Fascinating, and thanks. I'm trying to tamp down thought (time for sleep!!), so I'll need to reread this later.

(The Irish alphabet-like structure ought to be ogham, though if it was tree-based perhaps someone was thinking of the elder futhark (Norse), some of whose letters are (or begin) names of trees. Ogham and futhark lack curves, for the most part, which makes them good for scratching with a point into a twig or carving into stone. I'd be interested to know whether Gilman cited the alphabet in question--)