Senna pendula

Title

Senna pendula

Authors

Howard S. Irwin, Rupert C. Barneby

Scientific Name

Senna pendula (Willd.) H.S.Irwin & Barneby

Description

126.  Senna pendula (Willdenow) Irwin & Barneby, comb. nov. Cassia pendula Humboldt & Bonpland ex Willdenow, Enum. Pl. Hort. Berol. 440. 1809.—Typus infra sub var. pendula indicatur.

Diffuse or awkwardly assurgent shrubs m, in brush-woodland, savanna and margin of gallery forest arborescent to 7 m, in close competition sarmentose and reportedly reaching (in Pacific Ecuador) 20 m, the older trunks brown or castaneous lenticellate, the pliantly virgate, commonly ± geotropic, simple or randomly branched annotinous stems terete striate, green or anthocyanic, the whole plant varying from fully glabrous to pilosulous with incumbent or spreading straight hairs (described in greater detail under vars.) but the lfts almost always glabrous above, the foliage bicolored, dull bright green or olivaceous above, paler and sometimes glaucescent beneath, the inflorescence a ± exserted simple or branched thyrsiform panicle of racemes.

Stipules caducous before maturity of associated lf, consequently little known, absent from most fl and all fr spms, submembranous pallid linear-lanceolate or -oblanceolate 1.5-9 mm.

Lvs (depauperate ones in inflorescence ignored) 5-13 cm; petiole including moderately swollen, when dry wrinkled and often discolored pulvinus 1.5-4(-5) cm, at middle 0.4-1.1 mm diam, shallowly, most often openly sulcate ventrally; rachis l-6(-7) cm, its longer interfoliolar segments (5-)6-21 mm; glands always between proximal pair of lfts, sometimes also between the second pair, or between all but the distal pair, sessile or shortly stipitate, in profile 0.7-2.5(-2.8) mm, the livid or greenish body varying from oblong- or lance-ellipsoid to oblong- obovoid, ovoid, obovoid or globose and in any case either obtuse, acute or apiculate, (0.25-)0.4-1.2 mm diam; pulvinules 1-2.7 mm; lfts (2-)3-6(-7), in most populations 4-5 or exactly 4 or 5, only locally 6-7 pairs, accrescent distally, the proximal pairs shorter and proportionately broader than the distal, these from variably asymmetric base varying in outline from broadly to narrowly obovate, oblong-elliptic or broadly oblanceolate, (1.8-)2.2-5.5(-6.5) x (0.75-)0.85-2(-2.4) cm, (1.7-)1.8-3.5(-3.9) times as long as wide (in local Peruvian var. scandens ovate-elliptic), at apex broadly rounded, obtuse mucronulate-apiculate, or shallowly emarginate, the translucent pallid or orange margin plane or incipiently revolute toward the pulvinule, the midrib with (5-)6-12(-13) pairs of camptodrome (+ random intercalary) secondary veins prominulous either on both faces or beneath only, the tertiary venulation variable, sometimes faint, open and irregular, sometimes forming a sharply raised, fine and regular reticulum.

Racemes (2-)4-27(-35, exceptionally -50)-fld, the peduncle (from geotropic branches appearing recurved) with raceme-axis together becoming (2-)3-18(-33) cm, the ascending buds either corymbose or racemose in praefloration; bracts membranous pallid or pallid-margined lance-subulate 1-4 mm, caducous as the pedicel begins to elongate; pedicels at and after full anthesis 8-33(-37) mm; hypanthium turbinate 1-2.2 mm; fl-buds obliquely obovoid obtuse, commonly glabrous, sometimes densely minutely puberulent; sepals almost always membranous subhyaline-margined, exceptionally (in w. Brazil) subcarnosulous and wrinkled when dry, varying from yellowish to fuscous or reddish-castaneous, moderately or well graduated, the outermost ovate or lance-elliptic 3-9.5(-10) mm, the amply obovate or elliptic-suborbicular innermost one mostly 7-15.5 (in n.-w. Argentina 5.5-6) mm; corolla zygomorphic, the subsessile glabrous petals golden- or orange-yellow fading pale yellow or stramineous dark-veined, the vexillar one broadly obovate-flabellate emarginate or deeply notched, its 2 neighbors ovate or obovate obtuse, the 2 abaxial ones ovate- or oblong-elliptic, either a trifle longer or shorter than the rest, the longest petal (11-) 12-26 mm; androecium glabrous, the yellow blade of the 3 staminodes varying from linear-oblanceolate to rhombic-orbicular, quadrate, trapeziform (broader upward), pandurate, inversely deltate, or broadly cuneiform 1.2-4.5 x 0.8-3(-3.5) mm; filaments of 4 median stamens 1.4-3(-4) mm, of the centric abaxial one 1.6-6(-7) mm, of the 2 long latero-abaxial ones dilated and (6.5-)7-20 mm, the 4 smooth or papillate, almost straight median anthers slenderly or plumply flask-shaped, including beak 3.5-7 x 0.7-2 mm, the body constricted 0.3-1.4 mm below apex and thence dilated into an obliquely truncate orifice in profile 0.7-1.5 mm wide; centric abaxial anther sterile or almost so, variably intermediate in length between the 4 median and the long abaxial ones, the body narrower than all of them, often only the beak dilated; 2 long abaxial anthers either brown yellow-tipped or uniformly yellow, lunately lanceolate from shortly sagittate base, including the beak (4.5-)5.5-11(-14) x 0.9-2.2 mm, slightly or strongly constricted 0.3-1.2 mm, in var. advena 1.1-2 mm below the symmetrically truncate apex into a broadly drum-shaped or more slender tubular beak at orifice 0.5-1.2 mm diam; dehiscence of fertile anthers by coarsely U-shaped slit; ovary glabrous, strigulose or densely pilosulous, the vesture white or less commonly lutescent; style linear-filiform or slightly dilated and up to 0.5 mm diam just below the minute stigmatic orifice, 1.5-10 mm long, either gently incurved distally or (when short) more abruptly hooked; ovules (48-)50-152.

