Monographs Details: Diomma
Authority: Stern, William L. & Brizicky, George K. 1960. The morphology and relationships of Diomma, gen. inc. sed. Mem. New York Bot. Gard. 10: 38-57.
Family:Simaroubaceae
Scientific Name:Diomma
Discussion:

Diomma iilei Engler ex Harms remained the only species until 1!)52, when Steyermark described a second species, D. fruticosa. Diomma comprises trees of striking appearance for dicotyledons. The unbranched trunk is surmounted by a crown of large pinnately compound leaves and the trees greatly resemble palms in habit. According to Maguire's field observations, at maturity the plants are crowned by a large compound paniculate inflorescence. Apparently the plants die after the fruits mature.

Harms (1931a) describes Diomma ulei as possessing large pinnately compound leaves. Flowers are white, regular, bisexual, 5-merous, and borne in short panicles (sic). The free stamens oppose the sepals, and are broadened at the base of the filament. The broad ovary is laterally compressed, 2- or rarely 3- locular, and borne on a broad, cushion-like, nectariferous base. Locules are nniovulate. Ovules are anatropous, and hang from the upper portion of the septum on a short, thick funiculus so that the raphe is dorsal and the micropyle inside (fig. 1A). This condition has been termed apotropy. Styles are 2, very short and stout, each terminated by a capitate stigmatic surface. Samaras are 2, rarely 3, winged, flattened, broadly ovate, rounded at the base, and attenuated at the tip into a bifid apiculum. Seeds lack endosperm and have a straight embryo. Harms (1931b) added that he did not observe any resin canals, that the bark and leaves were weakly bitter, and that tufted hairs occurred on the edges of sepals and petals. Steyermark's (1952) description of D. fruticosa, based on non-flowering material, is essentially the same as that for the genus as given by Harms (1931a), but he mentioned the occurrence of punctae in the leaves. The geographic distribution of Diom.ma appears to be restricted to the Rorainia sediments of the Guayana region of north-central South America.

Ule (1914), in describing the vegetation of Roraima, mentioned " . eine merkwiirdige neue Gattung der Simarubaceen, Diomma Vlei Engl." Harms (1931a), in validating Engler's description of this species, remarked that the position of Diomma is uncertain. He stated that the apotropous ovules differ from those of genuine Simaroubaceae and from those of the Geraniales as a whole. However, he placed it under Simaroubaceae because Engler had considered it under that family. Nevertheless, later in 1931, Harms delved further into the relationships of Diomma and averred that the apotropous ovules suggested an alliance with Sapindales, but that a close relation in this order would be difficult to find. Diomma was placed in the simaroubaceous tribe Picramnoideae next to Alvaradoa by Cronquist (1945). Steyermark (1952) mentioned that, "The large leaf-scars and long-pinnate leaves of this genus resemble some genera, such as Ailanthus, of the Simarubaceae . but the punctate leaves of D. fruticosa may show a closer affinity or connection with Rutaceae, to which family it is here assigned."