Glossary terms starting with "A"
Acutely rounded
Valves with apices that are sharply rounded, at less than 90°
Alveoli
Transversely linear chambers in the valve that have small openings externally and large openings internally
Annulae
Annulae are comprised of 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 a diatom is considered the apical axis.
Apiculate
Valve ends that are abruptly tapered to a fine point
Araphid
The araphid diatoms are pennate forms that lack a raphe system, hence lack motility. 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. The plural form is areolae.
Attenuated - drawn-out
Becoming thin or slender
Auxospores
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 plate
The axial plate is a structure within some species of Gomphoneis. The term refers to the internal 'plate' of silica that occludes the areolae internally. The margin of the axial plate results in the longitudinal line, visible in light microscopy.