Visit us at

Diatoms of the United States is now known as Diatoms of North America.
This site has been redesigned and updated, and will be closing soon.

Glossary terms starting with "L"



The direction away from the valve face along the mantle.


Acutely rounded

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



The direction toward the valve face along the mantle.


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.


Annulus (centric taxa)

In some centric diatoms, a hyaline ring separating the areolae in the valve center from the rest of the valve face. For example, the valve face of the genus Discostella has an annulus.

Annulus (Geissleria)

In the genus Geissleria, a circular or semi-circular hyaline area on the valve face which interrupts two or more distal striae. An annulus may be distinctive or barely discernible, depending on species. Plural form is annuli.

In Krammer et al. (2000) the definition is in the key for the Navicula group Annulatae. The species in this group have been elevated to the genus Geissleria.



A small projection opposite the ligula that covers gaps in abvalvar (younger) split rings.

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.



Valve apex that is abruptly tapered to a fine point.


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



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.

Aufwuchs - epiphyton , periphyton

A German term for “surface growth” or “overgrowth”. A community of small animals and plants that adhere to surfaces in aquatic environments. Green algae and diatoms make up the dominant component of an aufwuchs. Small crustaceans, rotifers, and protozoans are also commonly found in both fresh water and marine aufwuchs faunas, while insect larvae, oligochaetes and tardigrades are peculiar to freshwater aufwuchs faunas. An English pronunciation might be “OWF-vooks”.



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 with two internal 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.


A structure that is divided into two branches.



Pennate diatoms having a raphe on each valve. Examples of biraphid genera include Amphora, Epithemia, Gomphonema, Navicula, and Mastogloia. See also araphid and monoraphid.

Biseriate - doubly punctate

A stria having two rows of areolae.


A valve end, or apex, having the shape of a head, or the shape of a rounded knob.

Other terms for valve ends include subcapitate (less strongly capitate), rostrate, and subrostrate (less strongly rostrate). Subcapitate and subrostrate are quite similar and many taxa may be described as having subcapitate to subrostrate valve ends.

In general, capitate and subcapitate ends are more expanded than the most narrow point on the valve, while rostrate and subrostrate ends are not more expanded than the most narrow point.


A type of central process unique to the genus Orthoseira. The internal openings are simple, while the external openings are composed of well-defined collars.

Central nodule

In raphid diatoms, the thickly silicified area located between the proximal raphe slits. Often thickened, both in valve and girdle views. It is the site where the silica deposition vesicle deposits the first silica during valve formation.


Central tube

An internal tubular expression of the fultoportula


The series of linking, siliceous bands associated with a valve. Each band is called a girdle band, or copula (from Latin for band or link; plural is copulae). Plural form is cingula.

Each valve has its own cingulum and the valves of most diatoms have a cingulum. The epivalve has the epicingulum and the hypovalve has the hypocingulum. The epicingulum always overlaps the hypocingulum. The two cingula together compose the girdle, or cincture.

Girdle bands may be secreted after the vegetative cell division that produced the valve. In other words, while the deposition of the valve is tied to cell division, the deposition of its girdle bands may not be.


Having the shape of a club. Usually applied to describe the asymmetry of Gomphoneis and Gomphonema in valve view.



A valve surface covered with small bumps or elevated bits of silica.


The narrow, hyaline area on the valve mantle in the genus Aulacoseira. A small furrow, or sulcus, separates the collum from the portion of the valve mantle with areolae. See also ringleiste.


A thin flap of silica on the external valve face, lying along the apical axis. Hyaline or finely porous. Proximal margin formed by the edge of the raphe slit. Extends unsupported over the valve face. Distal margin is free. May lie flat or may be slightly to distinctly elevated. May partially or completely cover the striae. Found in genera such as Fallacia, Lyrella, Mastogloia, Microcostatus, Nitzschia and Sellaphora. Plural form is conopea. From Greek for canopy.

Details of the conopea are best viewed with the scanning electron microscope, but may be visible with the light microscope as longitudinal lines near the raphe or as unornamented areas of the valve face.

Conopea have been found to contain symbiotic nitrogen-fixing cyanobacteria.

