Mimosa rufescens

  • Title

    Mimosa rufescens

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Mimosa rufescens Benth.

  • Description

    6. Mimosa rufescens  Bentham, Trans. Linn. Soc. London 30: 418. 1875 & in Martius, Fl. bras. 15(2): 362. 1876.—Typus infra sub var. rufescenti indicatur.

    Macrophyllidious lianas attaining 12-25(-35) m but precociously fertile and in disturbed habitats flowering first as a diffuse bush or incipiently sarmentose shrub (2-)3 m upward, the trunk said to be strongly armed but the subterete purplish-brown homotinous branchlets together with lf-stks and axes of ample efoliate panicle either unarmed or commonly thinly armed with scattered recurved aculei less than 1 mm, the young stems with lf-and inflorescence-axes densely minutely rufous-lepidote throughout and commonly also puberulent or sordid-pilosulous, the lvs conspicuously bicolored, above brown (when dry), either lustrous and glabrous or dull and puberulent, beneath paler brown, golden-green or olivaceous and minutely reddish-lepidote, proximally pallid-barbellate along primary nerves, the effuse pyramidal panicle of small white globose (exceptionally short-spicate) capitula exserted 1.5-4 dm above foliage. Stipules subulate 0.5-2 x 0.2-0.45 mm, early deciduous, perhaps sometimes obsolete. Leaf-stalks (5-)7-19(-23.5) cm, the petiole (2.5-)3-8(-10) cm, at middle 1-2.4 mm diam., the ventral groove either continuous or bridged between pinnae (but spicules 0), the longer interpinnal segments l-3.5(-4.5) cm; a laterally compressed pyramidal, low-conical or low-mounded, small-pored nectary near base of petiole and similar smaller ones near tip of all lf-stks and of most pinna-rachises; pinnae (3-)4- 10(-l 2)-jug., distally accrescent, the rachis of furthest pair 4-8 cm, the longer interfoliolar segments 7-20(-22) mm; lfts of longer pinnae 3-9 (-1 l)-jug., strongly accrescent distally, the first pair 1-7 (-11) mm distant from obscure conic paraphyllidia not over 0.3 mm, the blades passing upward from small rhombic to much larger and broadly obliquely rhombic-obovate or almost semi-obovate, at base asymmetrically cuneate, asymmetrically rounded or obliquely sub- truncate, broadly obtuse at apex, variable in amplitude according to position on lf, those of shade-lvs larger and of thinner texture, the terminal pair (14-)16-45(-55) x (8-)10-35(-42) mm, (1.25—)1.3—1.7(—2) times as long as wide, all 3(-4)-nerved from pulvinule, the slightly displaced, straight or forwardly arched midrib giving rise on each side to 3-5 secondary nerves brochidodrome well within the revolute margin, the inner posterior primary nerve incurved-ascending nearly to or short of mid-blade, the weak outer one narrowly intramarginal, much shorter, the tertiary venulation faint, all nerves immersed on upper face of lft, prominulous dorsally. Peduncles fasciculate by 3-7, the longer ones 4-9 mm, minutely bracteolate; capitula without filaments 3.5-4.5(-5) mm diam., prior to anthesis moriform, the fl-buds usually puberulent, sometimes glabrous, the receptacle normally ±1.5 mm, exceptionally elongating to 6 mm; bracts ovate 0.35-0.65 mm, persistent; flowers mostly 5-, some 4-merous, all diplostemonous, all or almost all bisexual, sweetly fragrant; calyx submembranous campanulate (0.4-)0.5-0.8 mm, puberulent externally, the rim minutely deltate-lobulate; corolla turbinate-campanulate 1.4-2 mm, whitish, the ovate or lanceolate, faintly 1-nerved lobes 0.5-0.9 x 0.35-0.45 mm, recurved at anthesis; filaments white or creamy-white, subequilong, the longest 1-2 mm longer than corolla-lobes; anthers (0.2-)0.25-0.3 mm. Pods l(-2) per capitulum, sessile or abruptly contracted at base into a stipe less than 2 mm, in profile oblong or broad-linear, straight or subdecurved, piano-compressed, when well fertilized (6-)7-12(-l 3.5) x (1 -7—) 1.8—3.3 cm, 10-16-seeded, the crenately indented, puberulent replum unarmed or rarely (A. Gentry 10709, NY) remotely randomly retro-aculeolate and 0.5-0.8 mm diam., forming an elevated rim around the flat papery brownish- olivaceous valves, these discolored and raised as a pimple over each seed, minutely red-lepidote overall but otherwise glabrous, when ripe breaking up into free-falling, individually indehiscent, narrowly oblong articles 4.5-7(-8) mm long and 3-5 times as wide; seeds transverse at middle of pod, in broad profile elliptic-obovate 5.5-7 x 2.5-3.5 mm, the brownish-olivaceous testa smooth; cotyledons green, between plates of endosperm.

    Mimosa rufescens is closely akin to M. guilandinae sens. lat., from which it appears essentially different only, although as a rule decisively, in more numerous and smaller leaflets. Leaf-formulae as low as iii-iv/4 (from Sa. dos Carajás in Pará, e.g., M. F. F. da Silva 1311, NY) or ii-iv/ 4-5 (from Loreto, Peru, Croat 17527, NY) meet or overlap the range observed in M. guilandinae var. spruceana, which is then only mechanically separable by a characteristic pilosity of the leaflets. It is theoretically arguable that M. rufescens is no more than a variety of M. guilandinae, or even the generalized prototype from which M. guilandinae arose through modification and simplification of the foliage and through progressive elaboration of the peltate trichomes. The geographic dispersal offers some support for this view, for M. rufescens is dispersed over almost the whole range of the several races of M. guilandinae sens. lat. and might have given rise by parallel evolution to the paucifoliolate mimosas here classified as units within M. guilandinae.

    By chance it was the lowest extreme of leaf- formula that was first discovered and described as M. micracantha, a variant that has proved to be elusive and highly localized in southwestern Venezuela and adjoining Brazil, and that may represent no more than a brief extension of the curve of variation in leaf-formula manifested by M. rufescens var. rufescens. Mimosa micracantha (a posterior homonym) is here reduced to a variety amnis-nigri of M. rufescens, but I am very uncertain as to its biological significance. Observation of whole populations is needed to establish modes of variation in leaf-formula between individual plants at different stages of development and between leaves of single plants produced low and high on the vine. The ambiguous nature of the type of M. micracantha is apparent from the fact that Bentham associated it not with multifoliolate M. rufescens but with early collections of M. guilandinae var. duckei obtained on the Solimões and lower Madeira rivers by Poeppig and Riedel.