Cojoba arborea
Rupert C. Barneby
2. Cojoba arborea (Linnaeus) Britton & Rose, N. Amer. Fl. 23: 29. 1928. Mimosa arborea Linnaeus, Sp. Pl. 519. 1753. — Typus infra sub var. arborea proponitur.
Unarmed, multifoliolate trees (5-)7-25(-60) m tall with trunk attaining 1.5-10 cm dbh and gray furrowed bark, the new stems, lf-axes, and peduncles densely pilosulous with loosely incurved-ascending or straight, widely spreading, brown or in age sordid-gray hairs to 0.15-0.35 mm, the lf-buds dark-brown-velvety, the lvs bicolored, the thin-textured lfts rich green sublustrous above, pale beneath, commonly quite glabrous or facially so and minutely ciliolate but sometimes remotely minutely strigulose on either face, when dry usually loosely involute and shallowly concave ventrally, the dense globose capitula of greenish, white-stamened fls rarely solitary, more often fasciculate and arising sequentially in axils of mature coeval and of recently new-fallen lvs, hence either immersed in or surpassed by foliage. Stipules obsolete Lf-formula iii—xviii(—xxii)/( 11—) 13—42(—50); lf-stks of well-developed lvs (2-)2.5-28(-36) cm, the petiole ±1.5-4.5(-7) cm, the longer interpinnal segments (6-)7-27 mm; a sessile, round or elliptic, cupular thick-rimmed nectary 0.6-1.7 mm diam between (or immediately below) each pair of pinnae and often l(-2) near midpetiole, exceptionally a similar but minute nectary at tip of some pinna-rachises; pinnae decrescent proximally, less or scarcely so distally, the rachis of longest pair mostly 5-10 cm, in amplest lvs (especially of sapling- or shade-branches) to 12-16 cm, the longer interfoliolar segments 1.4-4 mm; lf-pulvinules of each lft-pair subcontiguous on ventral side of pinna-rachis, 0.3-0.6 x 0.5-0.8 mm, transversely few-wrinkled, the lfts sessile against rachis; lfts a little decrescent proximally, thence subequilong, the blade inequilaterally lanceolate or narrowly oblong from shallowly semicordate-auriculate base, either obtuse, deltately subacute, or more often acuminate, those near mid-rachis (6.5—)7—15(—17) x (1.5-) 1.6-5 mm, (2.8-)3-5.2(-5.6) times as long as wide; venation weak and often indistinct except for slender, subcentric or slightly forwardly displaced, straight or distally falcate midrib, when evident (on hypophyllum only) palmate and pinnate, the posterior nerves from pulvinule 2-3, the inner one expiring or faintly brochidodrome short of midblade, the few secondary nerves from midrib widely ascending or subhorizontal, an extremely fine random tertiary venulation sometimes raised on lower face. Peduncles (l-)2- 8 per fascicle, the longer ones (2-)3-8 cm, all charged near or above middle with a caducous bracteole; capitula (25-)30-60-fld, the sessile homomorphic 5-merous fls spreading in all directions from a subglobose or plumply clavate receptacle 2—5.5 mm; bracts spatulate with incurved tip, 0.45-0.85 mm, brown-puberulent dorsally, deciduous; perianth greenish sometimes red-tinged, externally glabrous except for nearly always brown-puberulent calyx-teeth and often minutely puberulent tip of corolla-lobes; calyx deeply campanulate or subcylindric, faintly 5-nerved, 1.8-3.6 (-3.9) x 0.7-1.3 mm, the depressed-deltate or almost obsolete teeth 0.05-0.3(-0.6) mm; corolla slenderly tubular, scarcely dilated upward, (5.5-)6.4—8.8(-10.5) mm, the ascending ovate lobes (0.3-)0.5-1.4 mm, internally ciliolate with minute dilated pallid trichomes; androecium 20-42-merous, 10-15.5(-17.5) mm, the stemonozone ±1 mm or less, the tube (2-)5-9(-10) mm, sometimes a little thickened internally at base but lacking nectarial disc; ovary subsessile, slenderly ellipsoid, at anthesis glabrous; style a little surpassing the longer stamens, the stigma poriform. Pods solitary or 2-3 per capitulum, pendulous, subcylindric or moniliform, in profile either undulately linear or (when relatively long or when ovules abort) pinched between seeds into narrow isthmi, either gently or strongly falcate and in addition often randomly twisted, when well fertilized 8-18(-22) x 0.55-1.4 cm, 7-14-seeded, the seeds either almost contiguous end to end or in longer pods mutually distant, the sutures and venation of valves immersed, the smooth fleshy, minutely densely puberulent or velutinous, bright red valves becoming stiff but brittle-leathery and brown or fuscous- red after dehiscence, the endocarp smooth, light tan; dehiscence through both sutures, the valves gaping to expose seeds dangling on a pallid threadlike funiculus, this compressed but not sigmoid next to seed, simply bent backward within the fruit and after dehiscence straightening and wiry-persistent; seeds basipetal, basifixed, plumply ellipsoid and only a trifle compressed, in broader view 6—13(—18) x 5.5-11 mm, the thin testa lustrous black becoming dry and fragile, closely investing the whitish or greenish embryo.
This is a widely dispersed species variable in leaf- formula and venation of leaflets, in size of seeds (and seed-locules), and in length of stamen-tube, but the modes of variation, while overlapping, are clearly related to dispersal and seem worthy of some taxonomic expression.
References: [Article] Barneby, Rupert C. & Grimes, James W. 1997. Silk tree, guanacaste, monkey's earring: A generic system for the synandrous Mimosaceae of the Americas. Part II.