XX. LYSILOMA BENTHAM
Lysiloma Bentham, London J. Bot. 3: 82. 1944. — Generitypus (Hutchinson, Genera Fl. Pl. 1: 296. 1964): L. schiedeana [sic] Bentham = L. divaricatum (Jacquin) Macbride, the only species that could at the time have provided the generic character of "the pod of a Mimosa with . . . monadelphous stamens." The earlier lectotypification (Britton & Rose, 1928: 76) by L. bahamense Bentham = L. latisiliquum (Linnaeus) Bentham was an arbitrary selection based on page-priority that contradicts the essential differential character of the genus by having indehiscent fruits. The lectotype was, nevertheless, confirmed by Isely (Castanea 35: 249. 1970) on the grounds that when L. bahamense turned out, by taxonomic opinion, to belong to Lysiloma Bentham, it became an appropriate generitype.
Lysiloma sensu Bentham & Hooker, Gen. Pl. 1(2): 595. 1865; Bentham, 1875: 533-536, exclus. spp. 1 (= Pseudosamanea), 5 (= Acacia), 10 (= Acacia); Standley, 1922: 366-391; Britton & Rose, 1928: 76-84; Mohlenbrock, 1963: 436, 441; Isely, 1970: 249-252; 1973: 96-99; R. Thompson, 1980; Nielsen, 1981: 182; McVaugh, 1987: 188-193, fig. 26.
Unarmed, deciduous or semidedicuous trees with one or more trunks and often flaking bark, the sympodial branchlets thickened at the nodes by a prominent buttress under each lf-scar, commonly lenticellate, sometimes canescent in age; indumentum, when present, of white or sordid white (never brown or golden) hairs. Stipules evanescent, absent from all fruiting and most mature flowering specimens, commonly submembranous, varying (sometimes within a species, sometimes between lower and upper nodes of a young branchlet) from linear to obliquely dilated, semicordate, or flabellate. Lf-formula i/1.5 to ±xxx/50, the lfts consequently from exactly 6 to 1000+ per leaf; petiolar nectary near or well above midpetiole, sessile, either obesely cupular or mounded small-pored, and smaller nectaries often between some distal pinna-pairs and toward tip of pinna-rachises; lfts linear to elliptic or obovate, the size reciprocally adjusted to number, the venation simple, or weakly pinnate, or palmate-pinnate. Inflorescence of two sorts: (a) precocious, the usually bracteolate peduncles solitary or fasciculate at earliest efoliate nodes of flush- growth and often also axillary to earliest lvs; and (b) axillary to distal lvs of flush-growth and assembled beyond these into a short efoliate (bracteate) pseudoraceme; units of inflorescence either spicate-racemose (amentiform), or spherical capitate, or hemispherical-umbellulate, the individual fls bracteate in all but L. sabicu; fls homomorphic or almost so, the central fl(s) of some capitula sometimes a little thicker, but no longer, than the rest, its androecium not modified; calyx campanulate short-toothed; corolla ±0.5-1.5 times as long as calyx; androecium whitish 12-32- merous, 7-16(-20) mm, the tube 1.5-7 mm, the anthers 0.15-0.25 mm; gynoecium sessile or almost so, glabrous at anthesis (but the fruit occasionally puberulent). Fruit either a craspedium or an indehiscent legume, long persistent on tree (often into anthesis of second season), stipitate, in profile broad-linear or oblong-elliptic, acute or obtuse, framed by prominent sutures, the papery-crustaceous valves piano-compressed except where crumpled or low-bullate over seeds, straight but sometimes twisted through ±180° either above or below middle, the exocarp usually livid-purple-castaneous but when ripe nigrescent and exfoliating to reveal the stramineous endocarp, less often green ripening pale-brown and not exfoliating, the two endocarpic panels sealed around the seeds and, following craspedial dehiscence, falling together as a unit; dehiscence in most species craspedial, the valves then separating from the sutures and falling together out of the marginal frame, the seeds then released on the ground either through gaping margins of the endocarpic panels, or, when dehiscence none, through decay of the panels themselves; funicles filiform; seeds transverse, compressed-ellipsoid, rather lustrous brown, the pleurogram on middle of each seed-face commonly U-shaped, in L. candidum very small and semicircular, in L. sabicu complete. — Spp. 8, adapted to monsoon and desert climates at low and submontane elevations in Mexico and Greater Antilles, one of these reaching subtropical Florida, one western species extending N into SE Arizona; records from South America are based on misidentified Acacia.Contrary to a tradition that goes back to the first description in 1844 and persists to this day, Lysiloma is not characterized as a genus by craspedial dehiscence of the fruit. Bentham correctly observed and described craspedia of two species; but cautiously remarked that the generic affinity of the rest, and the status of the genus itself, required confirmation from further study. The myth of uniformly craspedial dehiscence in Lysiloma, like that of the arillate seed in Klugiodendron, illustrates the vitality of misconceptions that accumulate verisimilitude by reiteration, and thus resist correction.
