The nation provides the spatial framework for one of the most important decision-making bodies in agriculture—government. Agriculture in most Third World countries is the largest sector of production, the biggest contributor to gross national product and in consequence the chief target for taxation to pay for social and other services and to pay for government development projects, whether in agriculture or in other sectors of the economy. The national economy may be regarded as a single, spatially bounded, economic system, of which agriculture is a major part. Agriculture depends on services and more especially on a national trading and marketing system with a considerable regional variation in quality, and containing its own variations in the geographical distribution of its enterprises and systems of production. In the Third World land tenure exists in considerable variety and several of the tenure forms have been considered obstacles to agricultural change and to economic growth.