Senna birostris var. birostris

  • Title

    Senna birostris var. birostris

  • Author(s)

    Howard S. Irwin, Rupert C. Barneby

  • Scientific Name

    Senna birostris (Dombey ex Vogel) H.S.Irwin & Barneby var. birostris

  • Description

    110f. Senna birostris (Vogel) Irwin & Barneby var. birostris. Cassia birostris Dombey ex Vogel, 1837, l.c., sens. str.—"Hbt. in Chili: Dombey leg. (. . . Hrb. Kth. . . .)."—Holotypus, †B = F Neg. 1657; neoholotypus, collected by Dombey at Chinchina, prov. Huarochiri, dept. Lima, Peru, P! isotypus, hb. Pavon. s.n., MA! presumed isotypus, Dombey 836, G!—Chamaefistula birostris Dombey ex Macbride, Field Mus. Bot. 13, part 3(1): 158, pro syn. 1943.

    Cassia birostris sensu Bentham, 1871, p. 540; Macbride, l.c., exclus. pl. amazonica.

    Bushy shrubs at anthesis 4-15 dm, the young stems, lf-stalks and inflorescence thinly strigulose-pilosulous with pale straight or incurved hairs up to 0.2-0.7 mm, the dull olivaceous, relatively thin-textured lfts glabrous above, either glabrous or remotely pilosulous beneath, the racemes either lateral or corymbose-paniculate but all leafy-bracteate, either early elongating, the open fls then standing well below the racemose buds, or compactly pseudo-corymbose; stipules narrowly linear-attenuate 4-11 x 0.3-0.7 mm; lvs 7-15(-21) cm; petiolar gland either between or shortly above proximal pair; lfts 7-12 pairs, on pulvinules 0.7-1.4 mm, ovate- or elliptic-oblong and obtuse to deltately subacute or broadly ovate- or obovate-emarginate, the longest 11-24(-32) x 5-ll(-12)mm, 1.5-2.7(-3) times as long as wide, veinless above, the dorsally prominent midrib giving rise to 4-6(-7) pairs of fine, immersed and discolored or, toward the pulvinule, sub- prominulous secondary veins; racemes (5-)7-15(-18)-fld; sepals glabrous or almost so, little graduated, the outer (4.5-)5-7 mm, the inner 6.5-8.5 mm; longer petals 9-13 mm; filaments of 2 latero-abaxial stamens 6-11 mm, of the centric abaxial one 3-7 mm, the anthers of all 3 subequal 5-7.8 x (1-) 1.3-1.8 mm; ovary pilosulous; style 1.4-2.2 mm; ovules ±12-14; stipe of pod 4.5-7.5 mm, the body ±6-8 x 1-1.2 cm.—Collections: 17.

    Stony clay hills and quebradas in the loma formation, 370-2000(-2250) m, interruptedly frequent along the Pacific slope of the Peruvian Andes from s. Ancash (Huarmey) to w. Arequipa (Caraveli), lat. ±10°-16°S.—Fl. irregularly through the year.

    In this variety the androecium of S. birostris attains its greatest asymmetry, the long pair of abaxial stamens protruding their relatively massive anthers well beyond the petals, an arresting feature of the flower that must have suggested the epithet. These long stamens are, however, only on the average slightly longer than those of var. arequipensis, which see for comment, and var. helveola, which replaces var. birostris at slightly greater elevations on the Andean plateau to the east and northeast. The latter differs from var. birostris principally in the more abundant, often yellowish pubescence of foliage and inflorescence and in the strongly graduated, dorsally pilosulous sepals. The pod of none of these varieties is well known, but the few examples we have appear essentially similar.

    Many populations of var. birostris differ from other races of the species in the early elongating raceme axis, which carries the developing flower buds well beyond the level of the open flowers; others, however, have more condensed corymbiform racemes normal for the species. The different types of inflorescence are not related to elevation of habitat, for both are known from the foothills below 750 m and from near the altitudinal limit of 1800-2000 meters. Dombey’s original Cassia birostris was of the condensed, apparently less common type.