Senna hirsuta

  • Title

    Senna hirsuta

  • Authors

    Howard S. Irwin, Rupert C. Barneby

  • Scientific Name

    Senna hirsuta (L.) H.S.Irwin & Barneby

  • Description

    138.  Senna hirsuta (Linnaeus) Irwin & Barneby, Phytologia 44(7): 499. 1979. Cassia hirsuta Linnaeus, Sp. PI. 378. 1753.—Typus infra sub var. hirsuta indicatur.

    Coarse erect or diffuse, simple or several-stemmed, amply leafy malodorous herbs becoming softly woody in age, at anthesis (3-)5-20(-24) dm, highly diverse in pubescence, that of stems, lfts and pods independently varying from lustrously spreading-hirsute to pilosulous or incurved-puberulent, rarely minute and almost 0, the longest hairs 0.15-2.3(-2.8) mm, when over 1 mm mostly erect, sometimes septate, when shorter incurved or subappresed, in either case mixed with (or almost replaced by) minute thickened yellowish or reddish trichomes, the relatively thin-textured lfts not or not strongly bicolored, dull dark or yellowish- olivaceous on both faces, the inflorescence a narrow thyrse of usually few-fld, often pseudo-umbellate, but sometimes up to ±35-fld and then densely fld racemes, the proximal ones almost always subtended and greatly surpassed by fully developed lvs and the distal ones subtended by vestigial lvs consisting of rudimentary lf-stalk with scarcely diminished gland, the distal ones forming a ± continuous panicle.

    Stipules erect or falcately incurved to erect, thinly herbaceous, linear-oblong, -attenuate or linear-elliptic-acuminate 3-12 x (0.2-)0.4-1.6 mm, deciduous before the lf, absent from most fruiting spms.

    Lvs below (and at base of) inflorescence 8-28(-33) cm; petiole including shrunken pulvinus (1.2-) 1.5-6.5 cm, at middle 1.1-2.8 mm diam, bluntly 3-ribbed dorsally, narrowly thick-margined and openly shallowly sulcate ventrally; gland at base of petiole proper (contiguous to pulvinus), ascending at narrow angle to petiole, sessile or commonly very shortly stipitate, in profile (0.8-) 1-2.5 mm tall, the slenderly or plumply ovoid, ellipsoid, fusiform, claviform or globose body 0.4-1.7 mm diam, when both broad and stipitate then ± dorsoventrally compressed or phalloid, a similar gland exceptionally between proximal pair of lfts; rachis (3-)4.5-12(-16) cm, the terminal appendage either dry setiform or fleshy- thickened subuliform; pulvinules 1.3-3.5 mm, moderately turgid, when dry either green or nigrescent; lfts 3-7(-8) pairs, strongly accrescent distally, the distal pair subsymmetrically ovate-, rhombic-ovate-, lance- or lance-elliptic-acuminate or -caudate (3-)3.5-10.5 x 0.8-4 cm, (1.7-)2-6 times as long as wide, at base inequilaterally rounded or cuneate, the margins plane or incipiently revolute toward the pulvinule, the centric midrib and 7-16 pairs of fine camptodrome (with random intercalary) secondary veins nearly always immersed above, prominulous beneath, the faint tertiary venulation commonly visible (either faintly raised or merely discolored) beneath only, in one variety prominulous on both faces.

    Peduncles (0-)l-30 mm; racemes commonly (n. of the Equator almost always) 2-8-, in Argentina and s.-w. Brazil commonly to 15-43-fid, the expanded fls raised to or above level of nodding buds, the axis becoming 0-8 cm; bracts lanceolate or linear-attenuate 1.5-4.5 x 0.25-1.2 mm, caducous as the pedicel begins to elongate; mature pedicels 9-25 mm; fl-buds plumply obovoid-globose becoming asymmetrically obovoid obtuse, when young usually ± pubescent but sometimes glabrous; sepals submembranous yellowish, greenish or brownsih ± strongly graduated, obovate or the inner ones oblance-obovate, the smallest outer one 4-7 mm, the longest inner one 6.5-10 mm; petals glabrous, yellow drying stramineous or whitish brown-veined, of subequal length but the flabellate or broadly obovate emarginate vexillar one broadest, the rest obovate or oblanceolate, the longest 8-15 mm; androecium (except for random puberulence of long anthers) glabrous, functionally 7-merous, the paddle-shaped staminodes 1.1-2 mm wide, the filaments of 4 median stamens 1.3-2.3 mm, of 2 latero-abaxial ones membranous- dilated at middle, 4-7 mm, of the centric abaxial one 2-3.5 mm, the body of 4 median anthers including short oblique beak 3.8-5.4 mm, that of the 2 long abaxial ones, measured from sagittate base to constriction below beak, 3.8-5.4 x 1.4-2.4 mm, the beak itself 1.2-1.5 mm, very asymmetrically dilated on exterior side into a thickened tongue-shaped, internally 2-grooved pollen-cup 0.7-1 mm long, the centric one similar but scarcely half as long or wide and sterile, the fertile anthers commonly brown yellow-beaked, sometimes yellow throughout; ovary densely white-hirsute, short-pilosulous, or minutely strigulose; style (1.8-)2-3.5 mm, openly sulcate ventrally, at apex dilated, strongly incurved through 90-180° and 0.5-0.8 mm diam, the crateriform stigmatic cavity introrsely directed, its margin barbellate; ovules 50-108.

