Mimosa debilis

Title

Mimosa debilis

Authors

Rupert C. Barneby

Scientific Name

Mimosa debilis Humb. & Bonpl. ex Willd.

Description

326.  Mimosa debilis Humboldt & Bonpland ex Willdenow, Sp. pl. 4(2): 1029. 1806.—Typus infra sub var. debili indicatur.

Herbs and weakly frutescent subshrubs, at first or permanently erect, or as often diffuse or scrambling and finally thicket-forming or sarmentose, at maturity commonly 8-20(-25) dm but potentially flowering precociously and then appearing monocarpic, rarely reported laticiferous, the terete or (var. amnis-mortium) quadrangular stems and lf-stks commonly at once armed with scattered recurved brown-tipped aculei 1-3.5 mm and hispid with fine yellowish or sordid setae to ±1-2 mm mixed or not with fine puberulence (but aculei, setae, and fine trichomes, or some of them, not infrequently lacking), the bicolored lfts brownish-olivaceous or leaden-glaucescent above, paler beneath, either subequally strigose-pilose on both faces, or on dorsal face only, rarely quite glabrous, the solitary or geminate, rarely 3-4-nate globose capitula forming a leafless or basally leafy-bracteate, simple or few-branched raceme exserted ±1-4 dm from foliage. Stipules lanceolate or ligulate (2.5-)3.5-8 x 0.7-2 mm, the firm blades prominently 3-5(-9)-nerved, dorsally either densely pilose, or puberulent, or glabrous, setose-ciliate, glabrous within, persistent. Leaf-stalks (0.6-)l- 6.5(-7) cm, at middle 0.4-1.3 mm diam., 1- or exceptionally 2-sulcate ventrally, commonly espiculate (but spicules usual in var. amnis-mortium and random elsewhere); pinnae 1-jug., the rachis including pulvinus 4-20 mm, the first of 2 pairs of lfts 0.5-2 mm distant from linear-lanceolate or narrowly ovate paraphyllidia 1- 2.5(-3) mm; lfts of distal pair obliquely obovate to oblanceolate, less often lance-elliptic, elliptic, or ovate-acuminate (17-)20-60(-65) x (6-)7- 23(-30) mm, at base semicordate, at apex obtuse mucronulate to deltate-apiculate, all palmately 3-5-nerved from pulvinule, the secondary nerves from the variably excentric, forwardly incurved midrib incurved-ascending nearly or quite to margin of blade, either not or feebly brochidodrome, the discontinuous margin with forwardly appressed setae free for ±0.6-1.5 mm, all venation prominulous and usually pallid beneath, above scarcely raised or fully immersed. Peduncles 5-20(-22) mm; capitula without filaments (4-)4.5-7.5 mm diam., prior to anthesis either conelike and canescently setose-pilosulous, or moriform; bracts linear-attenuate or linear 0.6-3.5 x 0.2-0.5(-0.6) mm, distally either glabrous or dorsally gray-pilosulous with very slender setae to 0.2-1.4 mm, densely or at least residually setulose-ciliolate, persistent; fl-buds distally gray-puberulent, sometimes minutely so; flowers 4-merous 4-androus, most or all bisexual; calyx 0.15-0.4 mm, the rim glabrous or minutely ciliolate; corolla narrowly funnelform or subtubular 2-2.6 mm, the ovate concave, scarcely thickened 1-nerved lobes 0.5-0.7 x 0.4-0.6 mm; filaments lilac-, rose- or magenta-pink, free to base, exserted 4-7.5 mm; ovary at anthesis either glabrous or puberulent. Pods radiating in a nearly always dense cluster, sessile, in profile oblong-elliptic or broadly linear 10-18(-20) x 3-4.7(- 5) mm, 2-4-seeded, the scarcely constricted replum 0.3-0.6(-0.7) mm wide, at apex abruptly short-mucronate or submuticous, except in var. parapitiensis armed on back and sides with erect brown- or purple-tipped, tapering setae to (0.6-)1.5-3.5(-4) mm, the thin greenish, later stramineous or purplish-brown and papery valves low-colliculate over each seed and variably setose or both puberulent and setose (glabrous), when ripe breaking up into free-falling articles 3-5 mm long; seeds plumply compressed-ovoid 2.3-3.5 x 1.8-3 mm, the smooth testa fawn-brown or fuscous, dull.

The foregoing circumscription of M. debilis embraces all so-called Sensitivae that have small narrow hispid pods combined with leaflets lacking the continuous rimlike marginal nerve characteristic of M. nuda sensu lato. It is arbitrarily distinguished from M. albida by hispid, not strigose pods, a feeble character of convenience that is, however, strengthened by vicariant dispersal. When much material of M. debilis and M. nuda is assembled, it is possible to trace a gradual transition between a continuous and a discontinuous marginal nerve, but the percentage of ambiguous individuals is small, and where the two species are sympatric, on and to the south and west of the Brazilian Planalto, the distinction is ordinarily sharp.

My definition of M. debilis allows within the species the same visually distracting but, on analysis, clearly random variation in armament and hispid or puberulent vesture that occurs in M. albida and M. nuda, and an equivalent or even greater range in outline, absolute size, and relative width of the terminal leaflets. The floral bracts vary from much shorter than the flower-buds to much longer than them, when they form a cone in praefloration. Leaflet-outline and bract length appear to be stabilized in parts of the whole range of the species, but elsewhere are independently erratic. Strictly typical M. debilis, found in lowland savanna habitats in the Orinoco and Amazon basins, has ample obovate leaflets and conelike capitula. Morphologically similar plants occur locally on the Brazilian Planalto, especially around Belo Horizonte in Minas Gerais and in the Federal District, where they are sometimes directly sympatric with M. nuda var. glaberrima. A form with small oblanceolate leaflets and moriform capitula, found to the exclusion of other members of subser. Mimosa in Costa Rica, Panama, and northeastern Colombia (Meta) has been segregated in Central America as M. panamensis (Benth.) Standi., and in that limited geographic context seems distinct indeed. But entirely similar plants are frequently encountered in eastern Bolivia, in a region where the whole group of Sensitivae is especially diverse and populous. Attempts to define these ample- and small-leaved populations are defeated by random occurrence, out of geographic context, of intermediate states; moreover plants combining ample leaflets with moriform capitula and small leaflets with conelike capitula are known.

It remains an open question whether morphologically similar but remotely disjunct populations of M. debilis represent independent but parallel modifications, in situ, of a common ancestor, or are genetically homogeneous and artificially dispersed. Many of the Sensitivae are adapted to unstable ecotonal habitats and flourish in disturbed environments. They seed prolifically, and the hispid one-seeded articles of the fruit seem designed to promote distant zoochorous dispersal. It is a suspicious circumstance that the known range on the Planalto of broad-leaved, cone-headed var. debilis is confined to a small segment of Sa. do Espinhaço in Minas Gerais and to the Federal District in the central highlands, at elevations between 900 and 1600 meters. Except for Pohl 2213 (W) collected long ago near Goiás Velho there is no record known to me for the present state of Goiás, and no obvious explanation for the failure of an often ruderal plant to colonize more widely the ostensibly hospitable cerrado province. These planaltine populations may exemplify secondary dispersal in M. debilis, and may eventually become more widespread on the Planalto. Similarly the Central American "panamensis" could be derived from Bolivian parentage. But speculation on this topic has little factual basis.

While I feel obliged to disregard some of the more eye-catching diversity, which seems impervious to organization into discrete units, I find it convenient and probably realistic to disentangle from the mass what appear to be five distinctive races of M. debilis, separable as follows: