Like an obelisk. This apparently applies only to long thin objects, including trees. Origin is obscure -- clearly Latin, but which word? Per the OED, it's from cacūmen, tree-top -- but they think its first use is 1871, and this blogger finds a use from 1633; this dictionary of entomology traces it instead to cacuminis, limit, which I can't find in Lewis & Short. So who knows. Either way, we now know there's a word for it. Taking that 1633 citation as an example:
Though every day alone he did repair,
And 'mongst the cacuminous thick beeches shade,
In vain, this idle stuff, to hills, and woods bewray'd.