Monographs Details: Senna obtusifolia (L.) H.S.Irwin & Barneby
Authors:Howard S. Irwin, Rupert C. Barneby
Authority: Irwin, Howard S. & Barneby, Rupert C. 1982. The American Cassiinae. A synoptical revision of Leguminosae tribe Cassieae subtrib Cassiinae in the New World. Mem. New York Bot. Gard. 35, part 1: 1-454.

69.  Senna obtusifolia (Linnaeus) Irwin & Barneby, comb. nov. Cassia obtusifolia Linnaeus, Sp. Pl. 377. 1753.—"Habitat in Cuba. Lectoholotypus (Brenan, Kew Bull. 1958(2): 250. 1958): Dillenius, Hort. Eltham. Pl. Rar. 71, t. 62, 1732! typotypi, OXF (hb. Dillen.), G (hb. Burman. ex hort. eltham.)!—C. tora var. obtusifolia (Linnaeus) Haines, Bot. Bihar & Orissa 304. 1922.

Cassia tora ß [C.] humilis Persoon, Syn. Pl. 1: 456. 1805, based on Cassia tora ß Linnaeus, Sp. Pl. ed. 2, 582. 1762.—Holotypus, Plumier, Pl. Amer. Fasc. 4, t. 76, fig. 2. 1756!—C. humilis (Persoon) Colladon, Hist. Cass. 96. 1816.

Cassia toroides Rafinesque, Med. Bot. 96. 1828.—"Georgia to Kentucky. —No typus known to survive but the description, in context of the dispersal, decisive.—Diallobus falcatus Rafinesque, Sylva Tellur. 128. 1838, nom. illegit.

Senna toroides Roxburgh, Fl. Indica 2: 341. 1832.—"The seeds of this plant were sent from Mysore to the Botanic Garden at Calcutta by Dr. Buchanan in 1800 and about the close of 1801 the plants bloomed."—No typus seen, but the description and discussion decisive.— Cassia toroides Roxburgh, Hort. Bengal. 31. 1814, nom. nud.

Diallobus uniflorus Rafinesque, Sylva Tellur. 128. 1838.—"C bicapsularis [sensu] Miller &c. . . . Antilles, Madera."—No typus known to exist, the name referred here by Britton & Rose (1930, sub Emelista tora).

Cassia obtusifolia sensu De Wit, 1955, p. 254; Brenan, 1967, p. 77; Isely 1975, p. 115, 206, map 50; Mulick & Krishna, 1978, fig. 6 (seed).

Cassia tora sensu Bentham, 1870, p. 115 & 1871, p. 535, quoad pl. amer., exclus. syn. plur.; sensu Schery, 1951, p. 69; Gleason, 1952, p. 386 (cum fig.); Dimitri & Rial Alberti, 1954, fig. 4 (optima!) et auct. plur. post-benthamian. Emelista tora sensu Britton & Rose, 1930, p. 242, exclus. basionym. et syn. plur.—Non C. tora Linnaeus (1753).

Essentially monocarpic but sometimes of long duration, in extratropical climates plainly annual, when starved or crowded flowering as a diminutive, singlestemmed plant less than 1 dm but normally 3+ dm and amply leafy, commonly erect, branched distally or bushy-branched from base, sometimes diffuse, in favorable conditions becoming softly fruticose, the roots black with yellow growing tips, the terete or obscurely angulate pale green stems up to 20(-24) but commonly less than 12 dm, these with lf-stalks and pedicels either glabrous or thinly strigulose (pilosulous) with appressed, incurved or (rarely) erect hairs up to 0.2-0.6 mm mixed with (or replaced by) minute thickened or verruculiform trichomes, the thin-textured malodorous foliage slightly bicolored, the always ciliolate lfts either glabrous on both faces, or glabrous above only, or pubescent throughout, the 1-2-fid racemes subsessile in axils of cauline lvs and shorter than them, sometimes late in season forming a small terminal corymb.

Stipules herbaceous, incurved to erect, linear attenuate at both ends (3.5-)5.5-15(-17) x 0.4-1.5 mm, strongly 1-nerved or (when broad) obscurely penniveined, deciduous before the lf.

