Mimosa arenosa

  • Title

    Mimosa arenosa

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Mimosa arenosa (Willd.) Poir.

  • Description

    61. Mimosa arenosa (Willdenow) Poiret, Encycl. suppl. 1: 66. 1810.—Typus infra sub var. arenosa indicatur.

    Potentially arborescent shrubs and trees attaining 6 m with trunk to 15 cm diam., sometimes precociously flowering as diffuse or scrambling subshrubs 1-2 m, either unarmed or erratically armed on intemodes (but only exceptionally on back of lf-stks) with erect, recurved or (var. lysalgica) antrorsely curved, broad-based aculei (1—)2—6 mm, the young branchlets, lf-stks and axis of inflorescence pilosulous with soft fine incurved or spreading, whitish or cream-colored hairs to 0.2-0.5 mm intermixed with few or many minute livid granules, the thin-textured plane, facially glabrous or thinly pilosulous lfts bicolored, olivaceous above, paler and minutely pallid-dotted beneath, the moderately dense spikes of white, mostly glabrous but rarely puberulent fls at first axillary to expanded lvs, often later forming a shortly exserted pseudoraceme or narrow panicle. Stipules firm, narrowly lanceolate or linear-attenuate 2-5 x 0.4-0.9 mm, 1-3-nerved, the midrib becoming bluntly prominulous dorsally, the blade long persistent. Leaf-stalks 2-11 cm, the petiole including livid pulvinus 5-15 mm, at middle 0.3-0.7 mm, the longer interpinnal segments (4—)5—12(-l 3) cm, the ventral groove bridged between pairs of pinnae and spiculate or not; pinnae of larger lvs (cave: absent from some specimens) 4-10(-12)-jug., decrescent proximally, the rachis of longer ones 16- 45(-60) mm, the longer interfoliolar segments 0.7-2.5 mm; lfts of longer pinnae 12-30(-42)- jug., the first pair 0.6-1.5 mm distant from subulate paraphyllidia directed retrorsely from axis of pinna, all in outline narrowly oblong or linear, acute or abruptly acuminulate, straight or subfalcate, the longer ones 3-6.5 x 0.7-2 mm, 3-6 times as long as wide, all veinless or almost so on upper face, beneath finely 2—4-nerved from pulvinule, the slightly displaced costa discolored but mostly immersed except just above the pulvinule, the 1 or in var. leiocarpa 2-3 posterior nerves weak and short. Flower-spikes solitary or commonly geminate (at times paired with a branchlet), the axis including very short peduncle (3-)3.5-7 cm; bracts linear or linear-elliptic 0.350.9 mm, caducous; fl-buds plumply obovoid, glabrous or rarely puberulent; flowers 4-merous 8-androus, in some spikes many staminate; calyx membranous, shallowly or shortly campanulate 0.35-0.75(-0.85) mm, contracted at base into a minute drum-shaped pedicel no longer than wide, the low-deltate, glabrous or ciliolate lobes 0.1-0.2 mm; corolla white, broadly turbinate 2-2.5 mm, the ovate subacute lobes 1-1.4 x 0.7-0.9 mm, not or scarcely callous at apex; filaments white or cream-white, exceptionally pallid-roseate in fading, exserted (3-)3.5-5(-6) mm; ovary either glabrous or thinly pilosulous laterally, commonly granular. Pods loosely ascending, stipitate, the slender glabrous or thinly pilosulous, straight or arcuate stipe 4-8 mm, the planocompressed linear body (26-)30-60(-70) x 4- 6(-7) mm, cuneately contracted at base, more abruptly apiculate at apex, the obscurely or shallowly undulate replum 0.3-0.5(-0.8) mm wide, the glabrous or early glabrate, often remotely granular but not viscid, delicately venulose valves low-colliculate over each seed, at first green lustrous, then fuscous-stramineous, scarcely 0.05 mm thick in section, when ripe separating from replum and breaking up into 7— 10(— 11) segments 4-6 mm long; seeds obtusely rhombic in wide profile, 3.2-4.3 x 2.5-3.5 mm, the fuscous-olivaceous or light brown testa microscopically granular.

    Mimosa arenosa resembles in many respects the partly sympatric M. tenuiflora, which differs, however, in resinously glandular foliage, hooded calyx- and corolla-lobes, and viscid-glandular pod bullately distended over each seed. Like M. tenuiflora, and for no more pressing reason, the present species has been known by different names in its widely dissevered areas of dispersal north and south of Amazonia, but the hazy distinctions in form of inflorescence and proportions of flower-parts by which Bentham (1875: 416) distinguished a Brazilian M. malacocentra from a Venezuelan M. arenosa can no longer be maintained. Most Venezuelan populations of var. arenosa have slightly longer (0.5-0.8, not 0.35-0.5 mm) and therefore more deeply campanulate calyces, but I have found no supporting differences.

    Alike in Venezuela and Brazil the leaflets of M. arenosa vary from glabrous ciliolate to pubescent on both faces, but the modes of variation are different. In Venezuela the two types are erratically sympatric; both occur commonly around Caracas, and have been collected together in Lara (Pittier 13095, NY). In Brazil pubescent leaflets seem to occur only locally in north-central Minas Gerais and are commonly accompanied by cauline aculei curved upward, of a type foreign to the species elsewhere. I recognize this form as a weakly differentiated var. lysalgica.

    In arid northeastern Colombia and northwestern Venezuela the species is represented by plants of a slightly anomalous type to which the name M. leiocarpa has been applied. Bentham (1875: 416) maintained this provisionally as a species but admitted to an inexact memory of the typus, collected at Sta. Marta by Bertero. He thought that M. leiocarpa might be separable from M. arenosa by greater number of pinnae and glabrous foliage, but these distinctions have lost cogency in the course of time. We now know that the leaf-formula on Sta. Marta is quite plastic and, as already noted, glabrous and villosulous forms of M. arenosa are sometimes sympatric and otherwise identical. Nevertheless M. leiocarpa does differ from typical M. arenosa in some small features of the foliage and may appropriately be maintained as a third variety. Disjunct populations of M. arenosa in southern Pacific Mexico seem also to belong here.