Glossary terms starting with "a"

 

Acutely rounded

Valve apex that is sharply rounded, at less than 90°.

Alveolus

A stria in the shape of an elongated chamber. The external wall of the alveolus is perforated by many areolae and the internal wall is perforated by one long opening. Plural form is alveoli. An example genus is Pinnularia.

Annulae

One to four transapical striae that interrupt the typical striae at the poles. These structures are restricted to the genus Geissleria. The annulae may be distinctive or barely discernible.

Apical axis

The long axis of the valve face of a pennate diatom, passing through the apices. Follows the midline of the valve and may be curved, as in the genus Cymbella. See Pervalvar axis and Transapical axis.

 

Apiculate

Valve apex that is abruptly tapered to a fine point.

Araphid

Pennate diatoms lacking a raphe on either valve. Examples of araphid genera include Diatoma, Fragilaria, and Synedra.

 

Arcuate

Curved like a bow or bent along the apical axis.

Areola - punctum

Perforation, or pore, in the diatom valve. Usually many are grouped to form a stria. The plural form is areolae.

The shape of the areola can be important in diatom taxonomy, and may be lineolate, punctate, loculate, or in the shape of a letter C.

 

Attenuated - drawn-out

Becoming thin or slender.

 

Auxospore

A special cell that develops and expands before producing a new frustule. Maximum size of a population is restored through auxospore formation. Auxospores are usually associated with sexual reproduction.

Axial costa - longitudinal rib

A narrow ridge of silica along the axial area on the inside of the valve bordering the raphe.

Mastogloia smithii is an example which has two axial costae. Together they form the sides of a groove, or gutter (Paddock and Kemp, 1990), widening slightly at the central nodule and stopping short of the distal raphe ends.

See also costa.

Axial plate

Internal ‘plate’ of silica that occludes the inner openings of the areolae. Found in some species of Gomphoneis where its margin appears as the longitudinal line that is visible in light microscopy.