Mimosa microcephala

  • Title

    Mimosa microcephala

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Mimosa microcephala Humb. & Bonpl. ex Willd.

  • Description

    285. Mimosa microcephala Humboldt & Bonpland ex Willdenow, Sp. pl. 4: 1039. 1806.—Typus infra sub var. microcephala indicatur.

    Amply microphyllidious shrubs l-4(-5) m, armed or not and in habit from free-standing and either erect or diffuse to sarmentose or vinelike, the stiff livid-brown stems in subsp. microcephala unarmed, in subsp. cataractae erratically armed on some or all intemodes and also along back of lf-stks with usually recurved, sometimes straight, spreading or antrorse aculei l-2.5(—4) mm, in both subspp. the stems, lf-stks and axes of inflorescence coarsely strigose with appressed or narrowly ascending yellowish-brown setae to 0.5-2 mm attached laterally near or below middle and tapering at each end, these mixed or not with gray puberulence, the foliage subconcolorous, the narrow, closely imbricate lfts facially glabrous, ciliolate or not, the inflorescence an efoliate or proximally few-lvd pseudoraceme or panicle of pseudoracemes exserted 1-2.5 dm above foliage, the very small globose or ellipsoid capitula fasciculate by 2-5 at each node. Stipules erect firm, triangular or lanceolate (1—)1.5—5 x 0.6-1.4 mm, either finely 3-5-nerved dorsally or nerveless. Leaf-stalks of major lvs 7-14(-16) cm, the petiole including firm livid pulvinus 4-21 x 0.8-1.8 mm, the longer interpinnal segments 3-11 mm, the narrow shallow ventral groove interrupted between each pinna-pair by an ascending linear-lanceolate glabrous or puberulent spicule 0.7-2.4 mm; pinnae of major lvs mostly 10-30-, rarely 7-10-jug., subequilong or a little decrescent proximally, the rachis of longer ones 1.2- 5(-5.5) cm, the longer interfoliolar segments 0.3-0.9 mm; lfts of longer pinnae 20-60-jug., decrescent only at very ends of rachis, the first pair 0.5-1 mm distant from pulvinus (paraphyllidia 0), the blades linear or linear-elliptic acute or obtuse, the longer ones (2-)2.3-6.5 x 0.4-0.9 mm, 4-9 times as long as wide, all veinless above, beneath finely 3-nerved, the midrib subcentric, the lateral nerves (faint in young lfts) submarginal, produced nearly to blade apex. Peduncles very slender 3-11 mm; capitula without filaments 2.5-4.5 mm diam., prior to anthesis moriform, the ovoid or narrowly ovoid obtuse fl-buds either glabrous or puberulent; bracts ovate or lance-subulate 0.4-0.9 x 0.2-0.6 mm, 1-nerved ciliolate, persistent; flowers 4-merous 8-androus, fragrant; calyx 0.3-0.5 mm, the obscurely or very shortly deltate-lobulate rim either glabrous or minutely ciliolate; corolla (brown or pinkish-brown when dried) turbinate, in some capitula all staminate, in others staminate proximally and bisexual distally, the larger ones 1.6-2.5 mm, the plane triangular-ovate, 1 - or 3-nerved lobes 0.8-1.3 x 0.55-0.8 mm; filaments either pink fading whitish or white from the first, free to base or almost so, exserted 3.5-5(-7) mm. Pods ±1—4 per capitulum, sessile or attenuate at base into a pseudostipe, cuspidate at apex, in profile linear, linear-elliptic or undulately linear, when well fertilized (9-)10-60(-90) x 3-5 mm, (2-)3-10(-16)-seeded, the replum 0.35-0.6 mm wide, together with stiffly papery valves strigulose or hispidulous with forwardly ascending or appressed setae to 0.5-1.4 mm and sometimes in addition minutely puberulent, when ripe breaking into rhombic-oblong or elliptic, biconvex, individually dehiscent, free-falling segments 4-7 mm long; seeds plumply lentiform 2.3-2.8 x 1.7-2.2 mm, the fuscous testa smooth lustrous.

    The concept of M. microcephala is here expanded to include aculeate M. cataractae Ducke and analyzed into two geographic subspecies and five varieties. As indicated by Irwin (1966: 103), the differences in leaf-formula and seed-number (inadvertently reversed in Irwin’s discussion) between M. microcephala and M. plumaeifolia Kleinh., which is indistinguishable from M. cataractae, are indecisive. The aculeate plants have been described as relatively tall sarmentose shrubs and the unarmed ones as smaller savanna bushes, but field-labels suggest that in open places such as rock-islets in river rapids the prickly ones can form either stiff free-standing bushes or trailing subshrubs, not materially different, except in presence of aculei, from genuine M. microcephala in comparable habitats.

    The close relationship of M. microcephala is with polymorphic M. somnians, likewise either armed or unarmed, from which it differs principally in the 1- or locally 3-nerved, not 5-several-nerved corolla lobes, and in the lateral attachment of the cauline setae, a feature that has passed unnoticed in the literature. Kindred of more local dispersal and native in similar riverine and outcrop habitats in Amazonia and the Guayana Highland are M. calliandroides, similar in all respects except for dilated and basifixed setae of stems and leaf-stalks, M. surumuensis distinguished by plumose setae, and M. brachycarpoides, similar in setae but different in greatly altered leaf-formula.