See also pseudoconopeum.


Striae that lean toward the central nodule, with the sternum as the origin. Convergent striae may occur throughout the valve, at the valve center, or at the valve apices. In contrast, striae are radiate, or divergent, when they lean away from the central nodule, with the sternum as the origin.


An unornamented, elongated thickening of the valve. Usually refers to a thickening parallel to the striae. Plural form is costae. From Latin for rib.

Morphogenetically, a costa parallel to the striae is a secondarily thickened virga.

See also axial costa.

Costa (primary, secondary, tertiary)

In genera such as Diatoma, Meridion, and Tetracyclus, costae can be qualified by the extent to which they traverse the valve face. Primary costae fully traverse the valve. Secondary costae start at the valve margin and end at the sternum. Tertiary costae start at the margin and end before reaching the sternum.

See also costa.



A ridge of silica on the side of the satellite pore away from the central tube.


An internal valve occurring in species in the genus Craticula. This craticular valve consists of a raphe-sternum and robust transverse bars. Frustules with normal morphology produce the craticular valve under conditions of high solute concentration. The formation of these internal valves is not linked to mitosis.

Cribrum - sieve plate, velum

A thin, domed, porous plate of silica covering the internal opening of an areola, found in genera such as Thalassiosira and Stephanodiscus. A cribrum is a type of velum. Visible only with the SEM. Plural form is cribra.

A similar structure with finer pores, found in many raphid diatoms, is the hymen.


Having the shape of a cross. Usually applied to describe the valve outline of some diatoms.



Having the shape of a wedge. For example, valves of Surirella amphioxys have cuneate apices. The genera Gomphoneis and Gomphonema have species with frustules that are cuneate in girdle view.


A crossing pattern of striae. The striae may cross in three directions with a transapical row and two oblique rows. The genera Decussata and Pleurosigma have decussate striae.

Distal raphe

The portion of a raphe branch that is near the valve end, or valve apex. See also proximal raphe.


The side of the valve that is wider and more convex than the ventral side. Occurs in genera that are asymmetrical to the apical axis, such as Cymbella and Encyonema. From Latin for back of the body.



Having notably more length than width. Having a long and slender shape.



Native, or limited to a specific geographic area.



Algae which grow within the subsurface of sediments, clays, and silt.


An organism that lives inside another, benefitting both individuals. For example, endosymbiotic cyanobacteria often inhabit living cells of the genus Epithemia.



Algae which grow on the surface of rock or stone.



Algae which grow on the surface of sediments, clays, and silt, i.e., residing at the water/sediment interface.

Epiphytic - periphyton

The growth habit of organisms that live on the surfaces of plants or other algae.


The growth habit of living attached to sand grains, or moving through sand.



The larger theca of the frustule. The epitheca overlaps the hypotheca. A theca is composed of a valve and its associated cingulum. From Greek, theca means sheath or case (plural is thecae), so epitheca means outer sheath.


External tube

An extension of a fultoportula on the valve face having the shape of a tube. Typically it is a simple tube, but may be variously shaped.


A hyaline area of thickened silica extending from the central area of some pennate diatoms. A transverse fascia is formed by secondary deposition of silica into depressions on the valve. Examples of genera having a transverse fascia include Luticola and Staurophora. Plural form is fasciae.

See also stauros for a similar structure.


Series, or bundle, of rows of areolae oriented radially in some centric diatoms.

Fibula - keel puncta

Internal strut that provides structural support to the canal that contains the raphe. Plural form is fibulae. The fibulae extend transapically from the raphe canal to the valve face. Examples of genera having fibulae include Denticula, Nitzschia, and Surirella. From Latin for clasp, pin, or brooch.



Having the shape of a thread.


The narrower apex, or pole, of a clavate valve. Often has a pore field. Example genera include Didymosphenia, Gomphoneis, and Gomphonema.



The large opening in one wall of a loculate areola. Located on the external or internal wall of the areola, depending on species, opposite the velum. Plural form is foramina.


The siliceous parts of the diatom cell wall. Composed of the larger epitheca and the smaller hypotheca. The epitheca overlaps the hypotheca similar to a pill box or Petri dish. From Latin for a little piece.

