CENTRIC DIATOMS Order Biddulphiales
Terminology specific to centric diatoms (Fig. 10)
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FIGURE 10 Valve striation in centric diatoms; (a) fasciculation ("curvatulus" type), striae parallel with long edge stria of the bundle; (b) fasciculation, striae parallel with long central stria;
(c) radial striae, running from the center toward the margin of the valve with shorter striae inserted;
(d) tangential straight striae; (e) tangential curved striae.
Radial striae run from the center of the valve toward the margin (Fig. 10c).
Decussating arcs—spiraling rows of areolae. Fasciculate striae are grouped in bundles (sectors, fascicles) and are parallel to a radial stria, either the edge stria of the bundle (Fig. 10a) or the central stria (Fig. 10b).
Tangential striae are either straight (Fig. lOd) or curved (Fig. lOe). Suborder Coscinodiscineae
Family Thalassiosiraceae Lebour 1930 emend. Hasle 1973
The family, as delineated by Simonsen (1979), comprises marine as well as freshwater planktonic diatoms, all having strutted processes as the main morphological, taxonomic character. The genera dealt with in this chapter were classified into two families by Glezer et al. (1988)—Thalassiosiraceae
Lebour emend. Hasle and Stephanodiscaceae Makarova, order Thalassiosir-ales—and into four families by Round et al. (1990)—Thalassiosiraceae Lebour, Skeletonemataceae Lebour emend. Round et al., Stephanodiscaceae Glezer 8c Makarova and Lauderiaceae (Schtitt) Lemmermann emend. Round et al., order Thalassiosirales Glezer 8c Makarova, subclass Thalassiosirophycidae Round 8c Crawford. Silva 8c Hasle (1994) proposed conservation of Thalassiosiraceae against Lauderiaceae and Planktoniellaceae (Schutt) Lemmermann.
Terminology specific to Thalassiosiraceae: (Figs. 6-8, scanning electron microscopy)
Strutted process—fultoportula—a process through which a thread of organic material (mostly chitan, as far as is known) is extruded, consisting of (1) a narrow tube through the basal siliceous layer ("strutted tube") surrounded by struts and satellite pores and (2) "the external tube" which may be missing except for the basal chamber (Syvertsen 8c Hasle, 1982). Special terms: "operculate" and "trifultate" strutted processes (Fryxell 8c Hasle, 1979a, p. 378). Occluded process—hollow external tube not penetrating the valve wall, sometimes (always?) at the top of an areola (Syvertsen 8c Hasle, 1982).
The genera treated here are characterized by: Cells in chains or embedded in mucilage.
Cells in chains linked by threads of organic matter from strutted processes or by external tubes of marginal strutted or occluded processes.
Valve outline circular.
Valve surface thin radial costae, rows of poroids or loculate areolae, or alveoli.
External parts of processes usually more conspicuous than the internal parts.
One or a few labiate processes. Internal cribra and external foramina (SEM). Chloroplasts small, rounded bodies. Resting spores/cells present.
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