    Pods 1-5 per raceme, stiffly ascending at narrow angles but when elongate outwardly recurved through up to 3/4-circle (in one var. also spirally twisted through 1-2 turns), in outline narrowly linear to linear-vermiform 10-28 x (0.25-)0.3-0.65(-0.7) cm, when fully ripe laterally compressed-tetragonal, the broad ventral suture 3-keeled, the valves green ripening brown and papery, faintly or shallowly depressed between seeds, the seed-locules separated by broad complete hyaline septa, (1-)1.2-3.5(4.5) mm long, as wide as the pod’s cavity, in consequence either longer or shorter than wide, the distal ones usually longer than the rest; seeds either transverse or obliquely basipetal, when not crowded plumply obovoid compressed parallel to the valves, when crowded becoming drum-shaped or variably rhomboidally distorted, the larger ones (1.8-)2-3.4(-3.8) in var. acuminata to 4.5 x 1.5-2.6 mm, the smooth testa dull brown, fawn, chocolate-brown or brownish-olivaceous, encircled in the plane of the hilum by a darker line, the areole on either the broad or narrow faces or even displaced across the angle between these, in outline elliptic or subcircular (0.55-)0.7-2.4 x (0.3-)0.4-1 mm.

    The material of Cassia hirsuta and its close relatives available for Bentham’s scrutiny a century ago could at the time be reasonably interpreted as representing two species, a C. hirsuta sens. str. defined by a relatively broad but not necessarily short pod combined with hirsute vesture and a C. leptocarpa defined by a narrow but only potentially longer pod combined with vesture varying from hirsute to pilosulous or obsolescent. In view of the essentially identical habit of growth, flower structure and seeds, Bentham’s distinction between pods 1-1 1/2 and 2-3 lines wide was at best finely drawn and in the light of our much more extensive collections has faded into a continuum of variation. Britton & Rose (1923, p. 255, in key to Ditremexa) characterized the pod of D. hirsuta as both shorter and wider than that of D. leptocarpa, but perplexingly allowed in the description of the latter’s fruit a range in length of 7 to 30 cm, fully encompassing the 8-20 cm range assigned to D. hirsuta. Correlations in the S. hirsuta complex between length and width of pod, and between the pod’s proportions and the vesture of foliage, ovary, or both, are so low that no combination of these features serves as specifically diagnostic.

    Study of the pod of S. hirsuta sens. lat. has brought to light some unexpected facts. The 60-96 ovules within the relatively broad and short pod of var. hirsuta sens. str. are not significantly less numerous than the 60-108 recorded from the long narrow pod of the so-called leptocarpa type. With increase in number, the individual seed-locules become shorter and broader because more crowded along the pod’s long axis, and the seed, accommodating itself to its confining cell and to pressure from its neighbors, becomes distorted from the apparently primitive, obliquely basipetal orientation and from compression parallel to the valves to a strictly transverse orientation and compression parallel to the interseminal septa. Thus drum-shaped seeds crowded into narrow stalls, as described by Brenan (1967, p. 80) for adventive African C. hirsuta, bear their areoles not on the broad faces but on the narrow rim. While two extreme forms of seed are strikingly different at first view, there are intermediate and irregular configurations where the areole is displaced to an angle between two faces; and furthermore two types of seed may be found in one pod, the compressed ovoid one at random points along the pod where neighboring ovules fail to develop and also very frequently in elongated locules toward the apex of even the shortest pods. The same variation was noticed by Bentham (1871, p. 532) in Cassia occidentalis. It appears that the differences in proportions of the pod and in orientation and compression of the seed can be traced to one cause, a slight modification in the ontogeny of the pod’s valves.

