Lichens form as a result of a symbiotic partnership between two separate organisms, a fungus and an alga or a cyanobacterium. Around 18,000 species of lichen have been described and identified worldwide. In Malta around 200 species have been recorded so far, 12 of which are endemic to the Maltese islands. Lichens come in a wide array of colours and various growth forms, including foliose (leaf-like), fruticose (shrub-like), crustose (forming a thin crust on rock or other substrates) or squamulose (intermediate between foliose and crustose). These organisms grow on soils, rocks or tree bark and occasionally on other substrates including metal. They are able to survive in all climates and altitudes but in general they need three things to grow; undisturbed surfaces, time, and clean air. Their growth rate is fairly slow adding to about 3 millimetres to their radius every year, but a few species grow about 10 times as fast. Their ability to absorb water and minerals from rainwater and directly from the atmosphere makes them ideally sensitive to air pollution. Different lichen species vary in their tolerance to pollution and therefore make very good biological indicators of levels of atmospheric pollution. Lichens are used for many things by both animals and humans worldwide. They provide forage, shelter and building materials for mammals, birds, snails, slugs and insects. In the past lichens were used as a food source in times of hardship by humans and before the advent of modern dyes they were important sources of dyes for clothing. Their unique chemistry has lead to the production the litmus dye which is widely used as an acid/alkaline indicator in chemistry. Furthermore because of their unique and varied biologically active substances, lichens are also used in the development of natural drugs, in the pharmaceutical industry and as food supplements. Two collections of lichens are currently housed within the National Museum of Natural History. One of the collections, donated by Surgeon Rear Adimral Sir Reginald Bankart dates back to the 1930s whilst the other collection was recently donated by Professor Denis De Lucca, son of the late Dr. Carmelo De Lucca, one of the former curators at the National Museum of Natural History.