Glossary terms starting with "R"
Radiate
Striae angled or curved toward the central nodule are called 'radiate' striae. Striae may be radiate in a specific part of the diatom valve. For example, the striae may be radiate throughout the valve, radiate only at the center valve, radiate only at the ends, etc.
Raphe
A slit through the valve face of monoraphid and biraphid diatoms, usually positioned along the apical axis. This is the structure which enables a diatom cell to move over substrates.
Rapheless valve - pseudoraphe valve, PRV, central sternum
In the cell development of monoraphid pennate diatoms, a raphe is initially formed on both valves. The raphe of one valve is secondarily filled with silica, leaving a thickened axial rib. The other valve lacks a raphe and is termed the "rapheless valve".
Rectelevatum
The proximal internal terminations of the raphe in a rectangular, elevated, elongate structure. This structure is found in Muelleria, Scoliopleura and Neidium.
Recurved - reflexed
This term describes a structure that is 'bent back' on itself.
Rimmed depression - hufeisen
Many Planothidium species, but not all, possess an asymmetrical central area, which bears internally either a rimmed depression or a hood. The difference between these two structures was not recognized before SEM studies, and both of them were termed ‘hufeisenförmige Fleck” in German or “hoof-mark” or "horseshoe-shaped area" in English. The rimmed depression is a structure found only in the genus Planothidium. It is a hyaline structure on one side of the central area of the rapheless valve.
Rimoportula - labiate process, jelly pore
The rimoportula is a tubular process found in some centric, araphid, and raphid diatoms, through which polysaccharides and other carbon-containing substances are extruded. Rimoportulae are characterized by a lip-shaped aperture on the internal face of the valve. On the external valve face, the opening of the rimoportula may be a simple, round opening or a tube. Rimoportulae are considered to have appeared relatively early in diatom evolutionary history, but have been lost in a number of more advanced, raphid groups. They are found in a number of centric genera, including Stephanodiscus and Aulacoseira, some araphid taxa, including Diatoma, Hannaea, Meridion and Tabellaria, and some eunotiod taxa, such as Actinella, Amphorotia, Eunophora, and Eunotia.
Ringleiste - sulcus
A structure found in the genus Aulacoseira. The ringleiste is an internal silica ledge that projects into the cell interior from the collum. The ringleiste is variously developed in different Aulacoseira species.
Rostrate
Valve ends that terminate in a 'rostrum", a blunt, tapered end resembling a beak are termed rostrate. Note that the shape terms subrostrate and subcapitate are quite close, and that many taxa can be described as possessing subrostrate to subcapitate ends. In general, capitate ends are more expanded than the most narrow point, while rostrate ends are not more expanded than the most narrow point.