This is Child #1 Riddles Wisely Expounded, also known as Lay The Bent To The Bonny Broom (a soporific) and all the rest. Gentian was (indeed IS) used as an anti-fungal agent and to treat burns and thus probably thought of as "magical".

As a teenager, I found the text in a library and thought I'd made a tremendous discovery and so sang it to death. Only later did I realise its banal ubiquity as Prof Child's #1 hit.