Senna sousana

  • Title

    Senna sousana

  • Authors

    Howard S. Irwin, Rupert C. Barneby

  • Scientific Name

    Senna sousana H.S.Irwin & Barneby

  • Description

    200bis. Senna sousana Irwin & Barneby, sp. nov., inter S. atomariam et S. mollissimam quasi intermedia, hanc foliolis ovato-acuminatis, illam racem- is ineunte anthesi erectis abbreviatis subcorymbosis ipsisque floribus parvulis simulans, ab ea inflorescentiae forma florumque magnitudine, ab ilia foliolorum ambitu racemisque omnibus e folii coetanei axillo nascentibus diversa, ab ambabus ecologia montano-temperata abstans.-MEXICO. Oaxaca, distr. Sola de Vega: Sola de Vega, 1300 m, 23.VI.1979 (fl), M. Sousa et al. 10489.- Holotypus, MEXU; isotypus, MO.

    Drought-deciduous trees (3-)4-14 m tall, the stout annotinous branchlets, foliage and immersed panicle of racemes densely pilosulous and tomentulose with fine curly and incumbent golden hairs up to ±0.5-1 mm, the membrano-chartaceous lfts bicolored, brownish-olivaceous and relatively thinly pubescent above, densely so or at first pannose-tomentose beneath, the racemes arising singly or geminate from axils of relatively short synanthous lvs, the later lvs longer.

    Stipules erect, subulate or lance-triangular 3-5 x 0.5-1.5 mm, firm, except from inhibited budlike brachyblasts early caducous.

    Lvs accompanying fls 9-22 cm, the later ones 15-40 cm; petiole 3-12 cm x 0.5-2 mm, appearing terete, the shallow ventral sulcus concealed by vesture; rachis 1.5-18 cm, its interfoliolar segments (0.6-)l-5 cm; pulvinules 2-5 mm; lfts 2-5 pairs, accrescent distally, ovate abruptly deltate-acuminate varying to ovate-subelliptic, the distal pair (2-)3.5-13 x 2.5-5 cm, at base subsymmetrically rounded or broadly cuneate, the margin re volute, the midrib with (8-)10-15 pairs of camptodrome secondary and reticular venules all at first immersed but sometimes becoming prominulous above, sharply raised beneath.

    Racemes ±10-30-fld, the several simultaneously expanded fls raised almost to the level of the crowded nodding subglobose, glabrate or pilosulous fl-buds, the erect axis at full anthesis 3.5-11 cm; bracts triangular-acuminate or lance-caudate 3-5.5 mm, early caducous; pedicels at anthesis 13-18 mm; sepals fuscous yellow- margined, strongly graduated, the outermost ovate 3.5-5 mm, the innermost broadly obovate concave 5.5-8 x 4.5-5.5 mm; petals yellow drying dark or brownish-yellow dark-veined, 4 subhomomorphic except for size, the 3 adaxial and 1 abaxial (opposed to pistil) broadly oblanceolate, the 3 similar ones ±9-11 x 4-5 mm, the abaxial one 12-16 x 5-6.5 mm, the fifth petal much larger and strongly oblique, its claw thickened, its blade both incurved through nearly 90 and folded inward over the androecium, measured along midvein ±15-17 mm; androecium of S. atomaria, the thinly pilose anthers 2.8-3.4 x 1.3-1.7 mm; ovary glabrous except for random hairs along ventral suture; style incurved 1-1.5 x 0.5-0.6 mm; ovules ±48.

    Pod pendulous, the stout stipe ±2 mm, the thickly linear, laterally compressed body (18-)25-37 x 0.9-1.3 cm, in all respects like that of S. atomaria or S. mollissima.-Collections: 10.

    Wooded mountainsides and stream banks in pine-oak or oak-alder forest, on limestone, 1300-1850 m, locally plentiful near the crest of Sa. Madre del Sur in distr. Sola de Vega and Ejutla, Oaxaca, Mexico.-Fl. II, IV-VIII.-Vainillo.

    This handsomely floriferous but taxonomically ambiguous senna suggests a hybrid derivative of S. atomaria × mollissima, and at first was tentatively so interpreted by us. Its mature foliage is that of S. mollissima, but in form of inflorescence and in size of the individual flower it appears much closer to S. atomaria. The abundant and uniform material assembled by Dr. Mario Sousa and his students at MEXU, however, cannot be passed over so lightly, for it testifies eloquently to the existence of a self-productive entity locally abundant in a cool montane environment (1300 m upward) in which neither S. atomaria nor S. mollissima could be expected to flourish. Both of these two species are primarily or obligately at home in the tierra caliente, S. mollissima below 300 (one record at 800) m and S. atomaria, a little more accommodating in its requirements, ascending from sea level, in matorral and low deciduous woodland or thornforest, up to about 1100 m, beyond that contour only feebly, on south slopes or in arid interior basins. MacDougall, to be sure, collected S. atomaria somewhere near Sola de Vega (NY, s.n.) where it may possibly come into marginal contact with S. sousana, but S. mollissima cannot be expected at such elevations. Moreover S. atomaria and S. mollissima var. glabrata are sympatric along the Pacific coast between southern Sinaloa and the isthmus of Tehuantepec without giving rise anywhere to known hybrid forms. Some of Dr. Sousa’s collections were provisionally named Cassia xanthophylla (B. & R.) Lundell, the type of which, an immature collection from Guatemala superficially similar because of the dense golden pubescence of the young foliage, is referred herein to S. mollissima var. glabrata, of which it has the precociously elongating and pendulous raceme-axis. Although distinguished from S. atomaria chiefly by leaf-form and habitat, S. sous ana seems to have acquired a specifically independent identity.

    We take pleasure in dedicating the species to Dr. Mario Sousa, Jefe del Herbario Nacional, Instituto de Biologia, UNAM, equally well known for his energetic and fruitful exploration of the flora of Oaxaca as for his generosity in sharing the results with colleagues.