XV. ALBIZIA Durazzini
Albizia Durazzini, Mag. Tosc. 3(4): 10. 1772. — Sp. typica: A. julibrissin Durazzini. — Etymology and orthography: named (Nielsen, Adansonia, n. ser. 19: 206. 1979) in honor of Filippo degli Albizzi, who brought seeds from Constantinople to Florence in 1749. The spelling Albizzia, adopted by Bentham in 1844 (London J. Bot. 3: 84) and much copied, is incorrect.
Pseudalbizzia Britton & Rose, N. Amer. Fl. 23: 48. 1928. — Sp. typica (unica): Ps. berteriana (de Candolle) Britton & Rose = Albizia berteriana Balbis ex de Candolle.
Ours (whether native or naturalized) micro- and (few) macrophyllidious, unarmed, mostly deciduous trees of sympodial growth, the branches of any season terminating either in an efoliate or distally efoliate, simple pseudoraceme of capitula or in complex panicles of pseudoracemes. Stipules mostly small (<5 mm), but in A. chinensis enlarged foliaceous, always caducous, absent from fruiting specimens and perhaps lacking in some species. Lf-formula i-xiv(-xix)/3- 63; 1ft-venation mostly palmate, in A. burkartiana reduced to simple midrib, in 2 Asiatic species palmate- pinnate and in anomalous A. procera pinnate. Fls dimorphic (obscurely so in some capitula of A. niopoides), homomorphic in A. procera only; the peripheral ones either sessile or pedicellate, in native species always small (calyx <3.5 mm, corolla <6.5 mm) but in naturalized ones larger (calyx to 5.5 and corolla ±7-12 mm); androecium 10-40(-46)-merous, <2 cm in native species and A. procera but well over 2 cm in other exotic species; intrastaminal disc known only in the terminal flower of A. lebbeck (Tomlinson, 1980). Pods sessile or shortly stipitate, in profile broad-linear, straight or nearly so, compressed or piano-compressed, variable in texture of valves and in mode of dehiscence: (a) papery and inertly dehiscent through both sutures, or (b) as the preceding but indehiscent, or (c) like the last but the valves segmented after fall of fruit; or (d) leathery, the valves segmented between seeds but retained by wiry sutures (resembling pods of Hydrochorea); or (e) pithy-lignescent, tardily segmented alike through valves and sutures (hence conventionally lomentiform), or (f) stiffly papery, the valves finally differentiated into continuous, inertly dehiscent exocarp and detached segmented endocarp encapsulating each seed in a buoyant envelope; funicle filiform or scarcely dilated; seed attached either basally or laterally above base (3.5-)4-l 1 mm, the exotesta mostly light brown or ivory-white or yellowish, occasionally brown, usually translucent when soaked, areolate, the pleurogram at middle (in A. chinensis below middle) of seed face, either U-shaped or less often complete. — A discontinuously circumtropical genus, most highly diversified in tropical America, Africa (including Madagascar), southeast Asia and Malesia, variously defined and its species consequently of indefinite number, represented in the Americas by 19 native species widely dispersed, many in monsoon climates or in riparian habitats within the lowland and low-montane tropics, one extending N in Mexico to desert foothills of S Sonora, three S into extratropical Brazil, N Argentina, Paraguay, and Amazonian Bolivia, two endemic to the Greater Antilles; four Asiatic species cultivated and two of these widely naturalized, one (A. julibrissin) frost-hardy northward in eastern United States to ±41° N.
The foregoing generic description of Albizia is deliberately tailored to those elements of the genus either native, or naturalized, or commonly cultivated in the Americas and should not be interpreted as definitive of the whole genus. Until a comprehensive worldwide study of Albizia can be undertaken, we adopt for it a provisional diagnostic syndrome of: sympodial growth; lack of armament; palmate or palmate- pinnate (not simply pinnate) venation of leaflets; inflorescence either: (a) composed in part of axillary, annual, sylleptic, strictly reproductive branches, (b) with ephemeral axes that do not persist as vegetative ones; or (c) both; dimorphic flowers, either capitulate or corymbose-umbellulate; and a straight or almost straight pod, never elastically dehiscent but diverse in texture and in release of the seeds. Our definition of Albizia is materially narrower than that proposed by Nielsen (1981: 180), which included among others our genera Chloroleucon (armed) and Pseudosamanea (indeterminate branches, pinnately veined leaflets). Our diagnostic syndrome assembled above purposely excludes the Asiatic A. procera, which is aberrant in Albizia in our sense by a combination of pinnately venulose leaflets and homomorphic flowers. The generic affiliation of A. procera is here left in abeyance, though we think it should be excluded from the genus.
The American native Albizia form a group that is homogeneous in most respects, but diverse in the late developmental states of the fruit. A similar but in detail different radiation has occurred in Madagascar (Capuron, 1970), and it is obviously not possible on a worldwide scale to define either Albizia itself, or any segregate from it, exclusively by characters of the legume. A telling example is offered by the American Aa. subdimidiata, multiflora, and inundata. Bentham knew all three from flowering specimens, which are so deceptively similar that he could not distinguish them even as species. Their pods, however, turned out to be so different at maturity (see description in the conspectus, spp. 16, 17, 19) that they have been transferred by subsequent authors to the genera Samanea, Arthrosamanea, and Cathormion, depending on relative importance attached to texture and dehiscence of the fruit. It now seems pretty certain that a more or less perfectly segmented fruit must have developed independently in several evolutionary lines of Ingeae, just as it has in Acacia (cf. Dugandia rostrata (Humboldt & Bonpland) Britton & Killip; Manganaroa articulata (Ducke) Spegazzini). The lomentiform (or in the case of A. inundata cryptolomentiform) fruits of African and Indian Cathormion and of American Hydrochorea and Albizia sect. Arthrosamanea are here interpreted as parallel but not homologous, and belong to taxonomic groups that can be readily separated and recognized by characters of inflorescence- architecture and leaflet-venation. Dispersal of papery pods is at least potentially anemochorous, whereas that of segmented pods is prevailingly hydrochorous. There is a strong correlation between method of seed-dispersal and microhabitat.
In summary, we recognize in the American flora two sections of Albizia: the native sect. Arthrosamanea (often a misnomer) distinguished by small flowers (corolla <6.5 mm) and short androecium (<2 cm); and a sect. Albizia, consisting of paleotropical species cultivated and naturalized, differing consistently only in relatively large flowers (corolla 7-12 mm) and elongate androecium (well over 2 cm). The first of these sections corresponds more or less neatly with Pithecellobium sect. Samanea ser. Subarticulatae (Americanae) and ser. Parviflorae of Bentham’s Revision of Mimoseae; the second to elements of his Albizzia sect. Eualbizzia ser. Obtusifoliae and Falcifoliae. In the pages that follow we divide sect. Arthrosamaea into four series, but introduce serial nomenclature of the Old World group, for purposes of orientation, only into the Conspectus following.
Cassens and Miller (1981) noted that Albizia sects. Albizia and Parviflorae share, along with Pithecellobium sect. Pithecellobium, Havardia, and Sphinga, the distinction of being the only members of the Ingeae that have septate fibers and nonconfluent parenchyma. They distinguished the wood of Albizia, though, on the basis of ray width, pore diameter, vessel element length, and claimed that the two types do not appear closely related. Baretta-Kuipers (1981) also stated that Albizia has septate fibers in the wood. This is apparently true for the American species; however, Chauhan and Dayal (1985) found several Indian species for which this is not true.