Lichen. It belongs to bryophytes and lives in dark and humid places. China is rich in bryophyte resources, about 3000 species have been reported, but less than 1.8% of them are used as medicinal materials, and they are rarely used as official medicinal materials. The chemical constituents of Rehmannia glutinosa mainly include terpenoids, flavonoids, bibenzyl compounds and coumarins, and the main flavonoids are apigenin, luteolin and their glycosides. Lichen is a stable and mutually beneficial association between fungi and photosynthetic organisms, and fungi are the main members. Another definition regards lichen as a specialized fungus, surrounded by mycelium, which grows with lower photosynthetic organisms with water as reducing agent, forming a variety of special primitive organisms to varying degrees. Traditionally, lichen is regarded as a special lower plant produced by fungi and algae.