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The name Bacillariophyceae has been used in several ways: to refer to all diatoms (e.g. when the diatoms are trated as a class within the Heterokontophyta, as by van den Hoek et al. 1995), or to refer to the raphe-bearing pennate diatoms (e.g. Round et al. 1990), or to refer to all pennate diatoms (Medlin & Kaczmarska 2004). Here, we adopt the third of these: all pennate diatoms.
The pennate diatoms are characterized morphologically by the feather-like organization of their valve pattern, which is evident in the title illustrations. The distinction between this kind of pattern and the centric patterns found in other clades of diatoms was first discussed by Schütt (1896) and Karsten (1928). In pennates, a long rib or strip of plain silica runs along the length of the valve, subtending transverse ribs and lines of pores on either side. In contrast, the clades of centric diatoms possess a radial organization of the pattern. Electron microscopy has revealed that, at the centre of the radial pattern, there is a ring-like annulus (although the annulus can be elliptical or elongate in some species, such as Odontella sinensis and Attheya species: Pickett-Heaps et al. 1990, Stonik et al. 2006). Because of the resemblance of the pennate valve to the human rib-cage, the longitudinal rib or strip is called the sternum.
Most pennate diatoms have elongate cells, shaped like boats, rods, spicules or bananas and it is sometimes stated that this, rather than the organization of the valve pattern, is the essential feature of the group. However, elongate shape is not confined to pennate diatoms, being found in several clades of polar centric diatoms.
Pennate diatoms are monophyletic according to molecular phylogenies (e.g. Medlin & Kaczmarska 2004, Sorhannus 2007): the sternum has apparently evolved only once. Another autapomorphy of pennates is the loss of all flagellate stages: during sexual reproduction, fusion takes place between large amoeboid or motionless gametes, which are usually ± equal in size (morphological isogamy).
The pennates are more species-rich than other diatom groups. Most species are benthic, growing attached to solid substrata or moving through sediments and over surfaces, but the group also contains some common and important planktonic genera, such as the toxin-producing Pseudo-nitzschia, and the remarkable hair-like Thalassiothrix, which can grow up to 5 mm long.