The major industrial centres of the world lie in cool temperate areas. On the other hand a number of valuable crops can only be grown in the tropics – such as cacao, bananas, coconuts, jute, sisal, hemp, rubber, coffee, oil-palm – or the subtropics – such as sugar-cane, cotton, tea, groundnuts and tobacco. All these crops had been cultivated by the inhabitants of the tropics long before the arrival of Europeans, and indeed spices had reached Europe from the Moluccas and Malabar at least as early as Roman times. However the expansion of European settlement and trade led to the production of these crops specifically for export to Europe. In the Americas, where, away from the Indian cultures in Mexico and Peru, there were very low population densities, the early settlers established the plantation system, based upon an abundance of land and imported slave labour, to produce high-value goods such as sugar and tobacco for sale in western Europe. Africa, however, was bypassed; its lack of good harbours, the prevalence of disease, and later the overwhelming importance of the slave trade, meant that there were few attempts to produce export crops until the late eighteenth century. In Asia the Portuguese and Dutch, the most active of the early explorers, were more interested in trade than settlement. They found dense populations and more advanced civilisations than there were in the Americas or Africa. Thus at first they were content to buy agricultural products, such as spices in the East Indies or tea in China, from the indigenous producers. It was not until the nineteenth century that Europeans took an active part in the production of these crops.