Monographs Details: Senna pallida var. shreveana 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 - Shrubby or potentially arborescent, xeromorphic, glabrous or residually strigulose-pilosulous, with habit and stipules of var. palmeri, the firm pallid glauces- cent foliage ample after rain, the 2-fld racemes then axillary and shorter than the subtending lf, in dry spring months flowering again from depauperately microphyllous brachyblasts arising from lvs already fallen, the fls then well exserted. Primary lvs (absent from many specimens) 5.5-9 cm; petiole 12-24 mm, ±2.5-3 times as long as first interfoliolar segment of lf-stalk; rachis 2.5-5.5 cm; glands between proximal and rarely also the second pair of lfts 1-2.2 mm, the head 0.30.5 mm diam; lfts 4-7(-8) pairs accrescent upward, obovate obtuse to oblanceolate deltate-acute, the distal pair 13-27 x (4-)6-10 mm, mostly 1.8-3, rarely 4-4.5 times as long as wide, the secondary venation of 5-8 pairs of camptodrome (with rare and random faint connecting venules) visible on one or both faces; lvs of brachyblasts much smaller, 1-5.5 cm, the longest lfts up to 6-16 mm. Peduncles (4-)6-26 mm; pedicels 8-25 mm; long sepals 5.5-8.5 mm; long petals 16-22 mm; anthers of 4 median stamens 3.5^.8 x 1.4-1.8 mm, the beak of 3 abaxial ones 1.3-2.3 mm. Stipe of pod 4-6 mm, the body (5-)6-11.5 x 0.4-0.55 cm; seeds 3-3.4 x 2.3-2.7 mm, the areole 0.4-0.7 x 0.4-0.55 mm.-Collections: 27. [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. Lfts or larger primary cauline lvs 2-8(-9) pairs, perceptibly and usually conspicuously accrescent distally, the terminal pair longest; areole of seeds 0.4-0.9 mm in greatest diam. 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 in desert and thorn-forest below 600 m in Sonora and Sinaloa; valves of pod simply mounded over the seeds."]

Distribution and Ecology - Thickets and stony hillsides along draws and dry washes in the first foothills of Sierra Madre Occidental and adjoining coastal plain, commonly in drought- deciduous thorn-forest, below 600 m, locally abundant between 26° and 28°30'N in s.-w. Sonora and n.-w. Sinaloa; in the middle foothills to the east, on Rios Bavispe and Mayo, passing freely into var. geminiflora and in lowland centr. Sinaloa into var. pallida.

Discussion:The range of var. shreveana lies in the zone of transition between the Sonoran Desert proper and the thorn forest of the Sinaloan coastal plain. It seems likely to represent a recent offshoot of the otherwise intertropical var. pallida which has invaded a genuine desert climate and become somewhat modified in the process, the leaflets being here of thicker texture so as to reveal superficially only the costa and its pinnate secondary venation. It has adapted further, like var. gaumeri on the seasonally desiccated karst limestones of Yucatan, to an abundant flowering in the dry spring season, January-March, from microphyllous brachyblasts, as well as a conventional fall one, September to January, from axils of primary leaves. In the latter stage var. shreveana closely resembles south-Mexican var. palmeri, but this becomes readily distinguished in due course by the broad pod. In the middle foothills of Sonora var. shreveana passes gradually also into the more mesophytic var. geminiflora, which see for further comment.
Distribution:Sonora Mexico North America| Sinaloa Mexico North America|