Monographs Details: Senna pallida var. isthmica 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 2: 455-918.
Description:Variety Description - In aspect quite like var. pallida, except for venation of lfts, at anthesis 2-3 m, the lfts usually pubescent beneath, sometimes glabrous above, rarely so on both faces except for costa ciliate beneath, the hairs up to 0.25-0.55 mm, the racemes mostly borne on leafless branchlets axillary to coeval lvs, in dry springs borne on brachyblasts furnished with greatly diminished foliage. Stipules herbaceous narrowly oblanceolate or linear-lance-acuminate 3.5-12 x (0.5-)0.6-2 mm. Major lvs 5.5-12 cm; petiole 9-20(-22) mm, at middle 0.4-0.8 mm diam, 2-3 times as long as first segment of lf-stalk; rachis (3-)3.5-6 cm; gland between proximal (exceptionally also second) pair of lfts stipitate, the stipe puberulent, the whole in profile 2-3.3 mm, the head 0.5-1 mm wide; lfts (6-)7-10 pairs accrescent upward, the distal pair obovate, oblong-obovate or oblanceolate 1530 x 6-13 mm, 1.9-3.1 times as long as wide, the secondary venation of ±6-8 pairs of camptodrome veins fully immersed above, discolored but not raised beneath, tertiary venulation essentially 0. Peduncles 7-17 mm; pedicels 10-18 mm; long sepals 5-7 mm; long petals 1422 mm; filaments pilosulous or glabrous, those of 4 median stamens free 0.7-1.2 mm, of 3 abaxial ones 2-3.8 mm, the anthers of 4 median stamens 4-5 mm, the beak 1.3-2 mm; style 1-1.7 mm. Stipe of pod 3-4 mm, the body 5-10.5 x 0.27-0.33(-0.35) cm, the interseminal septa 2.5-3 mm distant, the valves elevated over each seed as a sharply defined 4-armed pyramid; seeds 2.3-3.1 x 2.1-2.7 mm, the areole 0.4-0.6 x 0.3-0.4 mm.-Collections: 11. [Key: "Appendage terminal of lf-stalk subulate or setiform, not modified and dilated into a gland; range of whole species.Continental N. and S. America and Lesser Antilles, s. Windward Is. and Swan Is. in s. Caribbean; anthers nearly always stouter (1.2-)1.3-2 mm diam, abruptly truncate at apex and the tubules there commonly umbonate at the angle opposed to the laterally divaricate, often slightly infra-terminal beak, if whole anther less than 1.25 mm diam and umbo pronounced. Reticulate tertiary venulation imperceptible or almost so on both faces of lfts, these superficially either penninerved or veinless. Scattered over s. Mexico and Guatemala. Plants of Pacific Mexico to Chiapas and highland Guatemala. Pod 4-5.5 mm wide; either Sonora-Sinaloa, or Oaxaca-Guatemala. Habitat and dispersal differesnt, in Oaxaca, Chiapas and Guatemala, if below 1000 m the valves of pod raised over each seed into a sharply defined decussate pyamid. Matorral and thorn-forest at 5-700 m in e. Oaxaca and adj. Chiapas; valves of ripe pod pyramidally eleated over each seed."]

Distribution and Ecology - Disturbed thorn-forest, brushy hillsides and pasture thickets, 5-600 m, locally plentiful on the Coastal Plain and foothills of the Pacific slope of Isthmus of Tehuantepec in s.-e. Oaxaca, e. sparingly to the source valleys of Rio Grijalva on the Gulf slope in w. and n.-w. Chiapas.-Fl. X-V, sporadically later.

Discussion:The var. isthmica is precariously distinguished from var. pallida by the simple pinnate venation of the leaflets, but is peculiar for the syndrome of dilated stipules, relatively small flowers and very narrow pods, characters that occur, it is true, elsewhere in var. pallida but nowhere else, it seems, so constantly associated. The variety resembles distantly allopatric var. longirostrata and var. shreveana in leaflet venation, but differs in the broad stipules and in the sharply decussate pyramidal boss that develops early over each seed, a feature characteristic of var. pallida. It differs from var. longirostrata in ecology and therefore in dispersal, var. isthmica being an element of the xeromorphic lowland scrub and thorn-forest of the Isthmus coastal plain and neighboring foothills there and in lowland valleys of Chiapas, whereas var. longirostrata is adapted to the cooler, primitively wooded uplands of central and southern Guatemala. The var. quiedondilla, another form of S. pallida with similar foliage, has a wider flat pod and large seeds with correspondingly large areole. A few flowering specimens from middle elevations in Chiapas appear, nevertheless, for the present ambiguous between the two.
Distribution:Oaxaca Mexico North America| Chiapas Mexico North America|