Sphinctospermum

Taxonomy

Sphinctospermum J.N. Rose Contr. U. S. Natl. Herb. 10: 107. 5 Dec 1906.

Subfamily: Faboideae.
Phylogenetic Number: 3.8.12.
Tribe: Robinieae.
Group: Robinia.
Species Studied - Species in Genus: 1 studied; 1 in genus.

Description

Fruit: A legumelegume:
usually dry, dehiscent fruit derived from a single carpel that opens along two longitudinal sutures
; unilocular; 1.5–3.5 cm long; 0.25–0.35 cm wide; 1–1.5 cm thick; 2–9 times longer than wide, or more than 9 times longer than wide; with deciduous androecial sheath; with deciduous corolla; with persistent calyx, or deciduous calyx; with calyx shorter than fruit; without orifice formed by curving of fruit or fruit segments; straight; not plicate; not twisted; symmetrical; linear; not inflated; flattened; with beak (very short); straight, or declined; with solid beak the same color and texture as fruit; short tapered at apex; aligned with longitudinal axis of fruit; tapered at base; aligned with longitudinal axis of fruit; with the apex and base uniform in texture; chartaceous; seed chambers externally invisible; margin not constricted; margin without sulcus; margin embellished; margin with thickened sutural areas; wing(s) absent; nonstipitate; with all layers dehiscing; splitting along suture(s). Dehiscence of valves along both sutures; apical and down; active; with valves twisting. Replum invisible. Epicarp dull; monochrome, or multicolored; mottled; tan; with brown overlay; mottling color combination constant; with mottling over seed chambers; with surface texture uniform; glabrous; eglandular; without spines; not smooth; with elevated features; not veined; not tuberculate; striate; not exfoliating; without cracks; without embedded tissue, much thicker than epicarp, running from base to apex. Mesocarp present; trace; surface not veined; 1-layered; without balsamic vesicles; without fibers; without reniform canals; solid; chartaceous. Endocarp present; visible; dull; translucent; monochrome, or mottled; tan; with mottling more or less uniform; with brown overlay; smooth; without adhering pieces of testa; septate; with septa thin (tissue paper-like), flexible; with septa eglandular; chartaceous; not exfoliating; remaining fused to mesocarp and epicarp; without wings; entire. Seed(s) 3–12; length parallel with fruit length; neither overlapping nor touching; in 1 series. Funiculus ca. 0.5 mm long; of 1 length only; filiform; straight. Aril present; dry; when dry minute rim-aril; entire; cream.

Seed: 2–3 mm long; ca. 2 mm wide; ca. 1.5 mm thick; not overgrown; angular; symmetrical; rectangular (with central constriction); compressed (diamond-shaped); with surface grooved; transverse; without visible radicle and cotyledon lobes; without hilar sinus; without umbo on seed faces; without medial ridge on each face. Cuticle not exfoliating; not inflated; not wrinkled. Testa present; without pieces of adhering epicarp; not adhering to endocarp; free from endocarp; dull; not modified by a bloom; colored; monochrome; tan to brown; glabrous; not smooth; with elevated features; papillate; chartaceous. Pleurogram absent. Pseudopleurogram absent. Fracture lines absent. Rim absent. Wing(s) absent. Raphe not visible. Hilum present; fully concealed; concealed by aril; punctiform (elliptic); apical according to radicle tip but marginal according to seed length; recessed; not within corona, halo, or rim. Lens not discernible. Endosperm present; thick; not pluglike and not resembling tip of radicle; covering entire embryo; adnate to testa. Cotyledons smooth; both outer faces flat; both the same thickness; both more or less of equal length; not folded; margin entire 180 degrees from base of radicle; similar at apex; not concealing radicle; notched at radicle; without lobes; with the interface division terminating at base of radicle; without margins recessed; white to yellow; inner face flat; glabrous on inner face. Embryonic axis oblique to right angled; oblique to length of seed, or perpendicular to length of seed; without a joint evident between the radicle and the cotyledons. Radicle differentiated from cotyledon; bulbose; lobe tip curved; oblique to cotyledons to with 90 degree turn; centered between cotyledons; 1/2 to nearly length of cotyledons. Plumule rudimentary; glabrous.

Distribution

Southwestern United States and northwestern Mexico.

New World; United States (Arizona), or Mexico.

Generic Notes

Lavin (1987) transferred Sphinctospermum to Millettieae. Lavin & Doyle (1991), integrating morphological and chloroplast DNA data and analyzing it cladistically, concluded that Sphinctospermum should be returned to Robinieae. Lavin and Sousa (1995) monographed the genus. The strong constriction around the middle of the seeds is unique in legumes. The distinctive seeds have been referred to as hourglass shaped (Kearney and Peebles, 1951).

Tribal Notes

Tribe Robinieae

The genera, generic groups, and related data follow the tribal treatment of Lavin and Sousa (1995; Polhill, 1994a, 1994b), not Polhill and Sousa (1981).