SUMMARYThe development fruits (nutlets) and seeds ofSchoenus nigricans, a perennial sedge of wet coastal dune slacks, was compared between populations of the Mediterranean Sea and the North Sea, to examine the hypothesis of adaptation in a geographical gradient (Baker 1972). The development period of nutlets and seeds lasted several months. The serial insertion of nutlets on the spikelet caused a negative mass gradient from adaxial to abaxial nutlets. Final nutlet mass (first adaxial spikelet position) of Mediterranean plants was 30% higher than that of North Sea plants, extending Baker's hypothesis of the inverse relationship between seed (nutlet) mass and geographical altitude to geographical latitude.The difference in final nutlet mass between plants from populations of the Mediterranean and North Sea persisted, when plants were grown under the climatic conditions of the North Sea plants. However, the nutlet mass, but not the seed mass of the Mediterranean plants, grown under Dutch climatic conditions, was less than at their original size. Start of flowering of the Mediterranean plants under Dutch climatic conditions was delayed by 1–2 months, compared with their flowering at the Mediterranean sites. Nutlets of Mediterranean plants ripened later than those of Dutch plants under the Dutch climatic conditions.The differentiation in nutlet and seed masses and plant height in the geographic gradient in Europe and its persistence under experimental conditions indicates a strong genetic component of population differentiation. The data do not support the hypothesis of ‘general‐purpose genotypes’ (Schmid 1992) in this wet dune slack