Senna polyantha

  • Title

    Senna polyantha

  • Authors

    Howard S. Irwin, Rupert C. Barneby

  • Scientific Name

    Senna polyantha (Moc. & Sessé ex Collad.) H.S.Irwin & Barneby

  • Description

    144.  Senna polyantha (Colladon) Irwin & Barneby, comb. nov. Cassia polyantha Colladon, Hist. Casses 112, t. 2. 1816.—"Cassia sp. nova. Moc[ino] & Sesse, pl. mex. ined. ic. Hab. in Mexico."—Described from a drawing made by the artists of the Royal Expedition to New Spain, or from a copy of it made at Geneva for A. P. de Candolle.—Lectoholotypus, the protologue, including plate, as cited above!—Pterocassia polyantha (Colladon) Britton & Rose, N. Amer. Fl. 23(4): 244. 1930.

    Cassia browniana Kunth, Mimoses 135. 1819.—"Crescit prope Guanaxuato Mexicanorum, alt. 1068 hexapodarum."—Holotypus, collected by Humboldt & Bonpland in IX. 1803, labelled n. 4216 Guanajuato,’ P-HBK! isotypi, B (Hb. Willd. 7993), P (hb. Bonpland. 4216)!—Correctly equated with C. polyantha by Bentham, 1871, p. 547.

    Cassia marginata Sesse & Mocino, Fl. Mex. 100. 1894.—"Habitat in montibus Guanaxuati.’— Holotypus, Hb. S. & M. 1191, MA! = F Neg. 44357.—C. alata sensu Sesse & Mocino, Pl- Nov. Hisp. 60. 1893, quoad descr., exclus. syn. Linn. & Plum.—This last is identified by Sesse & Mocino with their Icon. No. 387, very likely the same as mentioned supra under S. polyantha.

    Cassia goldmani Rose, Contrib. U.S. Nat. Herb. 10(3): 98. 1906.—"[US] 365323, collected by [E. W.] Nelson and [E. A.] Goldman about 5 miles southwest of El Potrero, Lower California, October 31, 1905 (no. 7238)."—Holotypus, US! clastoholotypus + photo, NY!— Pterocassia goldmanii (Rose) Britton & Rose, N. Amer. Fl. 23(4): 244. 1930.—Equated with C. polyantha by Standley, 1922, p. 409.

    Pterocassia rubricaulis Rose ex Britton & Rose, N. Amer. Fl. 23(4): 244. 1930.—"Vicinity of San Luis Tultitlanapa, southern Puebla, July 22, 1908, C. A. Purpus 2657."—Holotypus, US! clastoholotypus + photo, NY!

    Cassia polyantha sensu Bentham, 1871, p. 547; Standley, 1922, p. 409.

    Shrubs and slender trees with commonly dense rounded crown of foliage, rough gray trunk and deeply canaliculate, usually red-castaneous hornotinous branchlets, at anthesis 1.5-7 m, the young stems, lf-stalks, dorsal (less often also ventral) face of lfts and axes of inflorescence all commonly softly pilosulous with weak spreading sinuous or straight gray or faintly lutescent hairs up to 0.2-0.45(-0.7) mm, but the vesture sometimes reduced to rudimentary puberulence of lf-stalk and pedicels, the foliage bicolored, the lfts either dark (when dried brownish-) green or bright yellow-green above, paler beneath, the inflorescence an immersed or shortly exserted subcorymbose panicle of racemes crowded in distal lf-axils, or some racemes subterminal to lateral branchlets.

    Stipules caducous (absent from many spms), erect submembranous brown or purplish, lance-acuminate (1.5-)2-6 x 0.7-1.4 mm, the margins not revolute.

    Major lvs widely spreading or decurved 4.5-14 cm; petiole including ovoid, often discolored pulvinus (4-)6-17(-19) mm, at middle 0.5-1 mm diam, narrowly margined ventrally, the sulcus almost closed; rachis (2-)2.5-11.5 cm; petiolar gland 0; lfts (5-)7-16(-19) pairs, of subequal length or decrescent proximally, in outline mostly oblong-elliptic, sometimes varying to -oblanceolate, linear-oblong or (when short) oblong-obovate, at apex obtuse-mucronulate or broadly deltate- acuminulate, the longest (11-) 12-22 x (3.5-)4.5-8 mm, 2.1-3.1 times as long as wide, the margin plane or (young) weakly revolute, the centric midrib immersed above, cariniform beneath, the 6-9 pairs of slender camptodrome (and random intercalary) secondary veins either finely raised or fully immersed on upper face, prominulous or merely discolored beneath, a fine weak tertiary or reticular venulation invisible above, raised or discolored beneath.

