Citrus fruits were introduced to the western world by traders from southern Asia during the period from the 12th to the 18th century. The most important citrus fruits are now produced in large quantities in regions where there is some seasonable periodicity of climate. They are able to withstand exposure to a few degrees below freezing in midwinter and seasonable periodicity of climate. They are able to withstand exposure to a few degrees below freezing in midwinter and they may be intensively cultivated in regions where occasional light frost occurs, as well as in the mildest subtropics.