Pod obliquely geotropic or pendulous, the stipe 2-5(-6) mm (caveat: pods sterile at base may appear long-stipitate), the cylindric, sometimes gently obcompressed-cylindric, less often laterally compressed but turgid, exceptionally (cf. var. tenuifolia) moniliform body when fully fertile (7-)8-18 (in var. tenuifolia -29) x 0.9-1.5(-l.6) cm, straight, gently curved or, when (as often) semisterile or bruchid-infested, variably distorted, the smooth green or fuscous, glabrous or early glabrate, rarely thinly pilosulous valves becoming pale brown and papery, the meso- and exocarp separating only when fully ripe and dry, the broadly margined sutures never prominulous, tardily if at all dehiscent; seeds commonly 2-seriate, locally (in s.-e. Brazil and n.-e. Argentina) 1-seriate, in either case turned with broad faces to the transverse septa, plumply obliquely obovoid 4.1-6(-6.6) x (2.8-)3-4.3 x ±2 mm, embedded in copious or scanty pulp, the brown, sooty or castaneous testa either smooth lustrous or microscopically granular dull, exareolate.

The limits of S. pendula as set in the foregoing description have no exact precedent. Our species is approximately equivalent to the residue of Bentham’s heterogeneous Cassia bicapsularis if this is purged of Senna bicapsularis sens, str., described below (no. 129) and of its var. chilensis, our S. candolleana, but is materially expanded in directions that could not be foreseen by Bentham in 1871. Its boundaries are more nearly coextensive with those assigned by Lasseigne (ined.) to Cassia pendula, but are expanded to include Cassia indecora (C. bicapsularis var. canescens Benth.) and contracted to exclude C. pendula var. bracteata Lasseigne, which is our S. cajamarcae. The species differs from the closely related and habitally similar S. bicapsularis sens. str. in its much longer pedicels and shorter hypanthium; from S. candolleana in the U-shaped (not discretely 2-porose) dehiscence of the fertile anthers; and from S. cajamarcae in the caducous stipules and bracts and in relatively numerous (±50-150, not 30-36) ovules with consequently many-seeded pod. In the Andes of Bolivia and southern Peru care must be taken to distinguish S. mandoni, easily mistaken at anthesis for some brevistylous form of S. pendula but different in the yet fewer (12-25) ovules and ultimately in the thin-textured, strongly compressed, internally pulpless fruit.

Among American sennas S. pendula is second only to S. pallida for multiplicity of subtle floral modifications, and far surpasses it in vastness of range, which extends continuously, except where interrupted by high mountains or closed forest, from latitude 27°N in northwestern Mexico to 27°S in northern Argentina. The centers of differentiation are, however, rather different, S. pallida being most diverse in southern Mexico and thence crossing the Equator only into northern Peru, whereas S. pendula is racially most complex in southeastern Brazil and on the Amazonian slope of the Andes, becoming more homogeneous, though no less populous in number of individual plants, in the Carib-Mexican region. The collections of S. pendula that have accumulated since Bentham’s revision of Cassia are heterogeneous in vesture of foliage, in venulation of the leaflets, in size of calyx and corolla, in form of the three staminodes, in length of the two long filaments and relative proportion of these to their anther, in length of style, and in number of ovules which, as they become more numerous and more crowded following fertilization, are displaced into two parallel ranks. In the bee-pollinated flower of Senna the staminodal flag standing erect behind the four short median stamens and the reciprocal positions at anthesis of the long stamens and the stigma are elements crucial to fecundation of the ovary. Theoretically one might expect rapid selection of particular floral patterns tailored to fit the needs of particular pollinators, certain to be different in the extremely diverse climates to which S. pendula sens. lat. has become adapted. This indeed is what we have found to be the case, strong geographic correlation with floral morphology emerging when the now extensive material of the species is examined in close detail. As elsewhere in the genus, we have found the density, quality and distribution of vesture to vary in so random and capricious a way as to become an impediment to a sound classification; but all the other variable features listed above contribute something to our understanding of adaptive radiation in this complex group of sennas.