Monographs Details: Calliandra houstoniana (Mill.) Standl.
Authors:Rupert C. Barneby
Authority: 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.
Description:Species Description - Amply microphyllidious (sub)shrubs and bushy trees flowering when 1.5—6(—8) m tall, either virgately single-stemmed or branching upward, the trunk rarely attaining 1(—1.4) dm diam, the terete branches and the lf-axes either glabrous or thinly to densely pilose- pilosulous with erect, ascending or appressed, either white, or sordid-gray, or brown to bronze or black hairs mostly 0.2-1 mm, the lvs bicolored, the lfts green and commonly glabrous above, glabrous to appressed-pilosulous beneath, appressed-ciliolate, the inflorescence a stout and narrow or a shorter pyramidal, efoliate or at base few-lvd pseudoraceme of either condensed or umbelliform capitula of relatively large fls terminal to the hornotinous stem or to occasional lateral branches; phyllotaxy distichous. Stipules thinly herbaceous, those associated with primary lvs lanceolate or narrowly ovate ±3-9 x 1-2.5 mm, early caducous, absent from many specimens, those of axillary buds similar but smaller, loosely imbricate. Lf- formula (v—)vi—xxiv(—xxxi)/24—65(—69); lf-stk of major lvs (4.5-)6-20(-21) cm, the petiole including pulvinus 8-35(40) mm, at middle 0.8-1.7 mm diam, the longer interpinnal segments (4.5-)6-15(-21) mm, the ventral groove continuous between pinnae or obscurely bridged; pinnae either decrescent near each end of lf-stk or only proximally, or randomly short and long, the rachis of longer ones (1.8—)2—10(—14) cm, the longer interfoliolar segments (0.4-)0.6-2 mm; lft-pulvinules 0.2-0.45 x 0.35-0.6 mm, not wrinkled; lft-blades linear or narrowly linear-lanceolate from either truncate or subacutely to obtusely auriculate base, acute or acuminate or obtuse mucronulate, either straight or ± incurved distally, less often sigmoid-falcate, the larger ones (2.5—)3—10(—11) x (0.4—)0.6-2.4(-2.7) mm, (3.2-)4-8(-9) times as long as wide, all plane or low-convex on upper face; midrib either subcentric or displaced to divide blade ±1:2, either simple or (in broadest blades) weakly branched on one or both sides, posterior primary nerves commonly 0, if 1-2 then very short and weak. Axis of pseudoracemes (0.3-)0.6-3 dm; stipuliform bracts at each node of pseudoraceme ovate 2-4 mm, early papery caducous; peduncles fasciculate by 2-6(-7), at anthesis 2—13(—16) x 0.7-1.7 mm, in fruit sometimes to 2.6 mm diam, all ebracteate; floral bracts ovate or obtusely deltate 1.2-2.4 mm, either discrete or shortly connate at base, caducous or sometimes persistent through anthesis; pedicels at anthesis 0.7-8 x 0.35-2.5 mm, either terete or when short turbinate and as broad as long, in fruit 1-3.2 mm diam; fls of each capitulum homomorphic or nearly so, the perianth 5-merous (calyx sometimes 6-7-merous), the corolla firm or subcoriaceous, the calyx and corolla alike glabrous to micropuberulent, strigulose-puberulent, or densely pilosulous, the indumentum white to sordid, brown, bronze, black, or of mixed colors; calyx campanulate or shallowly campanulate-patelliform (1-)1.2-3.6(4.5) x 2.2-5.4(-6.5) mm, bluntly 5- angulate and often in addition weakly several-nerved, the teeth deltate, depressed-deltate, or narrowly ovate to triangular, obtuse, 0.3-1.6(-3) mm; corolla (6.5—)7—12(—13) mm, the lobes (3-)4-7(-8) mm, toward tip 0.2-0.6 mm thick in section, often in age separating down to rim of stemonozone; androecium 36-76(-100)-merous, 4-7(-8) cm, the thickened stemonozone 2-3.5 mm, the tube ±24(-5) mm, the tassel usually crimson-scarlet throughout, sometimes pale crimson, sometimes bicolored, then pink-crimson only beyond middle; intrastaminal nectaries not differentiated, the smooth inner wall of the stemonozone copiously nectariferous. Pods erect-ascending, in profile mostly (6-)7-12(-16) x (0.9-)1.2-2(-2.3) cm, in var. colomasensis only ±3.5-5 x 0.5-0.6 cm, the sutural ribs in dorsal view mostly 2.54.5 mm wide, the ribs and valves alike glabrous to puberulent or densely velutinous to pilose with brown, mixed brown and white, or largely white hairs to 0.3-1.2 mm, the valves weakly cross-venulose; ovules 8-11 but seeds seldom more than 8 per pod, the seeds in broad view elliptic-obovate or broadly obovate ±7 x 4—7 mm, the smooth hard testa light brown, often dark-mottled or -speckled, pleurogrammic.