See also valve and cingulum.

Fultoportula - strutted process

A tubular process through the valve of some centric diatoms. The fultoportula consists of a central tubular process surrounded by two or more satellite pores. Externally, the fultoportula appears as either a tube or a simple pore in the valve wall. The plural form is fultoportulae.

Usually associated with the secretion of β-chitin to maintain buoyancy in the plankton.



Having the shape of a spindle. Broadest at the middle and tapering at both apices.

Ghost striae

Faint striae, composed of areolae that may not penetrate the valve wall.



Convex, rounded, protuberant; exceeding a semi-circle but less than a circle.

Girdle view

The view of the frustule when the girdle is uppermost. See valve view for an alternate view of the frustule and cingulum for a definition of the girdle.



A small rounded projection of silica on the outer surface of the valve.


The broader apex, or pole, of a clavate or cuneate valve. Example genera include Didymosphenia, Gomphoneis, Gomphonema, and Peronia.

Helictoglossa - terminal nodule, infundibulum, polar nodule

The internal, distal termination of the raphe in the shape of a pair of lips or a rolled tongue. The helictoglossa occurs in many raphid diatoms. If present, it can be distinguished in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). It may also be visible in girdle view as a thickened lip of silica. Plural form is helictoglossae. From Greek for rolled tongue.


A valve with poles, or apices, of the apical axis having different shapes. Heteropolar valves are asymmetrical to the transapical axis. Example genera include Gomphoneis and Gomphonema.

Heterotopic position

Applies to some concentrically undulate species of Stephanodiscus (e.g., S. aegyptiacus) where fultoportulae on the valve face are in the central area of the convex valve and in the marginal area of the concave valve. From Greek for “different place”.

See contrasting term, isotopic position.


Frustules in which one valve differs from the other morphologically. Frustules may be heterovalvar in their valve ornamentation or their raphe system. The term is often applied to the monoraphid diatoms, in which one valve has a normal raphe and the other is rapheless.



(1) The illustration, collection, or specimen that represents a taxon. (2) A type specimen. (3) A type material recognized by the International Code of Nomenclature for algae, fungi, and plants (ICN).

For diatoms, many authors designate an individual frustule or valve on a slide as the type specimen. They do this by imaging this specimen and circling it with a diamond objective marker, or marking its coordinates with a device such as an England Finder. While the ICN allows the whole slide containing the type specimen to be designated as the holotype, diatomists tend to avoid this practice in order to minimize later misinterpretation of the intent of the author.

The ICN uses a system called typification that makes objective identification possible by linking the name of a taxon with a holotype. Typification is intended to create a stable system of nomenclature by tying the published name to a holotype or a suite of holotypes.

Several other kinds of type material are recognized by the ICN, including isotype, lectotype, neotype and paratype. Types are not expected to be “typical” or “idealized” specimens, although workers may find such use of the terms.

See also synonym.

Hood - cavum

A silica extension with the shape of a hood on the inside of a valve. It occurs only in some species of Planothidium where it partly covers the inside of the asymmetrical central area of the rapheless valve. See also rimmed depression.


Any area of a diatom valve that is unornamented (lacking pores or other structures).


A very delicate, porous membrane of silica covering the opening of an areola or an alveolus. The pores of the hymen may be round or elongated and range from 5-10 nm (0.005-0.010 µm) in their shortest diameter. Found in many raphid genera such as Cocconeis and Neidiopsis. Visible only with the SEM. Plural form is hymenes.

A similar structure with larger pores, found in many centric and some raphid diatoms, is the cribrum.



The smaller theca of the frustule. The hypotheca is overlapped by the epitheca. From Greek for inner sheath.

Interfascicle - costa

An unornamented, thicked rib of silica located between fascicles. Usually applied to centric diatoms.


A valve with poles, or apices, of the apical axis having the same size and shape. Isopolar valves are symmetrical to the transapical axis.

Isotopic position

Applies to some concentrically undulate species of Stephanodiscus (e.g., S. niagarae) where fultoportulae on the valve face are in the central area of both the concave and the convex valves. From Greek for “same place”.

