Round et Bukhtiyarova 1996 Category: Monoraphid
TYPE SPECIES: Planothidium lanceolatum (Brébisson) Round et Bukhtiyarova
Frustules of Planothidium are heterovalvar, with slightly concave raphe valve and convex rapheless valve. Valves are elliptic to lanceolate, with rounded, rostrate or capitate apices. The striae are multiseriate. The raphe valve possess radiate striae and a prominent raphe. The terminal raphe fissures are deflected unilaterally.
Many Planothidium species, but not all, possess an asymmetrical central area on the rapheless valve. Internally, this central area bears either a rimmed depression, or a hood. Note that alternate terms for the hood are “cave” and “cavum”. The difference between the rimmed depression and hood was not recognized until the structures were examined under SEM. Previously, the structures were termed ‘hufeisenförmige Fleck” in German, or “hoof-mark” or “horseshoe structure” in English.
Cells have an adnate growth habit, in which they are attached by the raphe valve face to the substrata. The genus was split from Achnanthidium by both Round and Bukhtiyarova (1996) and Lange-Bertalot (1997). Details of sexual reproduction in P. lanceolatum are described in Geitler (1977).
Cite This Page:
Spaulding, S., Edlund, M., and Metzeltin, D. (2008). Planothidium. In Diatoms of the United States. Retrieved December 13, 2013, from http://westerndiatoms.colorado.edu/taxa/genus/Planothidium