Common traits include the presence (in addition to chlorophyll of the a-type) of specific carotenoid accessory pigments (carotenes and xanthophylls), which may or may not mask the green color of chlorophyll a, and the elaboration of oil or a peculiar carbohydrate, chrysose (leucosin), as a product of assimilation (in which nutrients are either transformed or incorporated into protoplasm). The pigments are localized in special structures called chromatophores. Impregnation of the cell wall with silica is widespread in the group, and movement by means of flagella (whiplike processes) is common in many of the unicellular forms and in the sexual reproductive cells. The divisions are distinguished by the following characteristics:
Chrysophyta - chromatophores are golden yellow to brown; unicellular to filamentous
Xanthophyta - chromatophores are green or greenish; unicellular to filamentous
Bacillariophyta - chromatophores are brown; unicellular with an outer sheath, or frustule, of silica.
Chrysophyta species typically move by means of two flagella of unequal length. Many formerly were regarded as protozoa. However, some types are nonmotile, and others show creeping amoeboid movement by means of protoplasmic extrusions. Most of the forms are freshwater.
Certain forms of Xanthophyta use two unequal flagella for motility; others use creeping amoeboid movements. Some (such as those of the genus Vaucheria) form a tubular, unpartitioned, thallus (body), producing multinucleate and multiflagellate zoospores. Xanthophyta contains mainly freshwater and terrestrial species.
Bacillariophyta is peculiar in the form of the silica sheath, or frustule, that it possesses. This consists of two distinct, overlapping halves and is ornamented with symmetrically arranged pores that have a complex ultramicroscopic structure. The symmetry may be radial (around a central axis), as in the order Centrales, or bilateral (right and left sides as mirror images), as in the order Pennales. Free-floating diatoms are an important type of marine and freshwater plankton. Some species form colonies by aggregation of the individual cells.
