Senna mexicana (Jacq.) 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.

Family

Caesalpiniaceae

Scientific Name

Senna mexicana (Jacq.) H.S.Irwin & Barneby

Type

Typus infra sub var. mexicana indicatur.

Synonyms

Cassia mexicana Jacq.

Description

Species Description - Shrubs, at anthesis commonly 0.5-3, rarely arborescent to 5("-8") m, varying from almost glabrous except for remotely puberulent young branchlets and lf- stalks to pilosulous throughout with forwardly incumbent, spreading or subappressed hairs up to 0.15-0.5(-0.8) mm usually mixed with some minute thickened or verrucular (granular), pallid or discolored trichomes, the foliage either concolorous or moderately bicolored, then dull yellowish-olivaceous above, olivaceous beneath, the lfts varying from equably pubescent on both faces or beneath only to glabrous (when sometimes dorsally verruculose) or glabrous except for a tuft of hairs in basal dorsal angle of midrib, when densely pubescent often ± distinctly papillate, the inflorescence of axillary racemes commonly at first shorter than lvs but distally, by reduction of foliage, forming a thyrsoid or corymbiform panicle. Stipules thinly herbaceous, early dry caducous, lance-elliptic, lance- or ovate- acuminate 1.3-6 x 0.3-1 mm. Lvs 3-20 cm, varying in size and number of lfts as described under the varieties, the lfts of larger lvs (2-)3-16 pairs; petiole, measured from stem to petiolar gland, (4-)6-23(-30) mm, at middle 0.4-1.4 mm diam, like the rachis trigonously 3-ribbed and openly shallow-sulcate ventrally; gland either between proximal pair of lfts or, these lacking or caducous, on lf-stalk ±1/2-way between pulvinus and proximal pair, sessile or subsessile (in var. shaferi distinctly stipitate), the reddish-brown body plumply or slenderly ovoid-globose obtuse 0.3-1.3(-l.5) mm diam, nigrescent when dry; pulvinules 0.7-2 mm; lfts at least slightly, often markedly accrescent and proportionately narrower distally, the distal pair obliquely lance- oblong, oblong- or ovate-elliptic, or ovate, 12-54 x (3.5-)4-20 mm, at apex abruptly deltate-acute to broadly short-acuminate or obtusely rounded mucronulate, at base asymmetric, rounded or cordate on proximal and cuneate to obscurely rounded on distal side, the lower lfts when shorter proportionately broader, the midrib immersed or depressed above, cariniform beneath, the (3-)4-11 pairs of camptodrome secondary veins either immersed or faintly prominulous above, immersed or sometimes sharply prominulous beneath, a tertiary venulation 0 or raised dorsally only. Racemes (2-)3-16-fld, the axis together with short peduncle becoming 0.5-7.5 cm; bracts lanceolate, resembling stipules, 1.2-3.5 mm, caducous; pedicels at maturity 7-23(-26) mm; fl-buds obliquely obovoid or subglobose, glabrous, puberulent or densely pilosulous, sometimes also minutely verruculose; sepals greenish or brownish with pallid membranous margins, moderately graduated, in outline obovate, oblong-obovate or broadly oblanceolate, the inner ones 5.5-8 mm; petals yellow glabrous, drying stamineous or whitish dark-veined; banner obovate-obcordate, the rest obovate or oblanceolate from short claw, the 3 adaxial either a trifle longer or shorter than the 2 abaxial, the longest 9-14 mm; androecium glabrous, functionally 6-merous, zygomorphic, the 3 adaxial members staminodal, the central abaxial one rudimentary sterile, the filaments of 4 median stamens 0.8-2.2 mm, of 2 latero-abaxial ones dilated, ribbonlike 4.8-6.5 mm, the anthers of 4 median stamens commonly yellowish, nearly straight (3.3-)3.5-5.2 mm, obliquely truncate 2-porose, those of 2 fertile abaxial ones lunately incurved, commonly brown (often yellow-tipped), (4.6-)5-6.5 mm, sagittate at base, dehiscent by 2 pores into a half-circular pollen-cup; ovary pilosulous or strigulose, rarely subglabrous; style 2-3 mm, at tip hamately incurved and dilated, at the ciliolate-barbellate stigma 0.3-0.5 mm diam. Pod ascending or erratically pendulous, the stipe 1-4 mm, the linear compressed but turgid body (5-)6-11 x 0.45-0.9 cm, straight or commonly gently decurved, the stiffly papery valves shallowly convex over the seeds, depressed between them (often more deeply so between every second seed on each face), the interseminal septa up to 0.4-1 mm wide, standing 2-3.5 mm apart; seeds obovoid or oblong-ellipsoid, moderately compressed, 2.8-4.5 x 1.7-2.5(-2.7) mm, pinched laterally at the hilum, the dull or moderately lustrous, brownish- olivaceous testa crackled, the linear-oblong or oval-elliptic areole 1.2-2.4 x 0.5-1.1(-1.3) mm.

