Senna versicolor

Title

Senna versicolor

Authors

Howard S. Irwin, Rupert C. Barneby

Scientific Name

Senna versicolor (Vogel) H.S.Irwin & Barneby

Description

95.  Senna versicolor (Vogel) Irwin & Barneby, comb. nov. Cassia versicolor Meyen [Reise 1: 483. 1834, nom. nud.] ex Vogel, Syn. Gen. Cass. 29. 1837.—Typus infra sub var. versicolori indicatur.

Densely leafy, clumped or mounded subshrubs becoming straggling shrubs or (in forest) spindly treelets, at anthesis (0.6-)1-4 m, appearing glabrous but the angulate hornotinous branchlets, lf-stalks and inflorescence at least thinly (rarely quite densely) pilosulous or strigulose with straight appressed or loosely ascending filiform hairs often mixed with short thickened livid trichomes, the thick- textured foliage strongly bicolored, the lfts dull olivaceous or yellowish-green above, darker green or brownish beneath, the inflorescence a squat pyramidal or subcorymbiform, immersed or shortly exserted panicle of racemes.

Stipules ascending or in age variably spreading or deflexed, thinly herbaceous becoming dry and brownish, in outline lance- or triangular-acuminate (4-) 5-18 x (1-) 1.5-6(-7) mm, symmetrical or at base quite inequilaterally dilated, the margins early replicate, the blades deciduous before the associated lf.

Lvs often crowded toward end of branches, (of some axillary branchlets (5-)8- 19(-22) cm, in expanded outline oblong-elliptic, the proximal and distal lfts slightly decrescent; petiole subangulate when dry, including discolored pulvinus (2-)4- 15(- 19) mm, at middle 0.8-1.8(-2) mm diam, openly shallowly grooved ventrally; rachis mostly 5.5-16(-18) cm, charged at insertion of lfts with a conspicuous tuft of spicules; gland between proximal pair of lfts erect or incurved stipitate, including stipe (1.5-) 1.7-3.4 mm, the ovoid to lancoid acute or obovoid claviform head 0.45-1 mm diam; pulvinules 1-1.7(-2) mm; lfts (6-)8- 13(- 15) pairs, in outline narrowly ovate, lanceolate or narrowly lance-elliptic, obtuse mucronu- late or subacute, the longer ones (1.8-)2-3.6 x (0.45-)0.5-1.1 cm, (2.3-)3-4.2(-5) times as long as wide, at base inequilaterally rounded or broadly cuneate, the margins plane, the coarse centric midrib prominent beneath only, the venation of upper face fully immersed, but 6-11 pairs of camptodrome with rare intercalary secondary veins sometimes faintly raised or immersed but discolored beneath.

Racemes (7-)12-45-fld, the 1-3 simultaneously open fls raised ± to level of the nodding buds, the axis including peduncle becoming 5-16(-22) cm; bracts resembling stipules in texture, lance- or ovate-acuminate (3-)4-9.5 x (0.8-)1.2-3 mm, deciduous as the pedicel elongates; pedicels at and after anthesis 1.4-3(-3.3) cm; buds subglobose, remotely strigulose-pilosulous or the sepals merely ciliolate; sepals submembranous pale yellow commonly fuscous-tinged, oblong-obovate not much graduated, the outer pair (4-)5.5-8, the inner (5-)6.5-9 mm; petals yellow glabrous, broadly oblanceolate or obovate-flabellate, the vexillary one often a little broader than the rest, the longest 11.5-16 mm; androecium glabrous, the filaments of 4 median stamens 2-3.5 mm, of 2 latero-abaxial ones (3-)5-8 mm, of centric abaxial one 2.5-5 mm, the anthers of 4 median stamens 3-5 mm, of 2 latero-abaxial ones (3.5-)4-7 x 1.2-1.7 mm, of the centric abaxial one (3.2-) 3.8-5 x 0.7-1.4 mm; ovary pilosulous; style linear incurved (0.8-)1-3.5 mm, at apex 0.2-0.5 mm diam, the stigmatic cavity terminal; ovules (10-) 12-28.

Pod obliquely spreading-declined from ascending pedicel, the stout stipe 5-15 mm, the broadly linear straight or decurved body 5-11 x 0.9-2 cm, bicarinate by the stout sutures, the stiffly papery brown valves low-corrugate over the seeds, the cross-section elliptic; seeds either narrowly or broadly, but always plumply obovoid 4.5-8 x 3-4.7 mm, the broadly elliptic-obovate areole 1.7-4.8 x 0.81.5 mm.

Along the central and southern Peruvian Andes between Ancash and Lake Titicaca, around which it extends a short distance into Bolivia, S. versicolor is the most common and abundant senna of the puna formation, descending only sporadically on river gravels and rocky quebradas below 2800 meters. Dried specimens of it are readily recognized by the curious reversed coloration of the leaflets which, contrary to the common rule in Senna, are brighter or lighter green above than beneath. In general habit it resembles the sympatric but less common S. birostris var. huancavensis, which see for comment on differences. Despite this resemblance, the sharply differentiated areole on the seed faces of S. versicolor suggests less affinity with S. biros tris than with the four Andean sennas described next below.

Beyond the range just mentioned S. versicolor reappears locally and, to our present knowledge, disjunctly near 7°S latitude in the Maranon valley in Cajamarca, and again near latitude 2-3°S in the Andes of Ecuador. The Ecuadorian plant, of which the pod is still as yet unknown, cannot at present be separated from common Peruvian S. versicolor, but that from Cajamarca differs abruptly in number of ovules and size of seeds, and cannot be taxonomically ignored. It is described below as var. heterosperma.

Our concept of S. versicolor differs somewhat from that of our predecessors. Bentham considered Meyen’s plant from Titicaca as the same as Bridges 10 and Mandon 751, relatively small-flowered sennas here referred respectively to S. aymara and S. birostris var. controversa, which see for discussion. The exceptionally clear photograph of the holotypus, now lost, includes a cleanly laid out flower from which the petals can be measured at near 15 mm long, altogether too long for either S. aymara or S. birostris. Furthermore we now have almost exact topotypes of S. versicolor in the shape of Ugent 5252 (NY) which to our mind leave no doubt as to the identity of the species, the only senna of its type which has been collected in the past century on the shores of the lake. Thus Bentham’s S. versicolor is a composite without fixed identity. Macbride, on the other hand, referred (1943, p. 168) C. versicolor and similar material from southeastern Peru to Cassia hookerana, which he thought perhaps only varietally distinct from C. latopetiolata, supposed to differ (in key) in the less unequal filaments of the stamens, the less acute leaflets, and the subsessile pod (the stipe described, nevertheless, as "scarcely 5 mm long"). We have found Macbride’s key characters of no assistance in defining two entities in the Peruvian material, and inaccurate if used to distinguish genuine Peruvian S. versicolor from S. birostris var. hookerana of southern Bolivia and Argentina. In equating C. versicolor with C. latopetiolata, which were published simultaneously, we have preferred to perpetuate in Senna the former epithet as the more descriptive and memorable, calling to mind the reversed discoloration of the leaflets.