Aicme Ailme is the fourth group, named for ailm, and its five letters are ailm, onn, ur, edad and idad, representing the vowels a, o, u, e and i. In the script these are cut as one to five notches across the stemline. A fifth group, the forfeda or supplementary letters, was added later to write sounds the original twenty could not.
The popular name tree alphabet needs qualifying: the medieval Irish glossaries do give tree names for many of the letters, and some letter names are certainly tree words, but McManus has shown that only about a third are securely trees, that others are ordinary words of quite different meaning, and that the systematic tree scheme is a later scholastic construction. The elaborate tree calendar and tree divination widely circulated today derive from Robert Graves's The White Goddess (1948), a work of poetic imagination that scholars of early Irish have consistently rejected as history.
Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.), The White Goddess: A Historical Grammar of Poetic MythRobert Graves