Endemic

Name honors William DeWitt Alexander (1833-1913), professor of Greek and president of Punahou School, member of the privy council under King Kalakaua and Queen Lili'uokalani, surveyor-general of the Territory of Hawai 'i, and distinguished author and historian.

Plants terrestrial. Fronds considerably more than 20 cm long at maturity, often longer than in f. erecta. Blades 1-pinnate-pinnatifid to occasionally 2-pinnate-pinnatifid. Pinnae 20-60 pairs, quite variable in cutting from deeply lobed to dissected, lobes or segments wedge-shaped, bases narrow or broad, margins entire or dentate. Sori at tips of expanded lobes. Indusia opening outward.

Found in small isolated populations in mesic forests, 900-1,700 m, known historically from Kaua'i, Moloka'i, and Maui, now known from only a few scattered populations.

This dissected form blends with the 1-pinnate D. erecta f. erecta over its range, and in one instance fronds typical of both f. erecta and f. alexandri have been found on the same plant. At its most extreme its cutting approaches that of D. mannii.

Diellia erecta f. alexandri may be distinguished from f. erecta by its deeply lobed, wedge-shaped ultimate segments that usually have narrow bases.