Caffeine crystals. Polarised light micrograph of a section through caffeine crystals. Caffeine is a central nervous system (CNS) stimulant and the world's most widely consumed psychoactive drug. It is chemically related to the adenine and guanine bases of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) and is found in the seeds, nuts, or leaves of a number of plants native to Africa, East Asia and South America, where it helps to protect them against insects and to prevent germination of nearby seeds. Besides the seeds of the coffee (Coffea .sp) bush, caffeine also occurs in over 60 other plants, such as tea (Camellia sinensis), guarana (Paullinia cupana), yerba mate (Ilex paraguariensis) and the kola (Cola sp.) nut. Magnification: x100, when printed 15cm wide.