The ovule of Potamogeton natans is crassinucellate, bitegmic and ortho-campylotropous. Both integuments are dermal in origin and develop by intercalary growth. The seed coat consists of the compressed layers of the integuments, of which only the outer layer shows cell wall thickenings. The campylotropous curvature differs from the usual types and is brought about by a proliferation of the adaxial carpel wall. The terminology, structure and functional aspects of the different ovule types are discussed.