These trees can grow very large, such as Alstonia pneumatophora, recorded with a height of 60 m and a diameter of more than 2 m. Alstonia longifolia is the only species growing in Central America (mainly shrubs, but also trees 20 m high).
The leathery, sessile, simple leaves are elliptical, ovate, linear or lanceolate and wedge-shaped at the base. The leaf blade is dorsiventral, medium-sized to large and disposed oppositely or in a whorl and with entire margin. The leaf venation is pinnate, with numerous veins ending in a marginal vein. Phyllotaxy is whorled i.e. two or more leaves arises at a node and form a whorl .
The inflorescence is terminal or axillary, consisting of thyrsiform cymes or compound umbels. The small, more or less fragrant flowers are white, yellow, pink or green and funnel-shaped, growing on a pedicel and subtended by bracts. They consist of 5 petals and 5 sepals, arranged in four whorls. The fertile flowers are hermaphrodite. The gamosepalous green sepals consist of ovate lobes, and are distributed in one whorl. The annular disk is hypogynous. The five gamesepalous petals have oblong or ovate lobes and are disposed in one whorl. The corolla lobes overlapping to the left (such as A. rostrata) or to the right (such as A. macrophylla) in the bud. The ovary has 2 separate follicles with glabrous or ciliate, oblong seeds that develop into deep blue podlike, schizocarp fruit, between 7–40 cm long. The plants contain a milky latex, rich in poisonous alkaloids. Fijians use the latex of A. costata (saurua, sorua) as a form of chewing gum.[2] The Alstonia macrophylla is commonly known in Sri Lanka as 'Havari nuga' or the 'wig banyan' because of its distinct flower that looks like a woman's long wig.
Many Alstonia species are harvested for timber, called pule or pulai in Indonesia and Malaysia. Trees from the section Alstonia produce lightweight timber, while those from the sections Monuraspermum and Dissuraspermum produce heavy timber.
Alstonia trees are widespread and mostly not endangered. However a few species are very rare, such as A. annamensis, A. beatricis, A. breviloba, A. stenophylla and A. guangxiensis.
Species
Alstonia has five distinct sections, each a monophyletic group; Alstonia, Blaberopus, Tonduzia, Monuraspermum, Dissuraspermum.
Alstonia scholaris (L.) R.Br. – pali-mari, dita bark, bitter bark, milkwood, milky bean, milky pine, white cheesewood, scholar tree, blackboard tree - E + S + SE Asia, Papuasia, N Australia
^Keppel, Gunnar; Ghazanfar, Shahina A. (2011). Trees of Fiji: A Guide to 100 Rainforest Trees (third, revised ed.). Secretariat of the Pacific Community & Deutsche Gesellschaft für Technische Zusammenarbeit. pp. 42–3.
Kade Sidiyasa, A., 3, 1992. A monograph of Alstonia (Apocynaceae).
Forster, P.I. (1992). "A taxonomic revision of Alstonia (Apocynaceae) in Australia". Australian Systematic Botany. 5 (6): 745. doi:10.1071/SB9920745. ISSN1030-1887.
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