The striae are the main structural-functional units in the diatom frustule, however, their morphology is still insufficiently studied and poorly used in Bacillariophyta taxonomy. New criteria for the stria architecture analysis rely on the data from electron microscopy investigations. For the first time hierarchical classification of the striae diversity in the diatom bipolar frustules was proposed. It reflects species ontogeny and the influence of ecology in evolution of different striae. Striae types were determined according to areolae types and originally defined stria parameters. Morphology of areolae determines macroareolar, postmacroareolar, combined and polyareolar striae types which are considered as the consistent stages in evolution of the striae in bipolar frustule. Our concept on the derivation of prevail in Bacillariophyta polyareolar stria from the macroareolar stria through the intermediate stria types is supported by similar morphological stages and their consistency recorded in frustule ontogeny of the species from different systematic groups of Bacillariophyta with bipolar symmetry. Basal and interbasal stria types were defined in accordance with the stria location within a basal siliceous layer. Conical type of areola and polyareolar stria with conical areolae are described. Their distinction from poroid areolae and polyareolar stria with poroid areolae is discussed. Proportional, compact and distant kinds of striae are defined on the ratio between stria and interstria areas and their meaning in species biology and ecology is indicated. All the definitions of the frustule structural units are based on the concept of functional morphology of the diatom frustule suggested by the author.
Keywords: functional morphology, db-element, df-morph, terminology, macroareola, postmacroareola, poroid areola, conical areola; stria: basal, interbasal, macroareolar, postmacroareolar, combined, polyareolar, proportional, distant, compact, transform
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