Morphology And Terminology

With an increasing amount of information on details of the siliceous diatom cell wall, especially that obtained with electron microscopy, a need for a generally accepted terminology became evident in the early 1970s. The first attempt along this line was published in 1975 as "Proposals for a Standardization of Diatom Terminology" (Anonymous, 1975; von Stosch, 1975) followed by "An Amended Terminology for the Siliceous Components of the Diatom Cell Wall" (Ross et al., 1979). These publications contain glossaries in Latin, English, German, and French. A Russian translation of Anonymous (1975) was published by Makarova (1977).

When the fine structure of pennate diatoms became more extensively studied, new terms were introduced (Mann, 1978, 1981; Cox & Ross, 1981; Williams, 1985, 1986). Terms specific to certain centric diatom families or genera, partly applicable to light microscopy, were also suggested (Hasle et al., 1983; Sundstrom, 1986; Rines 8c Hargraves, 1988).

The text of this chapter follows the current terminology, including, in part, that of Barber & Ha worth (1981). The gross morphology of the diatom frustule and structures more generally distributed within the class are defined in this chapter. Terms specific to particular taxa are defined in the introductory text to these taxa. The definitions of the terms may include elements not readily revealed by light microscopy. This does not exclude the possibility to recognize the presence of a particular structure. For example, the tubular parts of strutted processes may be visible in the light microscope, while the satellite pores are usually not observable.

Gross Morphology (Figs. 3 and 4) Apical axis—long axis of a bilateral diatom—axis between the poles of a frustule.

Pervalvar axis—axis through the center point of the two valves. Transapical axis—third axis of a bilateral diatom. Valvar plane—parallel to the valves—plane of division. Apical plane—perpendicular to the transapical axis.

Transapical plane—perpendicular to the apical axis. (If more specified terms are required, see Round et al., 1990, p. 23, Fig. 18.) Valve view—frustule seen from top or bottom. Broad girdle view—frustule seen from broad side. Narrow girdle view—frustule seen from narrow side. Frustule—the whole diatom box. Epitheca—upper overlapping part of frustule. Hypotheca—lower part of frustule. Valve—epi-, hypo-.

Valve mantle—marginal part of valve, set off from valve face at an angle. Valve face—part of valve surrounded by mantle.

Girdle—part of frustule between epi- and hypovalves consisting of epi- and hypocingula.

Cingulum—portion of the girdle associated with a single valve. Band or segment—a single element of the girdle.

Intercalary band(s)—copula(e)—element(s) nearest to the valves, different in structure from elements farther away from the valves.

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