The definition of C. houstoniana embodied in the foregoing description breaks with a long tradition of recognizing three, four, or several specific taxa in the group. Material available for study is now plentiful and lately has been enriched by intensive collections of D. J. Macqueen (OXF, generously shared with NY) in tropical Mexico and Central America. Since Britton and Rose’s summary in 1928, the close allies of C. houstoniana have been critically revised in their entirety only by Macqueen and Hernandez (1997), although McVaugh (1987) and Hernandez (1991) have recently presented thoughtful, even though inconclusive, analyses of variation within and between traditionally accepted taxa. The latter can still be recognized readily enough, but no longer as morphologically discrete entities, and are reevaluated here as varieties interconnected by intermediate forms. The main divisions of C. houstoniana sens. lat. are four, characterized as follows:

a. C. houstoniana sens. str.: Foliage relatively insensitive; leaflets distally incurved or falcate, highly lustrous adaxially, and the midrib decidedly displaced from mid-blade; vesture of inflorescence wholly or in part brown-bronze, fuscous, or black; pedicels relatively short and stout, the pseudoraceme consequently narrow and dense; and androecium uniformly red.

b. C. grandiflora, also known as C. anomala: Foliage more sensitive; leaflets straight or almost so, either acute or obtuse, with subcentric midrib; pinnae relatively short and numerous; vesture of inflorescence white through shades of gray, brown, and bronze to black, the color transitional in a northwest-southeast cline between Sinaloa, Mexico, and Honduras; and pedicels either longer or, if no longer, more slender than in the preceding, the pseudoraceme therefore more open; androecium of C. houstoniana sens. str.

c. C. calothyrsus: Foliage, lfts, and inflorescence of C. grandiflora, but perianth glabrous.

d. C. acapulcensis: Close to the preceding except for fewer pinnae and wider leaflets, puberulent perianth, and often bicolored androecium, the tassel rose-carmine in lower half, pallid proximally.

Each of the differential characters mentioned in this broad analysis is now known to vary independently of any other. Furthermore, each of the four idealized but only inexactly definable taxa varies internally in density, length, and orientation of vesture, in leaf-formula, in size and pubescence of leaflets, in size of perianth parts, in outline of flower-buds (from plumply to slenderly pyriform), and in depth of the sinuses between mature corolla-lobes. The number of potential character-syndromes seems indefinite, and those known from specimens are much more numerous than the available names. Most of the segregates described at the specific level by Britton and Rose (1928) are minor variants. Beside C. acapulcensis, only Anneslia colomasensis, notable for extremely short pinnae and small narrow pod, is provisionally preserved here as an independent taxon.

Particular notice must be taken that the North American species numbered 118 to 122 (incl.) in this account are coextensive with the seven species recently revised by Macqueen and H. Hernández (1997) under the title Calliandra ser. Racemosae. Contemporary but independent, these two revisions of one set of closely related taxa were undertaken and carried out in different contexts, mine within the framework of whole genus, the other more intensely focused on an assemblage of species of proven or potential value in tropical agroforestry, especially as sources of fuel. The synonymy and the definitions of taxa for the most part run parallel. Such conflict as exists is due to different emphasis on, or different interpretation of, a morphological polymorphism almost universal in the group, complicated by local hybridization between sympatric populations. The inconvenience of two competing taxonomies proposed almost simultaneously for one group of species is regrettable. In practice, that of Macqueen is strongly recommended for the taxa of economic importance. Insofar as my opinions presented herein are significantly different, they should be viewed in the light of a full generic survey. The classification of ser. Racemose by Macqueen and Hernández is supported by intensive fieldwork throughout lowland and tropical montane Mexico and Central America and is influenced by a study of seedling morphology. It is a pleasure and an agreeable duty to acknowledge Dr. Macqueen’s gift to NY of a substantial sample of his carefully selected and beautifully prepared specimens.