Narratives – The Barneby Catalogue

Calliandra Species Pages

Calliandra humilis

17. Calliandra humilis Bentham, London J. Bot. 5: 103. 1846. — Holotypus infra sub var. humili indicatur.

Functionally herbaceous, microphyll subshrubs with slender, loosely tufted or weakly assurgent to humifuse, mostly simple stems (0.1-)0.5-2.5(-3) dm from woody root and superficial or shallowly subterranean rhizomatous caudex, strigulose or pilosulous nearly overall with fine white hairs to 1 (—1.4) mm to nearly glabrous, the subconcolorous lfts glabrous on upper face, sometimes (especially in early lvs, sometimes in all) strigulose or pilosulous beneath, ciliolate or not, the small capitula either pedunculate or sessile and either solitary or paired in a succession of lf-axils (no brachyblasts), the lf-formula and lft-size also highly variable; phyllotaxy distichous. Stipules firmly herbaceous, linear-lanceolate or ovate-elliptic and straight to falcately lance-elliptic, 2-7 x 0.4-2.3 mm, coarsely 1-5-nerved, green or brown, turning dry persistent. Lf-formula (i—)ii/(6—)7 to x(-xi)/23(-27), most often within the extremes of ii-v/8-17, in n.-w. Mexico attaining vii-x/27-31, the number and size commonly mutually adjusted, the lvs often heteromorphic, the early ones (no further described) smaller simpler, the lf-stk of those associated with capitula 2-7(-8) cm, its ventral groove usually continuous between pinna-pairs, the petiole ±1-4 cm, the longer interpinnal segments 4—16(-20) mm; pinnae subequilong or distally accrescent, the rachis of longer ones (10-) 13-38 mm, the longer interfoliolar segments (0.45-)0.7-7(-9) mm; lft-pulvinules 0.2-0.45 mm diam, not wrinkled; lft-blades inequilaterally broad- elliptic, ovate-elliptic, lanceolate, or linear-lanceolate from shallowly obtusely auriculate base, obtuse when relatively broad but when narrow sometimes triangular-acute, the largest of a plant 3—10.5(—12.5) x 0.65-4(-6) mm, 2-3.8(-4.2) times as long as wide; venation of lfts primarily palmate, the primary nerves (l-)3 in smaller lfts, 5 in larger ones, the straight midrib forwardly displaced to divide blade 1:1.5-2, either 1- or 2-forked only well above mid-blade, the inner posterior primary nerve almost as strong, produced almost to tip of blade and enclosing with the midrib a nerveless elliptic areole, in ample lfts 1 anterior and 1-2 weak and short outer posterior primary nerves, all these immersed on blade’s upper face, obtusely prominulous beneath. Peduncles 0-32(-40) mm, either ebracteate or 1-bracteate close below capitulum, the capitula in some plants all sessile, in others all pedunculate, in yet others some of each, and then not rarely 1 sessile and 1 pedunculate from the same axil; capitula (2—)5—12-fld, the fls homomorphic, the receptacle not over 1.5 mm; bracts subulate or linear-attenuate 0.6-1.6 mm; pedicels mostly obsolete, but rarely 1 mm, exceptionally 1.5-2.5 mm; perianth 4- or 5-merous, gray-strigulose or -pilosulous overall to glabrous except for a few fine hairs at tip of lobes, the calyx 5-nerved or weakly 10-15-nerved, the corolla externally nerveless; calyx campanulate 1.9-3 x (0.75-)1.2-1.8(-2.4) mm, the orifice often asymmetrical, the longer teeth 0.3-0.7 mm; corolla narrowly campanulate (4.4-)4.8-6.4(-7.5) mm, the erect, often unequal lobes 0.7-1.3 mm; androecium (30-) 32-52(-78)-merous, (10—)11—15(—28) mm, the tube 1.8-4.4 mm, the stemonozone 0.55-1 mm, the tassel either pink or white; intrastaminal nectary in some bisexual fls to 0.4 mm; ovary subsessile, glabrous at anthesis. Pods subvertically erect, in profile (2.3-) 2.6-5.5(-5.8) x 0.5-0.7 cm, mostly 3-7-seeded, the sutural ribs in dorsal view 1.1-1.8 mm wide, the stiffly chartaceous valves becoming brown or stramineous, distantly transverse-venulose, either puberulent overall, or glabrous, or minutely granular; seeds plumply rhombic-oblong in broad view, 3.6-5.5 x 2.6-3.6 mm, the hard smooth testa gray, variably mottled or speckled, the pleurogram finely engraved.

In the short span of time between 1838 and 1853, botanists independently described as species four taxa that in recent years have been combined with good reason into one, taxonomically complex C. humilis, sens. lat. Two of these were encountered by European travelers at the south margin of the Mexican Plateau, in Hidalgo and Zacatecas, and were described in Europe. The two others were based by Engelmann and Asa Gray on collections of August Fendler and Charles Wright from New Mexico. Bentham (1875) perceived that only two substantial taxa, C. humilis and C. reticulata could be maintained, but as new material accumulated, from both southwestern United States and Mexico, these also proved to be geographically and morphologically confluent. Modern analysts (Benson, Isely, McVaugh) of the complex are unanimous in recognizing one species composed of two varieties differing ideally in leaf-formula, amplitude of leaflets, and some more trifling characters, but propose somewhat different definitions of these. Isely (1973) proposed for var. reticulata in United States a leaf-formula of i—ii(—iii)/5—6(—8) coinciding with leaflets 5—10(—12) mm long, in contrast to leaf- formula iii—vi/6—14(—20) in var. humilis, coinciding with leaflets 3-5 mm long. It is disconcerting to find in an isotype of C. reticulata (NY) pinnae bearing 11, 12, even 14 pairs of leaflets. In fact, leaf-formulae overlap between plants with relatively ample and distant, as opposed to small and crowded, leaflets. Supporting differential characters that have been sought in density of pubescence, prominence of venulation, and length of peduncles are all now known to vary independently and continuously, without any strong correlation with dispersal, for the range of var. reticulata is almost fully contained within that of var. humilis.

Nevertheless the extreme forms of var. reticulata are instantly recognizable. With their relatively low leaf-formula and especially with their larger, more prominently venulose leaflets are generally associated a condensed, even subacaulescent growth-habit, indumentum sparse or almost lacking, and capitula consistently sessile. The taxon reticulata may perhaps consist of neotenous variants of var. humilis that have arisen independently from the matrix of the species. Its strange, partly sympatric, partly vicariant dispersal within the whole range of C. humilis would harmonize with such a hypothesis.

Granted the significance of leaf-formula in separating vars. humilis and reticulata, it becomes necessary to recognize further a new variety, gentryana, which differs in yet more elaborately decompound leaf- blades and a vicariant dispersal on the west slope of Sa. Madre Occidental.

Calliandra humilis ranges further north than any other member of the genus, in Arizona north to the Coconino Plateau and in New Mexico to the upper Pecos valley in latitude 35°30'N, where it must survive frigid winter temperatures.

The key that follows will discriminate between most collections of var. humilis and var. reticulata, but obstinately individual specimens must be expected.

References: [Article] Barneby, Rupert C. 1998. Silk tree, guanacaste, monkey's earring: A generic system for the synandrous Mimosaceae of the Americas. Part III. Calliandra. Mem. New York Bot. Gard. 74: 1-223.