Senna pentagonia

  • Title

    Senna pentagonia

  • Author(s)

    Howard S. Irwin, Rupert C. Barneby

  • Scientific Name

    Senna pentagonia (Mill.) H.S.Irwin & Barneby

  • Description

    70.  Senna pentagonia (P. Miller) Irwin & Barneby, comb. nov. Cassia pentagonia P. Miller, Gard. Dict. ed. 8, Cassia no. 18. 1768.—Typus infra sub var. pentagonia indicatur.

    Coarse erect malodorous monocarpic herbs with habit of S. obtusifolia, at anthesis 2-24 dm, subcorymbosely few-branched distally, appearing glabrous but the lfts always minutely ciliolate and the young stems and lf-stalks often thinly minutely puberulent, the green membranous lfts glabrous on both faces, paler or subglaucescent beneath, the inflorescence of axillary subsessile 1-2-fld racemes immersed in foliage or in age forming a small terminal corymb.

    Stipules erect linear- or narrowly lance-attenuate 8-18 x 0.7-2 mm, the thinly herbaceous blades strongly 1-nerved, deciduous before the lf.

    Lvs 5-13.5 cm; petiole including discolored pulvinus (1.5-)2-5.5 cm, at middle 0.6-1.6 mm diam, carinate dorsally, openly shallow-sulcate ventrally; rachis (1.2-) 1.5-3 cm, usually a little shorter than the petiole; gland between proximal and rarely also between the second pair of lfts stipitate or subsessile, in profile (1.3-)2-4 mm tall, the narrowly lance-ellipsoid obtuse body 0.3-0.6 mm diam; pulvinules 1.2-2.5 mm; lfts 3 pairs, accrescent upward, the distal pair broadly obovate or subrhombic-obovate, obtuse mucronate or obscurely deltate-acuminulate (2.2-)2.5-6 x 1.3-2.6 cm, 1.4-2.4 times as long as wide, the margin and venation as in S. obtusifolia.

    Peduncles 0-3 mm; racemes 1-2-fld, the axis not over 1 mm; bracts lance- attenuate 3-5 mm, caducous; pedicels at anthesis 13-32 mm, in fruit much thickened and 14-40 mm; fls of S. obtusifolia, except (as described under vars.) sometimes larger, but the 3 abaxial anthers more strongly beaked, the beak (1-) 1.2-5 mm; ovary strigulose; style almost linear 3-8 mm; ovules 20-28.

    Pod stiffly ascending and slightly arched outward, the stipe 6-10 mm, the body 5.5-8 cm long, including the wings 8-11 mm diam, the body itself compressed quadrangular 4-5 mm diam, its cavity divided by complete septa into locules 2.5-4 mm long, the valves stiffly papery, the ridges parallel and approximate to the sutures produced as papery wings 2.5-4 mm wide; seeds descending obliquely across the cavity, obovoid compressed usually parallel to the septa, 3.5-4.2 x 2-3 mm, the castaneous sublustrous testa minutely pitted or granular, the linear or linear-elliptic areole 2-3 x 0.3-0.6 mm.

    While S. pentagonia becomes instantly recognizable so soon as its curiously winged ovary begins to expand, its smaller-flowered forms are at anthesis scarcely distinguishable from S. obtusifolia except by the longer (at least 1, not ±0.4-0.6 mm) beak of the three abaxial anthers. The likeness is so faithful that there can be little doubt that S. pentagonia originated by mutation from the common sickle- pod or an immediate ancestor. These related species were both first described by contemporaries of Linnaeus, but while the former has long been recognized as almost circumtropical in dispersal the latter, although similarly weedy in behavior, has proved singularly rare and local, the few known populations being loosely concentrated bicentrically in extra-Amazonian south Brazil and in northern Central America and adjoining Mexico. The plants of the northern hemisphere are all, so far as known, relatively small-flowered, as are those of Brazil southward along the S. Francisco valley from western Bahia to eastern Sao Paulo. From a still meager sample it appears that the anther-beaks of the Brazilian plants are shorter than those of North American ones, about 1-1.5, not 1.8-2 mm long, but the difference, if real, is inconsiderable, being unsupported by other characters. In the central Brazilian highlands the species is represented by a large-flowered race with extraordinarily long anther-beaks, described below as var. valens. Difficulty must be expected in separating fruiting material of var. valens from var. pentagonia, which are virtually identical in vegetative characters and in the pod; in fact a specimen from southern Goias (Goias Velho, Burchell 7062, LE, NY) already presents this problem. The range of var. valens overlaps that of the similarly large-flowered S. mucronifera, which may be separated without difficulty by the greatly elongate and very narrow wingless pod and at anthesis by the pubescent, emphatically venulose leaflets.

    Key to Varieties of S. pentagonia

    1. Fls relatively small, the longest sepal 7-9 mm, the longest petal 11-15 mm; style 3-4 mm; body of 3 abaxial anthers 3-4 mm, their beak 1-2 mm; dispersal bicentric: s. Mexico to Honduras; s.-e. Brazil.

    70a. var. pentagonia (p. 256).

    1. Fls larger, the longest sepal 13-15 mm, the longest petal 21-30 mm; style 6.5-8 mm; body of 3 abaxial anthers 9-10 mm, their beak 4-5 mm; Brazilian Planalto (s. Maranhao, e.-centr. Goias and w. Bahia).

    70b. var. valens (p. 257).