The general arrangement of cytoplasmic organelles in Achnanthes subsessilis is similar to that of other pennate diatoms so far investigated. The central nucleus is surrounded by perinuclear Golgi bodies which are believed to produce vesicles containing stalk material. The stalk is composed of four layers, is secreted from the raphe and in this region is surrounded by a mucilaginous collar. At the point of attachment the stalk spreads out over the surface and the outer layer terminates. The stalk is composed of polysaccharide without any associated protein, and the centre is more sulphated than the periphery. The stalk raises the cell well above the boundary layer on an antifouling-treated surface, thus permitting its continued growth and multiplication.