This specialized craspedial dehiscence, whereby the pods’ valves are detached as a unit, together with the seeds, from the sutural frame that often persists in the form of threads on the parent branch, is indeed characteristic of all the microphyll lysilomas widespread over continental Mexico and Central America, but in the two primarily West Indian species that are described at length on the following pages the entire pod falls indehisced, and the seeds are released only by decay of the valves on the ground. The carpological character that in reality distinguishes Lysiloma from other New World ingeoid Mimosaceae is the permanent adherence of the two parallel sutural ribs around the whole periphery of the pod, ribs that together form a fused keel that may or may not, depending on the species, separate eventually from the valves. The indehiscent fruits of L. latisiliquum and L. sabicu have been correctly described until now only by Ralph Thompson in his unpublished monograph of the genus. Thompson (1980) proposed a division of the genus into two subgenera based on this property of the fruit, but having accepted L. bahamense (which is L. latisiliquum) as generitype he was obliged to refer the genuine, craspedial species, which ought to constitute the subgen. Lysiloma, as a newly named subgenus. This we are unable to accept. Nomenclature and typification aside, we question the need for subgeneric division of the genus. The two West Indian lysilomas, while alike in almost all features of the fruit, differ from one another in several characters, some of which each shares with some member of the typical continental group, suggesting independent derivation. Thus L. latisiliquum shares a distally efoliate inflorescence only with L. candidum; while L. sabicu resembles L. tergeminum, and no other, in ebracteolate peduncles.
The phylogenetic relationships of Lysiloma can only be guessed at. The genus was thought by Nielsen (1981: 182) to be most nearly allied to Albizia, and perhaps better reduced to a section of that genus. It differs, however, from Albizia as defined herein, not only in the permanently sealed sutures of the pod but also in inflorescence architecture, in which it resembles Chloroleucon, Enterolobium, and Hesperalbizia. Its resemblance to Acacia is in most respects deceptively close, but Lysiloma is nevertheless effectively separable from habitally similar, unarmed Acacia by a syndrome of longer staminal tube (at least twice as long as diameter, generally longer) and the pods’ peculiar sealed frame.
Taxonomic opinion is contradictory about the size of Lysiloma, Hutchinson (1964) claiming 50 species, Nielsen (1981) about 35, and Thompson (following unpublished analysis, 1980) finding only nine major taxa. The common microphyll species are plastic in leaf-formula, pubescence, and width of pod, and have provided types of numerous microspecies based on single collections. Our perception of Lysiloma, which differs taxonomically from that of Thompson only in combining L. divaricatum and L. microphyllum, which are completely confluent in southwestern Mexico, is presented in the conspectus following. Only two species, both primarily West Indian, are here described and mapped.
Five taxa based on West Indian or South American types are excluded from Lysiloma:
1.Lysiloma ambiguum (Vogel) Urban, Ark. Bot. 22A, 8: 28. 1929 "ambigua!’. 1929. = Acacia ambigua Vogel, Linnaea 10: 600. 1836, homon. poster. = Acacia vogeliana Steudel, Nomencl. ed. 2,1: 9. 1840. = Lysiloma vogelianum Stehle, Bull. Mus. Hist. Nat. Paris, ser. 2,18: 193. 1946 "vogeliana".
2. Lysiloma guachapele Bentham, 1875: 533. —Treated herein as Pseudosamanea guachapele Harms.
3. Lysiloma marchianum Grisebach, Fl. Brit. W. I. 223, 1860 "marchiana". — Equated with Calliandra portoricensis (Jacquin) Bentham by Bentham, 1875: 543, which = Zapoteca portoricensis H. Hernández.
4. Lysiloma polyphyllum (Clos) Bentham, 1875: 535 ("polyphylla"), based on Acacia polyphylla Clos in Gay, Fl. Chil. 2: 254. 1847, homon. poster. — . . proviene probablemente del Tucumán. Se cultiva en San Isidro y otros varios puntos de Coquimbo." — Holotypus: C. Gay s.n., P (annotated by Bentham)!; isotypus, Gay 366, K!. — We equate this with Acacia visco Grisebach.
5. Lysiloma rostratum (Willdenow) Bentham, London J. Bot. 3: 84 1844 "rostrata". = Acacia rostrata Humboldt & Bonpland ex Willdenow = Dugandia rostrata (Willdenow) Britton & Killip, Ann. New York Acad. Sci. 35: 138. 1936.