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VALVE AND BAND MORPHOLOGY OF SOME FRESHWATER DIATOMS. IV. OUTER SURFACE MUCILAGE OF NAVICULA CONFERVACEA VAR. CONFERVACEA 1

The development of the mucilage on the outer surface of Navicula confervacea (Kütz.) Grun., a raphed, filamentous diatom, was studied with scanning electron microscopy. 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, valvo‐copulae 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. Other strands of the girdle arise from sutures during cell enlargement and continue to lengthen and intertwine until the individual frustules within a filament are obscured. Strands from sutures might originate from the advalvar row of pores of the girdle bands since these pores lie along the suture, but direct observation of this was not made. 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.