Variety Key - Key to the Varieties of S. mexicana 1. Larger lvs 7-20 cm; lfts (8-)9-16 pairs. Pod 4.5-6 mm wide; Hispaniola and Jamaica. 134a. var. berteriana (p. 416). 1. Larger lvs 3-12.5(-14) cm, if over 9 cm the lfts not over 7 pairs; lfts (2-)3-7(-8) pairs. Hispaniola, Cuba, Bahamas and Florida. 2. Margin of lfts not thickened; petiolar gland sessile or almost so; range as just given. 3. Lfts 5-7(-8) pairs, those of distal pair 13-28 x 4-10 mm; peduncle with raceme-axis together 1-3.5 cm; pod 4.5-6 mm wide; Hispaniola. 134b. var. mexicana (p. 416). 3. Lfts (2-)3-6, of most lvs exactly 4 or 5 pairs, those of distal pair 24-54 x 7-20 mm; peduncle with raceme-axis together (1-)1.5-7.5 cm; pod 5.5-8(-9) mm wide; Cuba, Bahamas, Florida. 4. Lfts relatively narrow, the distal pair prevailingly lance-elliptic, (24-)30-54 x ±715 mm, ±3-4.5 times as long as wide; Cuba, n.-w. Bahamas, Florida. 134e. var. chapmanii (p. 417). 4. Lfts relatively broad, the distal pair mostly ovate-oblong (22-)24-40 x 12-20 mm, 1.8-2.4 times as long as wide; s.-e. Bahamas, Caicos and Turks Is. 134d. var. latifolia (p. 417). 2. Margin of lfts thickened as a raised rim; petiolar gland stipitate, phalloid. Pod ±8 mm wide; Cuba (Oriente).

Discussion

Cassia chapmanii (=C. bahamensis sensu Bentham, non Miller), its var. latifolia Benth., C. shaferi B. & R., C. mexicana Jacqu. (including C. clarendonensis Britt.) and its var. moustiquensis Urb. form a replacement series extending from southern Florida through the Bahamas and Caicos islands to Hispaniola and the north coast of Cuba, with an outlying station on Jamaica. The flowers, except for some variation in pubescence of the sepals, their androecia, and the pods and seeds of all are essentially identical in form, and the tangible differences are in the foliage alone, the leaflets, relatively few and long northward, becoming progressively smaller, or more numerous, or both, to the south and east, a trend loosely correlated with shortening of the raceme-axis and narrowing of the pod. We find them so closely related as to be best treated collectively as forming a megaspecies consisting of five geographical races, with contiguous but well differentiated ranges of dispersal. The closely related S. benitoensis (q.v.) of eastern Cuba should perhaps also be included but is still too poorly known. Key to the Varieties of S. mexicana 1. Larger lvs 7-20 cm; lfts (8-)9-16 pairs. Pod 4.5-6 mm wide; Hispaniola and Jamaica. 134a. var. berteriana (p. 416). 1. Larger lvs 3-12.5(-14) cm, if over 9 cm the lfts not over 7 pairs; lfts (2-)3-7(-8) pairs. Hispaniola, Cuba, Bahamas and Florida. 2. Margin of lfts not thickened; petiolar gland sessile or almost so; range as just given. 3. Lfts 5-7(-8) pairs, those of distal pair 13-28 x 4-10 mm; peduncle with raceme-axis together 1-3.5 cm; pod 4.5-6 mm wide; Hispaniola. 134b. var. mexicana (p. 416). 3. Lfts (2-)3-6, of most lvs exactly 4 or 5 pairs, those of distal pair 24-54 x 7-20 mm; peduncle with raceme-axis together (1-)1.5-7.5 cm; pod 5.5-8(-9) mm wide; Cuba, Bahamas, Florida. 4. Lfts relatively narrow, the distal pair prevailingly lance-elliptic, (24-)30-54 x ±715 mm, ±3-4.5 times as long as wide; Cuba, n.-w. Bahamas, Florida. 134e. var. chapmanii (p. 417). 4. Lfts relatively broad, the distal pair mostly ovate-oblong (22-)24-40 x 12-20 mm, 1.8-2.4 times as long as wide; s.-e. Bahamas, Caicos and Turks Is. 134d. var. latifolia (p. 417). 2. Margin of lfts thickened as a raised rim; petiolar gland stipitate, phalloid. Pod ±8 mm wide; Cuba (Oriente). 134c. var. shaferi (p. 417).