Monographs Details: Tabebuia
Authority: Gentry, Alwyn H. 1992. Bignoniaceae--part II (Tribe Tecomeae). Fl. Neotrop. Monogr. 25: 1-370. (Published by NYBG Press)
Scientific Name:Tabebuia
Synonyms:Leucoxylon Raf., Potamoxylon, Proterpia, Couralia Splitg., Roseodendron, Handroanthus
Description:Genus Description - Shrubs to large trees. Leaves simple, 1-foliolate, or palmately 3-7(-9)-foliolate, the leaflets entire or serrate. Inflorescence a terminal panicle, sometimes reduced to a few-flowered raceme or single flower. Flowers with calyx cupular, campanulate, or tubular, truncate or bilabiate to slightly 5-lobed; corolla white, yellow, lavender, magenta, or red, tubular-infundibuliform to tubular-campanulate, glabrous or puberulous on outside of tube; stamens included or exserted, the anthers glabrous, the thecae straight, divaricate, glabrous; pollen grains single, globose, 3-colporate, the exine finely microreticulate; ovary linear-oblong, usually more or less lepidote, the ovules 2-multi-seriate in each locule; disk annular-pulvinate. Fruit a subterete capsule, elongate-linear to short and oblong, dehiscing perpendicular to the septum, smooth to irregularly verrucose-muricate, glabrous to lepidote or variously pubescent; seeds thin, bialate with hyaline-membranaceous wings or thick corky and essentially wingless.

Discussion:In the West Indies Tabebuia is a taxonomically difficult group, but on the mainland most of the species are fairly clear-cut. Despite the large number of (mostly poorly defined) species on Hispaniola and Cuba, all Antillean species belong to the same, otherwise small, section of the genus and are almost entirely vegetatively differentiated. Part of the taxonomic problem in Antillean Tabebuia stems from the apparent frequency of hybridization on Cuba (Gentry, 1989a). Many of the intermediate forms have been given specific epithets. Tabebuia has been called perhaps the most taxonomically difficult genus in the Cuban flora (Alain, 1957) and a thorough taxonomic treatment of the Cuban species will be possible only after extensive field work. The treatment of Cuban species proposed in this monograph should be regarded as no more than a first approximation. It is to be hoped that the treatment of Bignoniaceae for the new Flora of Cuba project, which I will co-author with Delby Albert of the Academia de Ciencias, will provide a more definitive treatment based on much additional field work. Tabebuia is an important genus horticulturally because of its very showy flowers (see Gentry, 1982b, 1984b) and in the timber trade (Amazonex Lumber Company, pers. comm.; Gentry, 1980). Some species have woods which are among the heaviest and most durable in the Neotropics (see discussion and references in Gentry, 1980). The bark of Tabebuia has recently become rather widely used as a cancer cure (Awang et al., 1991; Bencke, pers. comm.; Farnsworth & Gyllenhaal, 1989) and more recently in the treatment of candida infections (Duke, pers. comm.; Troth Ovrebo, pers. comm.). The pharmacologically active property in Tabebuia bark appears to be the naphthoquinone lapachol and related compounds (Awang et al., 1986; Farnsworth & Gyllenhaal, 1989; Girard et al., 1988). While Tabebuia bark clearly includes chemicals with antitumor properties, these are often wildly exaggerated in the herbal medicine literature. Excerpts from a recent article (Orbis 73 (1989)) capture the essence of this literature: “La-pacho bark contains ... lapachol. It is this substance that is able to enhance the immune system, help combat infections, improve vitality, build up immunity, strengthen cells, and help reduce and eliminate pain and inflammation. (Lapacho) grows where there is a high ozone content in the air where vital negative ions are also concentrated. It is therefore not contaminated by pollutants such as pesticides or exhaust fumes. The unusual thing about the flowers is that they are carnivorous and eat insects keeping the tree free from parasites and viral growths.” While I have not formally proposed sectional subdivisions of Tabebuia, several of the species groups indicated in the key clearly represent natural units as indicated below. Other subdivisions of the key-notably the separation of Group 3 from Group 4 and of Groups 7, 9, and 10-are entirely for practical rather than taxonomic reasons. The yellow-flowered species with mostly