See contrasting term, heterotopic position.



A specimen from the original collection of the holotype and of the same taxon. A type material recognized by the International Code of Nomenclature for algae, fungi and plants (ICN).

Junction line - separation line

In the genus Entomoneis, the line that appears where the keel meets the valve surface. The junction line may be straight or have irregular contours and may be variously ornamented.


A ridge of silica elevated above the valve surface. Formed by a folding of the valve wall. Keels occur in genera such as Nitzschia, Surirella, and Entomoneis.

The keel has the raphe on its outer edge, as in the illustration.


A space or gap in the apical silica of the partectal ring, between the apex and the subapical projections on the sides of the ring. May have a pyriform shape (have the shape of a pear).

Specific to the genus Mastogloia. Visible with SEM; usually not discernable with LM. From Latin for gap. Plural form is lacunae. An example species having a lacuna is Mastogloia calcarea.


A valve having an elongated outline, widest at the middle and tapering to both ends. Width may range from wide to narrow.

Lateral sternum

A thickened area of silica without pores on the valve face parallel to the sternum and interrupting the striae. One on each side of the sternum connected at the central node, giving the whole structure a lyrate shape (shape of a lyre). A freshwater example occurs in Mastogloia pumila.



A silica projection on a split ring girdle band. The ligula of a younger girdle band fills, or nearly fills, the gap caused by the split in the older band next to it. Found in many diatom taxa. From Latin for strap. Plural form is ligulae.


An areola that is elongated in the apical direction. From Latin for little line. Plural form is lineolae.

Lineolae compose the striae of the genus Navicula sensu stricto, or the Lineolate section of Navicula. This type of stria is called a lineolate stria.

Linking spines

In the genus Aulacoseira, linking spines join frustules together in a chain, or filament.

See also separation spines.


The acronym for light microscope. The study of diatoms relies on a compound light microscope in which a beam of light passes through optical lenses to view an image of the specimen. The light microscope provides images of the transparent features of the diatom.

Contributors to the Diatoms of the United States project use research grade light microscopes with the minimum specifications of a 100x, 1.3 numerical aperature (NA) oil immersion objective lens and 1.3 NA condenser lens.

The scanning electron microscope (SEM) is also important in diatom studies.



Having a locule, or chamber. A loculate areola is made of a chamber with a velum and a foramen for two of its walls. A loculate valve has loculate areolae. From Latin for small space.

Example genera with loculate areolae include Diploneis, Muelleria and Neidium.

Longitudinal canal

A chamber having the shape of a tube inside the valve, oriented along the apical axis. It occurs in genera such as Neidium and Muelleria, which have valves consisting of two layers of silica with the canal lying between them. The canals appear as longitudinal lines and may be central or marginal.

Longitudinal rib

A solid silica structure that runs along each side of the raphe in genera such as Frustulia.


A structure having the shape of a crescent moon. Applies to a thickened silica shape in the central area of certain species of Caloneis.


The downturned side of the valve surrounding the valve face. When a frustule is viewed in girdle view, the mantle is visible.


Marginal lamina

A siliceous membrane extending from the margin in many cyclostephanoid genera with raised internal costae


Pennate diatoms having a raphe on only one valve. A monoraphid diatom is a type of heterovalvar diatom. Example genera include Cocconeis, Achnanthidium, and Eucocconeis.

During the ontogeny of monorahid diatoms, a raphe begins to form on each valve. One valve completes its raphe, and becomes the raphe valve. The other valve fills its incipient raphe with silica, leaving a thickened axial rib, and becomes the rapheless valve.

The genus Nupela includes some monoraphid species and some biraphid species. In this web flora, Nupela is included in the biraphid group.


A genus with a single, validly published binomial. An example of a monotypic genus is Diprora Main, which contains only the species D. haenaensis Main.



A polysaccharide built of amino sugars and often associated with proteins. Secreted from a variety of openings in the diatom frustule. Mucopolysaccharides can be components of mucilage, a complex colloidal material used by diatoms for movement, protection, and adhesion.


Valve margin having the shape of a boat. The genus Navicula was described in 1822 by Bory for this shape.


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.