Lvs (disregarding those of depauperate dwarfs or of drought-inhibited late- season branches) 3.5-15(-17) cm; petioles including discolored pulvinus l-4(-5.5) cm, often progressively shorter upward along stem, at middle 0.5-1.7 mm diam, rounded or bluntly carinate dorsally, openly shallow-sulcate ventrally; rachis (0.8-) 1.5-4 cm, either longer or shorter than petiole; gland between proximal and sometimes between the second (but never between the third) pair, sessile or shortly stipitate, glabrous, in profile (1-)1.4-3 mm tall, the lance-fusiform obtuse or subacute head 0.3-0.8 mm diam; lfts of almost all lvs exactly 3 pairs, much accrescent upward, the distal pair broadly obovate to cuneate-obovate or broadly cuneate-oblanceolate, obtuse mucronulate or obscurely depressed-acuminulate (1.7-)2-6.5 x 1-4 cm, 1.6-2.5 times as wide as long, at base rounded or cordate on proximal and cuneate on distal side, the margin plane subhyaline, the centric midrib and (6-)7-10 pairs of camptodrome (with rare intercalary) secondary veins faintly prominulous above, sharply so but slender beneath, the tertiary venulation imperceptible above, often faintly discolored but hardly raised beneath.

Peduncles 0-5(-8, but few over 4) mm; bracts ovate-acuminate or lanceolate 2-5 mm deciduous; pedicels at anthesis filiform (7-)9-25(-28) mm, in fruit much thickened (9-) 12-35(-40) mm; fl-buds nodding subglobose, the thinly herbaceous sepals pale green, usually glabrous dorsally but ciliolate, sometimes fully glabrous or dorsally puberulent, the larger inner ones obovate or oblong-obovate (5.5-)6-9(-9.5) mm, 5-nerved from base; petals pale yellow, either glabrous or thinly puberulent dorsally, the vexillar one obcordate or cuneate-obcordate, the rest oblong-obovate, the 2 abaxial ones ± asymmetrical and either a little longer or shorter than the rest, the longest of all 9-15 mm, but the petals in some late-season fls apparently not fully expanding; androecium glabrous, the filaments of 4 median stamens 1-3 mm, of 3 abaxial ones (1.5-)2-4(-4.5) mm, the anthers of 4 median stamens oblong (1-) 1.4-2.8 mm, the biporose beak very short or almost 0, those of 3 median ones 2-4.5(-5) mm, contracted into a porrect or suberect beak (0.3-)0.4-0.6 mm, this usually dehiscent by one shallowly U-shaped slit, but a septum rarely dividing the orifice into 2 pores; ovary strigulose, rarely subpilosulous; style 1.7-4 mm, distally incurved and sometimes a little dilated, at the oblique stigmatic orifice 0.3-0.55 mm diam; ovules (16-)20-34(-38).

Pod stiffly ascending almost straight (when short) or more commonly arched out- and downward, sometimes through ± half a circle, narrowly linear attenuate at both ends (6-)7- 16(- 18) x 0.25-0.55(-0.6) cm, when first formed green and compressed-hexagonal, carinate by the sutures and on each face by two ribs parallel and approximate to the sutures, when ripe brown and more turgid, compressed tetragonal, the keeled sutures depressed and the parallel ribs becoming more prominent, the mid-strip of the valves faintly venulose, the valves becoming papery, tardily separating through both sutures, the interseminal septa well developed, the seed locules (3-)3.5-5.5(-6.5) x 2-4.5(-5.5) mm, varying from a little wider than long to twice as long as wide; seeds in broader pods obliquely descending across the cavity, in narrower ones almost parallel to its long axis, compressed-rhomboid or (when crowded) irregularly distorted, or in narrow pods subcylindroid-oblong, 3.2-5.3(-6) x 2-3.3 mm, the testa castaneous lustrous, crackled in age, the linear areole 2.5-4.2 x 0.25-0.5 mm; n = 13, 14 (Irwin & Turner, 1960, p. 311); 2n = 24 (Mahler, 1970, p. 71).—Collections: 339.

Lakeshores, river-banks and -beds, becoming a rank colonial weed of pastures, plantations, orchards, roadsides and waste places about farms and habitations, mostly 0-500 m, but up to 1100 m in Mexico, Colombia, the Brazilian Planalto and n.-w. Argentina, to 1680 m in Tropical East Africa, probably native of the Americas but now of almost circumtropical (and in N. and S. America and Asia warm temperate) dispersal: in North America circum-Caribbean and extending through the Mexican tierra caliente on the Pacific coast to s. Sonora, on the Gulf coast to Veracruz, apparently not recorded from Tamaulipas but n. through the Gulf coastal plain in United States from e. Texas to Florida, Missouri, Kentucky and tidewater Virginia (a fleeting waif n. of lat. 37°); in s. America scattered from n.-centr. Colombia to the Guianas and Tobago, discontinuously through the Amazon basin in Peru, Bolivia and Brazil, thence s. over the Planalto and the Atlantic states to Parana, Paraguay, and n.-e. (Corrientes, Misiones) and n.-w. (Salta, Tucuman) Argentina; lowland Pacific Ecuador and Galapagos Is. Widespread in Tropical Africa; tropical Asia (Indomalaya; Sri Lanka; Indochina), Malesia, s. China; Philippine Is.—Fl. in temperate U.S.A. VII-XII, in s. peninsular Florida, Antilles, Mexico, Central America mostly (IV-)VI-III, in equatorial latitudes throughout the year especially in wet periods, in s. Brazil and Argentina mostly X-V.—Hediondilla; matapasto; charamazca (Michoacan).