stellate pubescence and extremely hard heavy wood clearly constitute a natural unit (Group 3 + 4). Subdividing this group into less pubescent (Group 3) and more pubescent (Group 4) halves is merely an attempt to make the key less unwieldy. These two groups are the two halves of a continuum from less strongly pubescent to more strongly pubescent calyces and leaves and such species as T. billbergii, T. subtilis, and T. lapacho are intermediate. The two yellow-flowered species with lepidote leaves and calyces (Group 2) are probably not close to the other yellow-flowered species, to judge from their very different wood, and may not be close to each other, either. The yellow-flowered species formerly segregated as Roseodendron (Group 1) are characterized by thin calyces, a pyramidal inflorescence, dendroid and gland-tipped trichomes, and prominently costate fruit and are clearly closely related; however their distinctive medium-weight wood suggests that they may not be close to the heavy-wooded species. The four species with purple flowers and corolla tubes puberulous outside (Group 5) constitute a natural group for which the pubescence on the outside of the corolla tube provides a unique synapomorphy. These species have the heavy dark wood of the main yellow-flowered groups to which they may be more closely related than to other pink-flowered taxa. The numerous species with lepidote scales and glabrous white to purple or red corollas constitute a large natural group which is concentrated in the Antilles and has speciated profusely on Cuba and Hispaniola. These taxa are further held together by possessing light to medium weight wood. The subdivision of this assemblage indicated in the key to “groups” is completely artificial, however. Most of the “continental” species with exclusively lepidote indumentum (Group 7) are perhaps more closely related to each other than to the Antillean taxa. Moreover, all white-flowered species are clearly more related to the pink or lavender-flowered taxa than to the yellow-flowered ones. The two putatively hawkmoth-pollinated species (T. striata, T. stenocalyx) are extremely close to each other but otherwise rather isolated within the genus. Tabebuia obtusifolia and T. cassinoides are probably each other’s closest relatives despite the former’s very distinctive, putatively bat-pollinated, flowers. Tabebuia elliptica, T. roseo-alba, and T. reticulata are also a natural unit held together by their unique possession of calyces with pilose inner margins. They are also close to the variable T. insignis as are T. pilosa and T. orinocensis, at least the latter probably directly derived from the T. insignis complex, as may be the two water-dispersed species T. palustris and T. fluviatilis (=Couralia). That leaves only T. rosea among the continental species of Group 6 as being apparently more closely related to the Antillean species, and especially to widespread T. heterophylla, than to the other mainland taxa with exclusively lepidote trichomes. The two Hispaniolan species with unusually large and coriaceous bullate leaflets, simple trichomes (representing the stalks of “decapitated” stalked-peltate scales), and very short thick petioles and petiolules (Group 8) are closely related to each other but also to Groups 9 and 10, and their formal taxonomic recognition would make the remainder of the white- and pink-flowered lepidote taxa paraphyletic. Recognition of the simple-leaved Antillean taxa as Group 7 is entirely artificial, as most of these species seem closer to various members of Group 9 or 10 than to each other. For example, T. delriscoi seems to be a doubtfully stabilized hybrid between members of Group 7 and Group 10. The bulk of the Antillean taxa are extremely closely related and many species are difficult to differentiate, especially on Cuba. Separation of Groups 9 and 10 on the basis of the strength and color of the lepidote indumentum of the leaf undersurface is far from satisfactory and necessitates keying out such species as T. heterophylla in both keys. Perhaps that is unavoidable, since T. heterophylla as here interpreted is so polymorphic that it spans much of the range of morphological variation found in the entire Antillean portion of the genus.
Distribution:Mexico North America| Argentina South America| Dominican Republic South America| Haiti South America| Cuba South America|