Mimosa somnians Humb. & Bonpl. ex Willd.
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1991. Sensitivae Censitae. A description of the genus Mimosa Linnaeus (Mimosaceae) in the New World. Mem. New York Bot. Gard. 65: 1-835.
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Family
Mimosaceae
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Scientific Name
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Type
276. Mimosa somnians Humboldt & Bonpland ex Willdenow, Sp. pl. 4(1): 1036. 1806.-Typus infra sub var. somniante indicatur.
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Description
Species Description - Erect, assurgent, scrambling or rarely prostrate subshrubs, shrubs and treelets at anthesis (2-)4- 25(-40) dm, the trunk exceptionally attaining 2 cm diam., the wiry terete or strongly ribbed branches together with lf-stks and axes of inflorescence varying from subglabrous to densely pubescent and often in addition aculeate, the indumentum composed in variable density of: a) minute incurved villi, b) forwardly appressed strigae 0.2-2 mm, c) erect gland-tipped setulae 0.2-1 mm, d) erect or widely ascending plain setae, and e) scattered (not infrastipular) straight or gently curved, broad-based aculei to 1-4.5 mm, the firm plane lfts glabrous facially and ciliolate or not, rarely puberulent dorsally, the simply pseudoracemose or paniculate inflorescence of subglobose capitula either proximally foliate with diminished lvs or wholly exserted from foliage. Stipules ovate, ovate-acuminate, lanceolate or lance-attenuate 2-7(-10) mm, prominently several-nerved from base, the margin glabrous or ciliate, the dorsal face either glabrous or densely gray-puberulent. Leaf-stalks 2-18 cm, the petiole commonly 4-45 mm, rarely subobsolete or to 6-14 cm, the longer interpinnal segments (4-)6-25(-30) mm; pinnae (1—2)1—3-, 2-3-, 2-5-, 4-8-, exceptionally in some ample cauline lvs to 13(-15)-jug., the rachis of longer pinnae 10-55 mm; between each pinna-pair an ovate entire or denticulate (2-cleft), triangular-subulate or rarely setiform scale (spicule) 0.4-1.5(-2) mm; lfts of longer pinnae 13-19-, 20-30-, 25-45-, 3050-jug., inserted 0.4-1.3 mm apart, sub vertically imbricate in sleep, subequilong except at very ends of rachis, the first pair less than 1.5 mm distant from subulate paraphyllidia, the blades narrowly oblong or linear-oblong obtuse or sub-apiculate, those near mid-rachis (2-)2.5-7(-7.5) x (0.4-)0.6-1.4(-1.8) mm, all dorsally 2-4- nerved from pulvinule, the subcentric midrib giving rise on each side to (1—)2(—3) secondary venules, the venation pallid but not strongly elevated, in some smaller lfts obscure. Peduncles solitary or 2-3 together, (0.5-)0.8-3(-3.5) cm; capitula without filaments (4-)4.5-7(-8) mm diam., prior to anthesis commonly moriform but in uncommon varieties conelike, the fl-buds varying from obovoid to less often pyriform; bracts varying from narrowly elliptic-oblanceolate to ovate 0.5-4 x 0.2-2 mm, 1-several-nerved dorsally, usually glabrous dorsally, sometimes gray-pilosulous; flowers 4(-5)-merous diplostemonous, the lowest in each capitulum often smaller and staminate; calyx usually membranous (0.25-)0.3-0.55(-0.7) mm, with entire or obscurely lobulate rim glabrous or sometimes minutely ciliolate or gland-fimbriolate, rarely (subsp. longipes) firm striolate 0.9-1.6 mm denticulate; corolla turbinate, that of bisexual fls (1.7-)2. l-3.5(-4) mm, the submembranous tube 0.9—1.5(—1.7) mm, the firm stramineous or reddish, ovate or lanceolate obtuse or subacute, shallowly concave lobes (1-)1.2-2(-2.2) x 0.61.1 mm, striately 7-9-nerved; filaments pink, monadelphous through less than 1 mm, exserted (3.5-)4-7 mm. Pods usually several per capitulum, in profile linear, attenuate at both ends, straight or slightly curved, the stipe commonly (2-)3-23(-30) mm but at times no longer than the marcescent corolla and rarely obsolete, the body when well fertilized (25-)35-90(-l 40) x 35.5, rarely 6-7 mm, (6—)7—15-, rarely 18-20- seeded, the shallowly undulate replum 0.3-0.5 mm wide, the brown, red or purplish-castaneous, when ripe papery, dull or lustrous valves either glabrous (but then often granular), or simply setulose, or both strigose and villosulous, or both villosulous and ascending-setulose, or stipitate-glandular, finally breaking up into square, oblong or oblong-elliptic, bullately biconvex, free-falling, individually dehiscent articles (2.5-)3-8, rarely 11-13 mm long; seeds 2.5-3.6(-4) x 1.7- 2.9(-3.5) mm, turgid at middle, compressed toward margin, the brown or black testa smooth lustrous or minutely roughened.
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Discussion
The foregoing description attributes to M. somnians unprecedented variation in leaf-formula, in development of floral bracts, and in pubescence or armament of stems and leaves. Leaves of M. somnians are diverse in length of leaf-stalk, in proportionate lengths of petiole and rachis, in number and length of pinnae and, generally correlated with length, in number of leaflets per pinna. A great part of the whole range of foliar variation may be seen in some individual plants between lower, middle and upper leaves of one stem, the leaves at mid-stem being generally larger and more elaborately divided than the rest, that is to say composed of more numerous but not necessarily (or often) longer pinnae. The floral bracts of M. somnians are commonly narrow, 1-3-nerved and not over half the length of the associated flower-bud just prior to anthesis. Occasionally they may attain the length of the buds without acquiring accessory nerves, and quite rarely are dilated and several-nerved, when they resemble striate stipules in form and texture. In the extreme bracteose states, exemplified to varying degree by var. lupulina and var. velascoensis, the capitulum becomes conelike prior to anthesis, but this condition merely culminates a graduated series and provides an insecure specific criterion. I arbitrarily assign greater weight to the further modification of the lower bracts into a fused involucral collar and consequently exclude from M. somnians the clearly derived M. poculata and M. monacensis.
