Chambers associated with girdle band in Mastogloia.
Porelli. Small, closely packed perforations through valve. Perforations in ocelli, pseudocelli and apical pore fields are often referred to by this term.
Proximal raphe ends. Raphe ends on the central nodule (internally) and near the central portion of the valve (externally)
Pseudocelli. Group of areolae set off from the pattern of the rest of the valve, that decreases in size from areolae on the main part of the valve. Not physically separated from areolae by unornamented band or ring.
Pseudonodule. Differentiated area/structure on the valve whose form is variable. Essentially an area that may resemble an ocellus or pseudocellus, but is structurally different from them. Found only in Actinocyclus in freshwater diatoms.
Pseudosepta. Plate or lamina of silica projecting internally from apical portion of valve mantle.
Puncta. General term for pore/perforation through valve when substructure (i.e. sieve membrane) is unknown or lacking.
Raphe. Slit through valve along apical axis. Composed of (usually) two branches per valve.
Raphe branch. Continuous slit from proximal end to distal end.
Septum. Internally projecting plate on girdle band.
Seta. Elongate, external hollow tube/projection.
Spines. Conical or forked solid external projection.
Stauros. Central nodule (more heavily silicified) expanded to valve mantle.
Stigmata. Perforation through valve face whose external opening is rounded (or nearly so) and whose internal opening is slit-like or highly modified.
Stigmoids. Perforation through valve face whose external opening is similar to puncta of the valve and whose internal opening is slightly modified from the other puncta.
Striae. Rows of puncta/areolae, usually oriented along transapical axis, separated by unornamented ribs.
Terminal nodules. Older term used for internal distal raphe ends (helictoglossae), visible with the light microscope. Used when exact nature of internal distal raphe ends is unknown.
Valve. Siliceous part of the frustule containing most of the morphological features used to describe diatoms (taxonmically, morphologically, etc.). Each valve has two surfaces, the face and the mantle.
Valve face. Portion of the valve apparent in valve view (oriented to the valvar plane.
Valve mantle. Portion of the valve, differentiated by slope, that is apparent in girdle view (oriented to the apical plane).
Wing. Complex type of keel where two sides of valve have fused (or are partially lacking) below raphe.