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Diatoms of the United States is now known as Diatoms of North America.
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Glossary terms starting with "p"


Obovate (inversely ovate, or narrow toward the central area and wider toward the apices), with a concavity in each side, like the body of a violin.

Partectal ring

A valvocopula of the genus Mastogloia. The partectal ring includes the partecta, the partectal ducts, the partectal pores, and the lacuna.

Partectum - chamber, locule

A bulbous chamber on the inside of the valvocopula. Found only in the genus Mastogloia. Usually arranged in a row on each side of the valvocopula, together forming the partectal ring. Plural form is partecta.

The partecta secrete mucilage to the outside of the diatom cell through relatively large pores, the partectal pores, in the wall of the valvocopula. These pores are connected to the partecta by the partectal ducts.


The group of bilaterally symmetric diatoms. This is a heterogeneous group that includes araphid, monoraphid, and biraphid taxa.

In older taxonomic systems, pennate diatoms are contrasted with the centric diatoms. However, these two groups are not natural, evolutionary lineages.

Periphyton - aufwuchs, epiphyton

Algae growing on or around submerged aquatic plants. The term is often used in a more general sense to describe the algae growing on any submerged surface.

Pervalvar axis

The axis of the valve which is perpendicular to the center of the valve face. In centric diatoms, the center is the meeting point of the radii. In pennate diatoms, the center is the meeting point of the apical and transapical axes.


The collection of organisms, including algae, that float or drift in a body of water.

Plastid - chromatophore, chloroplast

In diatoms, the pigmented organelle that is the site of photosynthesis.

Plastids contain carotinoid pigments such as beta-carotene, diatoxanthin, diadinoxanthin, and fucoxanthin. They also contain a combination of chlorophylls a, c1, c2, and c3, depending on the species.

Most diatom plastids appear golden brown because the carotenoid pigments mask the color of the chlorophylls.

Polar bar - transapical thickening

A hyaline thickening of silica at the valve apices in some pennate species. The genus Sellaphora possesses variable development of polar bars.

Porca - ridge surface, corrugation ridge, transapical valve undulation

A transapical undulation, or elevated ridge, of the external valve face. The porca is particular to the genus Surirella. Plural form is porcae. From Latin for an agricultural term meaning the “ridge between two furrows”.

Pore field

An area of fine pores, or perforations, through the diatom valve. It is the site of the extrusion of mucopolysaccarides for making stalks and pads.


A small perforation through the valve. Plural form is porelli. Groups of closely packed porelli occur in ocelli, pseudocelli and apical pore fields.



A pointed, hyaline area at the end of a valve, formed by the joining of the longitudinal ribs that lie along each side of the raphe, surrounding the distal end of the raphe. Found in the genus Frustulia. From French for pencil holder.

Primary side

In raphid diatom ontogeny, the side of the valve formed from the initial branching of the raphe sternum. See secondary side.


The growth habit of lying flat, firmly attached by mucilage to a surface. Example surfaces include rocks, filamentous algae, macrophytes, and even other diatoms.



A valve apex having the appearance of being drawn out or pulled apart.

Proximal raphe

The portion of a raphe branch near the central nodule of a valve. The proximal raphe ends border the central nodule of a valve. See distal raphe.



A hyaline flap of silica forming a cavity on the external valve face, lying along the apical axis. Proximal edge, or edge near the raphe, arises a distance from the raphe. Distal edge joins the valve near the valve margin. Cavity formed by the pseudoconopeum is open at both ends. Striae on the floor of the cavity have fewer areolae than striae outside the pseudoconopeum. Areolae or other ornamentation are present between the pseudoconopeum and the raphe. Found only in some marine species of the genus Mastogloia.

By contrast, the proximal margin of a true conopeum forms the edge of the raphe slit. Compared to lateral sterna, pseudoconopea do not join at the central nodule.


A silica plate extending internally from the apical portion of the valve. To contrast, a pseudoseptum is part of a valve, while a septum is part of a copula, or girdle band. The pseudoseptum occurs in some species of Gomphonema, Gomphoneis, Stauroneis, and Navicula. Plural form is pseudosepta.



A structure embedded in or associated with the plastids, involved with fixing carbon dioxide. Assumes many shapes, from spheres to plates to rods. A diatom cell may have several. Visible with transmission electron microscopy, but usually indistinguishable with light microscopy. From Greek for the shape of a nut.

Pyrenoids are composed primarily of the crucial enzyme, rubisco, and are often traversed by thylakoid membranes continuous with stromal thylakoids in the plastids. Pyrenoids concentrate the often low amounts of carbon dioxide in water for use by rubisco, which incorporates carbon dioxide into the first step of carbohydrate production by the Calvin cycle.