Monographs Details: Enterolobium
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. & Grimes, James W. 1996. Silk tree, guanacaste, monkey's earring: a generic system for the synandrous Mimosaceae of the Americas. Part I. Abarema, Albizia, and allies. Mem. New York Bot. Gard. 74: 1-292.
Scientific Name:Enterolobium

Enterolobium Martius, Flora 20(2, Beibl. 8 [Herb. Fl. Bras.]): 116. 1837. — Sp. typica: E. timbouva Martius.

Enterolobium sensu Bentham, 1875: 598; Mesquita, Rev. Tax. Gen. Enterolobium 70 (antecedent literature). 1991.

Unarmed, deciduous, either macro-or microphyllidious trees, the young growth more or less densely pubescent with simple, pallid or brownish hairs. Stipules small, mostly caducous. Lf-formula ii-xxx/4- 80; petiolar nectary either cupular-patelliform, or mounded, or sunk in petiolar groove; lft-pulvinules <1 mm; venation of lfts palmate, palmate-pinnate, or (in narrowest blades) reduced to midrib. Inflorescence of either homo- or dimorphic, compact or sometimes umbelliform capitula borne either singly, or in fascicles, or in efoliate pseudoracemes either (a) at efoliate nodes below current year’s foliage and axillary to some early lvs of the annual cycle or (b) wholly in contemporary lf-axils, but not terminally paniculate, the meristem of major branches inhibited beyond the last unit of inflorescence and the whole ramification sympodial; fls 5-8-merous, the calyx of peripheral ones campanulate, turbinate- or cylindro-campanulate; corolla gamopetalous, twice or less than twice as long as calyx; androecium ("8"-)10-70-merous, the tube variable in length, the intrastaminal disc often well developed; ovary either tapering or truncate at apex; stigma minute, poriform; ovules 22-32. Pods sessile, in profile oblong or broad-linear decurved through Vi to 2 circles into a reniform-auriculiform, or annular, or compressed-helicoid figure, the sutures immersed, the outer (seminiferous) one either evenly arcuate or undulately recessed between seeds, the valves composed of (a) thin, fuscous or blackish exocarp, (b) thick, either dry mealy-fibrous or resinous- pulpy mesocarp, and (c) papery pallid endocarp inflexed to form complete, often partly membranous interseminal septa; dehiscence 0, the seeds released by weathering on the ground or excreted by herbivores; seeds transverse, uniseriate or (E. contortisiliquum) irregularly biseriate, borne on crumpled filiform funicle, plumply compressed-ovoid-ellipsoid, the hard testa castaneous or fuscous, some with pallid halo following the line of the complete or incomplete pleurogram, or pleurogram lacking; endosperm 0. — Spp. (at last revision) 10, but (see note following) perhaps a few more, widespread over the lowland and submontane American tropics from S Mexico and Greater Antilles S through the Magdalena valley in Colombia, the Orinoco basin in Venezuela, and Guyana almost throughout Brazil to E Bolivia, Paraguay, N Argentina, and Uruguay, 6 in Amazonian Brazil.

No account of American Ingeae would be complete without Enterolobium, but as this genus has been revised in detail by Mesquita (1990) we provide here no more than an outine of its taxonomy, in order to circumvent confusion between it and compatriot species of other genera. Below will be found a conspectus of the genus that may function as a multiple-entry key to the species, remarks on affinities in the tribe, and random reflections on taxonomic points still in need of clarification. For synonymy, typifications, and other relevant matters the reader is referred to Mesquita’s monograph.

Enterolobium was placed by Bentham (1875: 598) next following Pithecolobium sect. Chloroleucon (genus Chloroleucon of these pages), with the comments that it differed from Pithecolobium only in the broad, thick-walled, indehiscent fruit, and that it might as well be rated as a mere section of it. Mesquita (1990: 43) has suggested that Enterolobium is most closely related to Albizia, and that these genera might both be derived from Pithecellobium. Because he did not, however, define Albizia, his estimate of affinity lacks precision; and we think that Pithecellobium sens. str., an American group characterized by a specialized fruit and seeds, is an improbable ancestor for a genus, however circumscribed, that contains the type-species of Albizia, the Asiatic A. julibrissin.

