The Superstar of Lichens - Cladonia
Lichens are a symbiotic association of a fungus and algae or cyanobacteria, typically adhering tightly to rocks or tree trunks upon which they grow. Due to their unassuming appearance and vast variety, most people know very little about them. Yet one lichen - Cladonia - stands out with its complex and varied appearance. In addition, it is familiar to many who have used "litmus paper" in school science experiments to test pH levels. The litmus paper is made from chemical compounds extracted from Cladonia. This unique connection makes Cladonia a true "superstar" in the world of lichens!
Taxonomically, Cladonia belongs to the Family Cladoniaceae and Genus Cladonia in the Kingdom Fungi. At present, there are over 200 recorded species of Cladonia worldwide, with about 10 species found in Hong Kong. In terms of growth form, Cladonia is one of the most structurally complex of fruticose or bushy lichens. It typically grows in soil-filled crevices on rock surfaces, bearing a close resemblance to mosses.
The thallus of Cladonia is primarily green and can be divided into two main parts. The larger part, known as the "primary thallus", often appears as small leaf-like patches growing on soil. From this primary thallus grows another structure called the "podetium", which is cup- or branch-shaped and can reach a height of 4 to 10 cm. In some species, the tip of the podetium bears brown or red "ascocarp". The ascocarp is the lichen's reproductive structure containing spores that function like plant seeds, dispersing to new areas where they can grow into new individuals. The unique shape of the podetium serves as a key feature for identifying different species. For example, "Cladonia cristatella", one species found in North America, has a red ascocarp at the tip of its podetium, giving the lichen a striking resemblance to a British soldier wearing a red hat and green camouflage. This unique appearance has earned it the nickname "British Soldier Lichen".
Countless Uses Through the Ages
Like other lichens, Cladonia absorbs moisture from the atmosphere, enabling it to grow in extreme climates. Particularly in Arctic tundra regions, it is a major food source for reindeer and holds significant economic value for reindeer herders, such as the Saamis of Scandinavia and the Nenets in Russia.
Back in the 17th century, Irish scientist Robert Boyle, also known as the "Father of Chemistry", invented "litmus paper". According to legend, Boyle noticed violets turned red when exposed to acid. Inspired, he tested various plants in acid-base experiments and discovered that many plant and lichen extracts could distinguish acids from alkalis. Among them, Cladonia proved the most effective, turning red in acid and blue in alkali. Boyle then created a solution from Cladonia, soaked paper in the solution, and dried it to produce "litmus paper", solving the long-standing challenge of acid-base detection. Even today, "litmus paper" is still widely used, though modern versions are rarely made directly from Cladonia itself.
Modern scientists have also identified compounds in certain species of Cladonia with promising applications as natural antioxidants, antimicrobials, and even anticancer agents.