This is a special characteristic in Hong Kong cityscapes. In the early days of Hong Kong, in order to alleviate hazards from landslides, many slopes adjacent to main roads and developments in Hong Kong needed to be secured. Between the 19th century and World War II, the Hong Kong Government and other contractors constructed masonry stone walls to prevent these hazards. In early constructed walls, opened joints between stones allowed strong species like banyans to sprout. Traffic and human activities carry on despite the growth of these plants, which further stabilize the retaining walls they are growing on. As time goes by, sprouts mature into fully developed stone wall trees.