Enterolobium exhibits more variation in inflorescence structure than any other genus of New World Ingeae, and in pattern of inflorescence is intermediate between Chloroleucon, which differs in striate resting-buds and in narrower dehiscent (but sometimes spirally coiled) fruit, and Albizia ser. Arthrosamanea. The inflorescences of Chloroleucon and ser. Arthrosamanea are borne on anauxotelic (sympodial) axes. In Chloroleucon the axis continues to elongate beyond the furthest axillary inflorescence-unit (though determinate); the inflorescence at anthesis is never the terminal-paniculate inflorescence of ser. Arthrosamanea, but in contrast becomes a terminal panicle of capitula by suppression of the leaves (the capitula are subtended by bracts). Some species of Enterolobium (e.g., E. barnebianum), and individuals of other species, often have precocious inflorescences (i.e., the leaves are suppressed at the oldest nodes) and simple axillary capitula or fascicles of capitula at younger nodes, thus resembling Chloroleucon. Other species (e.g., E. contortisiliquum) or individuals will often have leaves suppressed at the lowest oldest nodes of some proleptic branch systems and also at the terminus of the meristem, thus forming at anthesis a terminal pseudoraceme of capitula. In Enterolobium, Albizia ser. Arthrosamanea and Chloroleucon, vegetative branching is by development of proleptic buds. In Chloroleucon, sylleptic branch-formation has never been observed. It is usual in ser. Arthrosamanea and common in some species of Enterolobium (e.g., E.contortosiliquum, barinense, and cyclocarpum) for sylleptic buds to develop, but they then form annual, axillary strictly reproductive branch systems with ephemeral axes; that is, the axes never contribute to the vegetative architecture of the tree.

The genus remains best characterized by the proportionately broad, thick-walled, often resinous, internally septate indehiscent pod decurved around a very short, annular or C-shaped dorsal suture into a ring or compressed spiral, but no one attribute of the fruit is peculiar to the genus. The fruit of Samanea is, for example, similarly thick-walled, resinous, septate, and indehiscent (but straight), whereas that of Abarema microcalyx var. enterolobioides is externally similar to that of Enterolobium but tardily dehiscent. Niezgoda et al. (1983) included E. contortisiliquum in a study of the ultrastructure of the pollen of Ingeae. They found that the pollen of E. contortisiliquum, unlike that of any other taxon they studied, is characterized by an ektexine-free interstitium. The tectum is imperforate, as is that of all species of Chloroleucon, and in contrast to all species of ser. Arthrosamanea except Albizia carbonaria.

Enterolobium contortisiliquum at anthesis closely resembles Blanchetiodendron blanchetii, which was, in fact, first described in Enterolobium, but the straight papery, dehiscent fruits and thin-skinned discoid seeds of the latter are decisively different. Enterolobium schomburgkii, on the other hand, is more similar to some African species of Cathormion (as delimited by Brenan & Brummit, 1965: 192, sequ.), especially to C. dinklagei (Harms) Hutchinson & Dandy, which scarcely differs except in the more nearly lomentiform fruit. Whether African Cathormion is congeneric with the Asiatic Cathormion Hasskarl (sens. str.), which is armed with indefinite axillary thorns and by spinescent stipules on sucker shoots, is a question that cannot be addressed in this context. It is worth notice, however, that while truly indehiscent the pod of most enterolobiums breaks apart under no great pressure along the line of the interseminal septa; it is, as it were, cryptically lomentoid, thus not fundamentally different from that of Cathormion.

The expansion of Enterolobium to embrace Samanea, proposed by Prain in 1897 (J. Asiatic Soc. Bengal, pt. 2 Nat. Hist. 66. 352), acknowledges an undeniable similarity in resinous-pithy texture of the pods’valves but takes no account of the profound differences in inflorescence-architecture and leaflet-venation; it has found no acceptance in the literature.

Eight of the 10 species of Enterolobium recognized by Mesquita, including the generitype, are uniform in essential structure, except that the flowers of each capitulum are dimorphic in E. barnebianum (? al

ways) and reported to be so in E. barinense, whereas they are homomorphic in the remainder. The three species E. oldemanii, E. schomburgkii, and E. glaziovii differ from the remainder in multiple characteristics emphasized in the conspectus that follows, and are segregated as a sect. Robrichia.