112.  Dalea aenigma Barneby

(Plate CIV)

Dwarf, diffuse subshrubs, with arching or creeping woody stems adventitiously rooting where they touch the ground, ± 1-2.5 dm long, distally verruculose and purplish, pilosulous nearly throughout with short, contorted or incumbent hairs, the foliage greenish, the leaflets glabrous above, pubescent and gland-sprinkled beneath, the inflorescence densely silky pilosulous; leaf-spurs 0.3-0.8 mm long; stipules triangular-subulate, 0.8-1.2 mm long; intrapetiolular glands 0; post-petiolular glands prominent; leaves 6-15 mm long, shortly petioled or subsessile, with narrowly margined rachis and (3) 4-7 pairs of oblanceolate to narrowly obovate, obtuse to emarginate, loosely folded leaflets 1.5-5 mm long; peduncles mostly terminal to branchlets, 2-5 cm long, sometimes one leaf-opposed on the main stems; spikes dense but not conelike, ovate-oblong or the smaller ones subcapitate, without petals ±8-11 mm diam, the pilosulous axis (0.5) 1-3 cm long; bracts persistent, rhombic-ovate- apiculate to lanceolate and shortly acuminate, 2-3 mm long, livid and gland-verruculose dorsally beneath the pilosulous vesture, silky-pilosulous within; calyx 4.6-6 mm long, densely pilosulous externally and internally about the orifice, the rather firm tube as long or more often shorter than the tube, the ventral pair slightly shorter and broader; petals bicolored, the banner opening whitish with purple-tipped basal lobes rubescent in age, the inner petals bright purple, inserted at 1.2-1.7 mm from the hypanthium-rim, the banner and keel gland-tipped, the banner-blade gland-sprinkled; banner 5.4-6.8 mm long, the deltate-ovate or spade-shaped blade 3.5-4 mm long, about as wide, hooded at apex, closed across top of claw to form a shallowly recessed cornet; wings 6.4-7.3 mm long, the claw 2-2.8 mm, the lance-oblong blade 4.4-4.8 mm long, ± 1.8 mm wide; keel 8.4-10 mm long, the claws 3-3.9 mm, the broadly oval-obovate to elliptic blades 5.5-6.2 mm long, 2.9-3.5 mm wide; androecium 10- merous, 7-9.5 mm long, the longest filament free for 2.6-3 mm, the bluish, gland- tipped anthers ± 0.6 mm long. — Collections: 4 (o).

Habitat not recorded, but to be sought on dry hillsides and gullied banks at ± 2400- 2500 m, known only from Valley of Mexico in Estado de Mexico and adjoining Hidalgo. — Flowering July and August. —Material: Hidalgo: Tulancingo, typus (cf. infra); Moore & Wood 4878 (BH, NY); Purpus 1377 (NY, p. p., mixed with D. bicolor). Mexico: Amecameca, Purpus 1743 (F, UC).

Dalea aenigma (a riddle, the status and affinity obscure) Barneby, Phytologia 26: 1. 1973. — "Mexico. Hidalgo: Tulancingo, July 22, 1905, Rose, Painter & Rose 8832.." — Holotypus, NY! isotypi, F, GH, US!

A small suffruticulose dalea, in habit suggesting Astragalus danicus L., D. aenigma falls within a circle of affinities dominated by protean D. bicolor, from which it is precariously separable by a syndrome of individually inconsiderable characters. The short procumbent stems, rooting where they touch the ground, are reminiscent of D. greggii rather than D. bicolor, but the foliage cannot be reconciled with what is known of D. greggii, the 4-7 (not 2-4) pairs of ventrally glabrous leaflets being abruptly different. Stems of D. bicolor var. bicolor, normally a much taller, shrubby or arborescent plant with lax and elongate spikes rising 5 dm or much more above the root, are nevertheless prostrate in one rare form, that described by Schlechtendahl as D. thymoides. This, however, still differs from D. aenigma in its spike at once short-pedunculate and lax of very shorttoothed and shortly pilosulous calyces. The banner of all forms of D. bicolor known to date from the high plains of Hidalgo and adjoining states is glandless except for an occasional, always minute, terminal blister; in D. aenigma the blade’s eye is charged with a conspicuous gland-crescent. A Purpus collection (no. 1377, NY) of D. aenigma, made at Tulancingo on the same day as the type-set, probably therefore an exact topotypus, is mixed with genuine D. bicolor var. bicolor in a phase similar to that collected often near Pachuca and vicinity. This suggests that D. bicolor and D. aenigma are sympatric, and strengthens the latter’s claim to recognition.

The riddle of D. aenigma has another facet. A hypothetical plant of the species shorn of vesture except for pubescent axillary short-shoots and with calyx-pubescence proportionately thinner and shorter would become essentially like the plant treated next in order as D. macrotropis, a point that can be appreciated by comparison of the two figures on Plate CIV. Such glabrate phases are commonplace among kindred daleas. However the stem of D. macrotropis is not known to be radicant and the source of the type remains a mystery. Pending rediscovery of D. macrotropis and study of new populations of D. aenigma it seems desirable to provide the latter with a name and, if only provisionally, specific status.