    While vesture has proved ineffective as a specific criterion, within S. hirsuta it is very definitely correlated with dispersal and with a few minor morphological features, and is therefore useful taxonomically at the varietal level. A thin puberulence often confined to the lower face of the leaflets characterizes a xeromorphic Mexican var. glaberrima and the original Cassia leptocarpa, which emerges as an independent variety highly localized in southeastern Brazil. A hirsute vesture of long, coarse, straight and shining hairs is proper to typical var. hirsuta and to what we here call var. hirta, partially equivalent to Bentham’s C. leptocarpa var. hirsuta (deceptively homonymous but heterotypic). A softer pilosulous vesture of shorter, mostly incurved or subappressed hairs marks the South American, largely extratropical vars. puberula and streptocarpa, in which survive, either facultatively or constantly, what we suppose to be a relatively primitive multiflorous raceme. Our six varieties of S. hirsuta are for the most part genuinely allopatric and vicariant. The species as a whole is, like C. occidentalis, prevailingly weedy even where native, and we suspect that random records of a variety from phytogeographically improbable stations and, indeed, some continuous or semi-continuous extensions of range are due to human interference. It is generally conceded that S. hirsuta is an American weed in Africa and Southeast Asia, but it is widespread there and has proved its ability to spread over great distances in ruderal habitats. Thus we have no real gauge by which to assess the extent to which any form of S. hirsuta is genuinely autochthonous except as a particular phase of it conforms to known patterns of geographical dispersal.

    The varieties of S. hirsuta are serially ordered below to conform with the hypothesis that obsolescent vesture, coarse hirsute vesture, an abbreviated, few-flowered, pseudo-umbellate raceme and a condensed pod with densely crowded seeds are derived or specialized features, and that the more primitive forms of the species are those found today in the Paraguai-Uruguai basin, near or astride the Tropic of Capricorn between southern Bolivia and Rio Grande do Sul in Brazil. It follows that var. streptocarpa, which closely resembles the vicariant S. neglecta, is placed first and var. hirsuta last. Generally speaking the varieties with more numerous derived features are dispersed correspondingly further north or northwest from the presumed focus of speciation for the section in southern Brazil.

    Key to the Varieties of S. hirsuta

    1. Lfts pubescent on both faces and cili(ol)ate, the longer hairs 0.4-2.8 mm.

    2. Vesture of lfts strigulose or softly pilosulous, the hairs when loose commonly incumbent or incurved, less often straight and spreading, in any case not highly lustrous nor more than 0.4-0.7 mm long; S. America in lat. 17-30°S.

    3. Pod both arched outward and spirally twisted, commonly sigmoid when flattened in press; racemes mostly 8-35-fld; fls small, the longer inner sepals 5-7 mm, the longest petal 8-13.5 mm; n.-e. Argentina (Misiones) and adjoining Brazil (Rio Grande do Sul).

    138a. var. streptocarpa (p. 429).

    3. Pod simply arched outward, not twisted; racemes in Bolivia and Paraguay 2-8-, in Argentina and e. Brazil (centr. Minas Gerais) to 6-14-fld; fls larger, the longer inner sepals 7-10 mm, the longest petal 12-16 mm; s.-centr. Bolivia and n.-w. Argentina (Salta, Tucuman, Santiago del Estero) to Paraguay, disjunct in s.-e. Brazil (Minas Gerais); adventive in Philippine Is.

    4. Pod 15-25 x 0.3-0.5(-0.55) cm; ovules 66-82; range as just given except Brazil.

    138b. var. puberula (p. 429).

    4. Pod 15 x 0.7 cm; ovules ±54; centr. Minas Gerais, apparently very local.

    138c. var. acuminata (p. 431).

    2. Vesture of lfts hirsute, the hairs straight, spreading or ascending, highly lustrous, the longest 0.6-2.8 mm; equatorial S. America n.-w. and n. to tropical Mexico and W. Indies, s. infrequently and interruptedly in Brazil and Bolivia to ±17°S.

    5. Pod slender, usually elongate and curved out- or both out- and downward, (14-) 15-27 x 0.25-0.45(-0.5) cm; Mexico and w. Cuba s.-e. through Central America to n. Colombia, thence s. along the Andes to n.-w. Bolivia, in S. America essentially cordilleran.

    138f. var. hirta (p. 433).

    5. Pod stouter, shorter and straight, 11-15 x (0.4-)0.45-0.65 cm; n. S. America from Colombia to the Guianas, s.-e. in Brazil to Maranhao and (disjunctly) s.-centr. Goias, n. to the Greater and Lesser Antilles; widespread as a weed in Old World Tropics.

    138g. var. hirsuta (p. 434).

    1. Lfts on upper face glabrous or very minutely puberulent, elsewhere thinly puberulent or strigulose, the longest hairs 0.2-0.4 mm; either s.-e. Brazil or Mexico and s.-w. United States.

    6. Venation of both faces of lfts finely prominulous, the blades openly reticulate; s.-e. Brazil (local in Rio de Janeiro and adjoining Minas Gerais).

    138d. var. leptocarpa (p. 431).

    6. Venation of upper face of lfts immersed, of lower face faintly penniveined, tertiary venulation 0 or merely discolored, prominulous on neither face.

    138e. var. glaberrima (p. 432).