dorno

The adjective is where this word earns its living. Dorna is thorny, and it went figurative early and everywhere: a dorna vojo is a hard road, a dorna demando is an awkward question, and neither has anything to do with plants.

The course has already sung it. Grabowski's 1913 anthem La Tagiĝo opens its second stanza with

Post longa migrado sur dorna la voj'

after long wandering on the thorny road, and the road in question is the movement's own: three decades of ridicule, poverty and official suspicion between the first booklet in 1887 and the song. The word order there is poetic licence with a purpose. Sur dorna la voj' instead of the prose sur la dorna vojo puts dorna before the article so the rhythm lands and the elided voj' can carry the rhyme, and it is a construction you will meet in nineteenth and early twentieth century Esperanto verse and essentially nowhere else. See poetic Esperanto.

Rozo kaj dorno. The pairing is a proverb in most European languages and Esperanto took it intact: ne ekzistas rozo sen dornoj, there is no rose without thorns. It is worth knowing because the community uses it, usually about Esperanto itself. The plant vocabulary around it is straightforward: dornarbusto is a thorn bush, dornokrono the crown of thorns, and sendorna describes the cultivated roses that have had the problem bred out of them.

Do not confuse dorno with pikilo, which is the sting of a bee or the spine of a hedgehog: an animal's weapon rather than a plant's. Both of them pikas.