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Trees. Tufted hairs absent. Leaves spirally arranged. Stipules lateral. Flowers perigynous. Petals persistent in fruit, imbricate (extra-Mal. also contorted). Fruit a capsule. Seed not persistent, with an obvious basal wing or suprahilar arillode.


The leaves of Ixonanthes have paracytic stomata, a simple vascular strand in petiole and midrib with sclerenchyma forming a complete cylinder enclosing parenchymatous ground tissue adaxially, and rhomboidal crystals and druses, as more or less constant characters. Variation has been recorded in presence or absence of anticlinal division walls of the epidermal cells, mucilage cells and extent of the vascular bundle sheaths of the minor veins. This diversity partly coincides with the boundaries between the sections Brewstera and Ixonanthes.

The wood of Ixonanthes is characterized by solitary vessels with simple perforations, fibres with numerous distinctly bordered pits, largely apotracheal axial parenchyma bands of 2-5 cells wide, and narrow l-3(-4)-seriate, weakly heterogeneous rays. The vessel-ray pits are large and simple.

The above characters are in fairly good agreement with the treatment of Ixonanthes, together with Allantospermum, Cyrillopsis, Ochthocosmus, and Phyllocosmus in one family of the Linaceae alliance. Allantospermum would fit here better than in the Irvingiaceae, although its wood stands out in the Ixonanthaceae on account of its minute, half-bordered vessel-ray pits.
— P. BAAS.


This small family of 2 or 3 Old World rain-forest genera was already recognized as a separate suprageneric taxon by PLANCHON (1847) and PLANCHON & KLOTZSCH (1856), who relegated it to the affinity of Ochnaceae, later correctly referred to Linaceae as a subfamily Ixonanthoideae by HUB. WINKLER (1931) and finally recognized as a family of its own by EXELL & MENDONÇA (1951).

As to the number of genera contained in the family, there is no unanimity of opinion. FORMAN (1965: 523) referred 8 genera to the family, but ΝΟΟΤΕΒΟΟΜ (1967) argued that several belong to Simaroubaceae. After careful consideration 3 genera are admitted here in Ixonanthaceae sensu stricto: Cyrillopsis KUHLM. from South America, Ochthocosmus BENTH. (incl. Phyllocosmus KLOTZSCH) from tropical America and Africa, and Ixonanthes JACK from Indo-Malesia. The three genera form a close-knit group. They are all small and the wide distribution in the tropics points to a high age of the group.

We must mention that there is a, doubtful, fourth genus, Allantospermum FORMAN, which the author reckons to the Ixonanthaceae. It occurs with one species in Borneo and another one in Madagascar. ΝΟΟΤΕΒΟΟΜ (1967, 1972) included this genus in Simaroubaceae. The morphology, chemotaxonomy, and palynology corroborate this affinity, but the anatomy of wood and leaf (VAN WELZEN & BAAS, 1984) is just in favour of affinity with Ixonanthaceae.

We refrain from a long discussion of the merits of HALLIER'S attempt (1923) to have Linaceae as a huge complex centre of affinities and confine ourselves to what is usually accepted nowadays in recognizing a few families grouped around Linaceae sensu stricto.

In the treatment of the family Linaceae (page 607, see there) the families Linaceae, Ixonanthaceae, and Ctenolophonaceae are opposed by concise diagnoses.

Leaf anatomy has clarified many points but a fair number remains unsolved, which should be the subject of further multidisciplinary studies (VAN WELZEN & BAAS, l.c.).
— C.G.G.J. VAN STEENIS 1985.