    Racemes closely (7-)10-50-fld, the 1-3 simultaneously expanded fls raised to or a little beyond level of ascending fl-buds, the axis together with peduncle becoming (2.5-)3-6(-9) cm; bracts lance-acuminate 2.5-5.5 x 1-1.8 mm, deciduous before or at latest immediately after anthesis; pedicels 12-24 mm; young fl- buds globose, glabrous or puberulent; sepals firm, fuscous yellow-margined or yellow overall, well graduated, the outer ovate 2.6-4 mm, the more broadly ovate or oblong-obovate inner ones 4.5-6 mm; corolla zygomorphic, the glabrous petals bright yellow drying dull yellow delicately brown-veined, subhomomorphic, oblanceolate, oblong-oblanceolate or ovate-oblong obtuse, the longest (7-)8-11 mm; androecium glabrous, the staminodes 0.5-1.1 mm wide, the filaments of 4 median stamens 1.4-3 mm (the pair further abaxial longer), of the 3 abaxial 3-5 (the centric a trifle shorter than its neighbors), the dark brown anthers slenderly lanceolate in outline from a bluntly sagittate base, those of 4 median stamens 3.4-4.7 x 0.8-1.2 mm, obliquely truncate at apex, of 2 latero-abaxial ones 4.5-6 x 0.8-1.2 mm, of the centric abaxial one 3.1-4.5 x 0.7-0.9 mm, the 3 abaxial gently incurved, the orifice produced on outer side into a blunt lip ±0.3 mm beyond the 2-porose orifice; ovary glabrous or thinly pilosulous at base; style (2.5-)3-4.5 mm, distally incurved through nearly 180°, grooved ventrally, 0.3-0.4 mm diam at the introrsely lateral stigmatic cavity; ovules 10-15.

    Pod obliquely ascending or randomly declined, the stout stipe 2-4.5 mm, the broadly linear-oblong piano-compressed, straight or slightly decurved body 7.5-11.5 cm, excluding the sutural wings 1-1.9 cm wide, bicarinate by the prominent sutures, these subundulately cristate all around by a wing 2-4 mm wide, the reddish, when ripe stiffly chartaceous pale brown or subpruinose, almost flat valves separating along the ventral suture and along the lines of interseminal septa into narrowly oblong panels 5.5-9 mm wide and as long as pod’s diam, permanently hinged along the dorsal suture to allow egress to the seeds; seeds strongly compressed parallel to valves, broadly obovate-disciform 6.5-8.5 x 4.5-5.5 mm, the testa glossy castaneous smooth or minutely pitted, the areole (sometimes indistinctly differentiated at one end) 1.5-3.3 x 0.5-1 mm.— Collections: 49.

    Matorral, drought-deciduous brush-woodland and thickets in grassland, sometimes on pedregal, on barranca terraces, or surviving cultivation along walls and fences, local and discontinuously dispersed at ±1400-2240 m over parts of s.- centr. continental Mexico and (lower and greatly isolated) Baja California: around the s. foothills of the Central Plateau and adjoining Bajio in extreme n. Michoacan, Guanajuato, s. Queretaro and w. Hidalgo; apparently disjunct in centr. and n. Oaxaca and extreme s. Puebla; and in canyons near 500-750 m in desert mountains of middle Baja California Sur (Sa. de la Giganta and vicinity, lat. ±25°45'-26°45'N).—Fl. in continental Mexico V-IX, in Baja California following rains.

    This species and closely related S. galeottiana are the only Mexican sennas characterized by a syndrome of glandless leaf-stalk, zygomorphic corolla and ventrally grooved style, and are unmistakable as soon as the broad flat, marginally winged pod takes form. The pod itself recalls that of some tropical forest caes- alpinioids of which the fruit is wind-dispersed; but it is actually shed from the raceme only reluctantly by weathering, and release of the seeds is accomplished by escape through the independently detached panels of the valves.

    Discontinuous dispersal has given rise to incipient racial differentiation in S. polyantha, but has not progressed to the point where definable taxonomic entities have emerged. The populations in Oaxaca tend to be more densely pubescent that those north of the Neovolcanic Range, but it is precisely there that the glabrate minor variant described as Pterocassia rubricaulis arose. We follow Standley (l.c.) in referring to S. polyantha the Baja Californian Cassia goldmani, an independently glabrate variant that is remotely allopatric and extratropical in dispersal. Britton & Rose attributed to Pterocassia goldmani leaflets narrower than those of Pt. polyantha and a glaucous pod. Subsequent collections have erased any difference in shape or width of leaflets between the continental and peninsular populations of the species, and the pod of C. goldmani is really not more than faintly pruinose. The leaflets of C. goldmani tend to be brighter green and of firmer texture than usual in interior Mexico, but this doubtless reflects the desert habitat. We can find nothing in the flower, pod or seed to distinguish it from genuine S. polyantha and speculate that it exists in protected canyons of Sa. de la Giganta as a relic of a once wider dispersal under moister conditions.