Valve and band morphology of some freshwater diatoms. IV. Outer surface mucilage of Navicula confervacea var. confervacea

This nonstructural cell wall material, present on the surface after critical-point drying and absent after acid cleaning, was of two types: strands and papillae. Strands were associated with the raphe system, areolae, elongated pores of the mantle, and all girdle sutures. Organic papillae were a common feature of valves, valvocopulae and pleurae, but their origin and distribution could not be explained since they often occurred between the obvious openings in the frustule. Strands from the raphe and areolae may function in attaching terminal cells to a substrate and adjacent cells to each other. Secretion between the bands also cannot be ruled out. Although mucilaginous papillae may sometimes occur at random on the entire surface of frustules, there is also a distinct, narrow multiseriate row of them around the edge of valves without marginal spines.