The cuticular emergences of M. somnians are of five sorts, all common in Mimosa: a) minute villi; b) forwardly appressed strigae; c) fine setae capped with a livid nectary; d) vertical or widely ascending plain setae, often tawny in color; and e) laterally compressed, straight or incurved prickles which appear to be homologous with plain setae and, when short or slender, not clearly distinguishable from them. Expression of these types of trichome is capricious, and each is encountered in almost every possible combination of absence, presence, density and dispersal in plants otherwise alike. In search of correlations between indumentum and other phenetic characters or with geographic dispersal I successively sorted all study material by presence/absence of each type of trichome, without a priori assumptions as to taxonomic significance. It turned out that in this complex fine villi, as expected from general experience of the genus, are absolutely fickle and independent of other characters. More surprisingly, the visually and tactually arresting viscid setae can come and go suddenly without correlated changes. For example, prickly var. somnians between Mexico and Panama is prevailingly glandular, but random eglandular populations (Hinton 11337 from Guerrero; E. Nelson 2723 from Oaxaca; Davidse 2329 from Nicaragua, all NY) not otherwise different, are not rare in this region. The type-collection of M. somnians, from the Magdalena valley in Colombia, is likewise a rare eglandular variant among prevailingly viscid stock. Again, copious collections of var. viscida and var. leptocaulis from the environs of Brasília contain pairs of plants identical except in this one respect. On the other hand, a dense hispid but eglandular pubescence, sometimes mixed with weak aculei, and linked with short petioles and often gray-villosulous stipules and corolla-lobes, appears to be characteristic of the planaltine subsp. lasiocarpa.
It is readily apparent that prickles vary greatly in size and number between plants of a given region and even between parts of one plant, waning at times to the proportions of scarcely dilated setae; but while prickly plants are encountered through the whole immense range of the species, from Mexico to Argentina, unarmed ones are not. A dispersal map differentiating solely between aculeate and non-aculeate M. somnians shows the former alone at and near the periphery of the species-range, but relatively rare in eastern Brazil; whereas the unarmed forms are concentrated in great numbers and variety on the Planalto and extend outside it only feebly to northeastern Brazil and Bolivia, and quite sporadically further. The disparate ranges of aculeate and unarmed M. somnians suggest divergent histories, or different proclivities for dispersal (or survival), or both.
Turning from indumentum to leaf-formula, I again sorted the material successively by number of pinnae, by length of pinna-rachis, and by number of leaflets per pinna. Number of pinnae appears to be higher on the average in aculeate than in unarmed M. somnians, yet is so capriciously variable, even at times between leaves of one stem, that it must be admitted an ineffectual, even deceptive character, despite tradition to the contrary. Length of pinnae and leaflet-number are at least approximately linked, and together are more stable than number of pinnae per leaf, both on a single plant and among otherwise like plants, whether aculeate or not. Flowers and fruits, the latter almost as variable in indumentum as the stems, are characteristic of ser. Somniantes sensu lato and offer no tangible characters at or below the specific level.
In his first paper on Mimosa Bentham (1842) described four aculeate and (in what I here define as M. somnians) six unarmed members. In his monograph (1875) and Flora Brasiliensis he acknowledged that supposed differences between members of the armed group had broken down and had no geographic plausibility; but he continued to maintain the unarmed ones, adding a seventh. Modem collections referable to Bentham’s unarmed group demonstrate intergradient morphologies of the same order as, though not quite coextensive with, those admitted to M. somnians sensu lato of Bentham’s last revision.
Little is known in detail about the ecology of M. somnians, but what is known and what can be inferred from dispersal and collectors’ comments is suggestive. Prickly subsp. somnians is a weedy opportunist often abundant in seasonally swampy lowland habitats, a precociously flowering herb potentially bushy or scrambling at maturity. The unarmed planaltine subspecies are primarily plants of upland campo habitats, with functionally herbaceous stems arising yearly from lignescent rootstocks, doubtless selected by intermittent fire but nevertheless still capable of acquiring, in some cases and in protected microhabitats, leafless woody trunks. This broad generalization must be qualified to admit particular exceptions, such as specialized unarmed var. aquatica, or specialized aculeate var. deminuta.
Reflecting on the morphological variation within M. somnians, which superficially appears chaotic, I discerned three major and one minor, approximately concentric circles of affinity, here titled subspecies, each successively less widespread spatially and here modified edaphically. In this view subsp. somnians, in which all types of indumentum are at least potentially realized and which has, by and large, the more elaborate leaf-formula, emerges as the primitive stock, which has proved sufficiently plastic to endure extremes of climate between subtropical and hy- laean without loss of identity. From this must have arisen by selection unarmed and commonly glabrescent subsp. viscida, ubiquitous in highland eastern Brazil and extending outside the cerrado climax only into ecologically related campo or rupestral habitats; the ecologically similar and sympatric but less widespread subsp. lasiocarpa, in which simple setose eglandular indumentum has become nearly stabilized; and finally the very local subsp. longipes, which has its own characteristic calyx and foliage, but may be aculeolate or not.