Our concept of obtusifolia is essentially that of Brenan (1958, l.c.), who also examined the nomenclature and rejected a faulty typification by De Wit; in this place we can add only some comments on internal variation in the New World.

The vernacular names Hediondilla and Matapasto provide a sort of shorthand description of this weedy malodorous senna which, like many other plants that have become, as it were, commensal with man, is genetically complex and phe- netically heterogeneous. Three chromosome levels have been reported in S. obtusifolia, but we have not learned to relate this to any phenetic formula or indeed to recognize them in dried specimens. Growth form is known to be potentially hereditary, for while in Texas the senior author grew side by side from seed strains of S. obtusifolia, one of which, originating in Texas, maintained the erect habit of its parent and flowered in late summer, while the other, originating on the white sands of the Guyana near-interior, remained diffuse like its parent and failed to flower at all when translated northward through 30 degrees of latitude. Moreover it seems probable that small genetic differences can be handed down indefinitely from one generation to the next, for the flower is commonly, perhaps always, fertilized in late bud, the style being then curved inward so as to present the stigmatic cavity directly into the face of the precociously dehiscent anthers.

From early times it has been known that the petiolar glands of S. obtusifolia may be either solitary between the first pair or one each between the first and second pairs of leaflets; and Bentham wondered (1871, sub C. tora) whether there might not be two real races different in length and curvature of the pod, a feature that had been supposed, as we now know erroneously, to distinguish S. obtusifolia from S. tora. In America a second gland is relatively rare and of sporadic occurrence through the range of the species; plants with or without it may occur in the same habitat, without relation to other variable features. The pod and its seeds, however, do show some interesting correlations with distribution. On the Pacific coast of Mexico and southward through Central America, Colombia, Ecuador and Venezuela, the pod is apparently always narrowly needlelike, (2-)2.5-3.5 mm diam when fully ripe and often strongly curved out- and downward. In pods of this type the seed locules are much longer than wide and the oblong-cylindroid seed lies almost vertical along the pod’s long axis. Throughout United States and the West Indies the pod is broader, 3.5-6 mm diam when fully ripe and on the average less curved; its locules are about 1-1.5 times as long as wide, housing compressed-rhomboid seeds obliquely tilted across the pod’s long axis. In the Guianas and Amazonia, eastern Brazil and Argentina, a narrow pod is prevalent, but a broad one appears at scattered points, perhaps chiefly suburban. On the other hand broad pods alone are on record from Bolivia and Paraguay. Whether these two fruiting types had originally discrete ranges which have become obscured by artificial dispersal we cannot now tell. And there are individual specimens with pods ±3.5 diam which would be hard to place in either category. The width of the pod has a bearing on the question of origin of the now almost pan tropical S. obtusifolia, which we suppose, following Irwin & Turner (1960, p. 316), to be aboriginally neotropical. All African specimens of which good fruit has been available for study have the broad pods of Antillean or United States S. obtusifolia, which suggests that the African races of the species are derived from Caribbean stock. The Indian, Indomalayan and Chinese material that we have seen is of the same type, and could have been carried eastward from Africa. But the Philippine form of S. obtusifolia has the needlelike pod of the plant prevalent in Pacific Mexico, and we cannot help wondering whether it is not derived from seeds brought across the Pacific over the Spanish trade route between Acapulco and Manila. Perhaps a biosystematic study will provide an answer to these questions. It may be worth mentioning that our study of American S. obtusifolia confirms Brenan’s view that genuine S. tora, which differs in its usually shorter pedicels, always truncate abaxial anthers and broad areole on the seeds, is foreign to the New World, ranging from peninsular India and Sri Lanka to Fiji and Samoa.

The foliage of S. obtusifolia is said to furnish a vegetable, a purgative drug, and poultices for sores, ulcers and insect-bites. The seeds have been used as a substitute for or an adulterant of coffee, and furnish a mordant for dyeing blue cloth. Burkart (1952, p. 169, sub C. tora) mentions a large-seeded form of the species, distinct from that wild in Argentina, which is cultivated for its laxative properties by Japanese settlers under the name of habuso. Not seen by us, it should be compared with the broad-podded Antillean-African-Indian forms mentioned above which might have reached South America independently from eastern Asia. The macerated leaves furnish the Bora Indians of Rfo Yaguasyacu in Loreto, Peru with a dye for Astrocaryum hammocks (Balick et al. 1037, MO).