A number of purine bases containing methyl substituents occur in plants and plant products and many of them have pharmacological properties. For example Theophylline is chemically 1,3-dimethylxanthine. It is found in tea. Theobromine occurs in cocoa. It is chemically 3,7-dimethylxanthine. Caffeine is present in coffee. It is chemically 1,3,7-trimethylxanthine.