Annual erect herb, unbranched, 3–13 cm. Stem quadrangular, with inconspicuous membranous wings, without nodes (exceptionally 1 node), often with 2 cotyledons at base. Any parts of the plants may be reddish due to the presence of anthocyanin in shadow habitats. Leaves not amplexicaulous, (2-) 3 pairs, opposite-decussate, with generally sub-verticillate aspect, shape variable depending on plant size and position of leaves; proximal pair of leaves sometimes on a lower node on the stem or absent, broadly ovate, narrowly attenuate at base, obtuse (sometimes rhomboid, orbicular or obcordate), (5-)10 (-l5) × (2-)5(-7) mm, lamina herbaceous, with 1–3 nerves diverging from base; medium pair of leaves generally large, subsessile, cordate, (4-)15(-25) × (3-)10(-15) mm, lamina herbaceous, with 3-5(-7) nerves diverging from base; distal pair of leaves generally reduced, very variable in size, shape and color (reddish), ovate to lanceolate, (5-)20(-37) × (3-)10 (-28) mm, lamina herbaceous to membranaceous, with 1(-3) nerves diverging from base. Flowers (1-)5(-14) in umbel-shaped, generally simple condensed cymes inflorescences, if composed 7–15 flowers per umbellule; pedicels (15-)25(-36) mm, erect at maturity, possibly cleistogamous (then reflected below the leaves). Calyx 5-lobed, the lobes shortly fused at base (< 1/5 of the length), 1.5–2.5 mm, linear, not winged, acuminate, accrescent in fruit. Corolla 5-lobed, light blue, white in throat, fused in the lower fourth, 2–3 × 1–1.5 mm, obovate, obtuse, acuminate, when withered with lobes forming a cone at the top of the capsule, accresent. Stamens 2–3 mm long; anthers c. 1 mm, yellow, rectangular, curved and slightly narrowing towards the apex, without papilla near the apex, opening by pores that later widen to slits along up to ½ of the anther length. Styles as long as the stamens. Fruit a sub-spherical capsule, 1.5–2 mm in diam., coriaceous, with accresent sepals appressed to the distal part of withered corolla, septicidally 2-valved with the partial septum.

Etymology. — The species is named in honour of Albert II de Monaco, Albert Alexandre Louis Pierre Grimaldi, Prince of Monaco, in recognition of his tireless support to the NGO Fanamby for their conservation activities in Northern Madagascar, especially in the Andrafiamena region where the species seems to be endemic.

Distribution and ecology. — Exacum alberti-grimaldii is only known from the Ampantsona river watershed occurring on seeping sandstone outcrops or on temporary humid river banks in semi-deciduous or sclerophyllous forest. It is an annual species completing its life-cycle in less than two and half months between March and May.

Conservation status. — Exacum alberti-grimaldii has been observed only three times in the Ampantsona valley and collected only once. With only one collection and three known subpopulations, an AOO of > 9 km² within the Protected Area under creation of Andrafiamena-Andavakoera and according to its annual life-cycle and rarity of proper habitats, E. albertigrimaldii is assigned a preliminary status of Vulnerable (VU D2) following IUCN Categories and Criteria (IUCN, 2001).

Notes. — Only two other species are characterized by verticillated upper leaves and umbella-shape inflorescence: E. dolichantherum Klack. and E. nossibense Klack. (Klackenberg, 1985: 94; Fig. 69, 96, 71). Exacum alberti-grimaldii can be easily distinguished from E. dolichantherum by its smaller discrete petals (2–3 × 1.0–1.5 mm vs 3–7 × 2–4 mm for E. dolichantherum) and from E. nossibensee by the absence of wings on the sepals (Fig. 2–4). The new species grows in a dry transition area on diverse substrates (limestone-sandstone) whereas E. dolichantherum grows in open areas on sand along the littoral of the east coast and E. nossibensee is endemic from the Sambirano Domain sensu Humbert (1955) on diverse substrates.