Astragalus mokiacensis A.Gray
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(1): 1-596.
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Family
Fabaceae
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Scientific Name
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Type
"Mokiak Pass, on the borders of Utah and N. W. Arizona, Dr. Palmer, 1877."—Holotypus, Palmer 105 in 1877, GH! isotypi, K. NY (2 sheets). POM (fragm.), US!
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Synonyms
Astragalus lentiginosus var. mokiacensis (A.Gray) M.E.Jones, Tium mokiacense (A.Gray) Rydb.
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Description
Species Description - Robust, caulescent perennial, possibly sometimes of short duration, strigulose with straight, appressed or subappressed hairs up to 0.45-0.7 mm. long, the stems glabrescent at base, cinereous distally, the herbage green or cinereous, the leaflets pubescent on both sides, or thinly so to glabrate, rarely truly glabrous above; stems erect and ascending in clumps, simple above the immediate base or branched at 1-4 nodes preceding the first peduncle, 1-4 dm. long; stipules sub-herbaceous becoming papery, broadly triangular-acuminate, 3-7 mm. long, semi-amplexicaul-decurrent, the upper ones mostly deflexed; leaves 5-13 cm. long, shortly petioled, with 7-21 broadly obovate, obovate-cuneate, or suborbicular, mostly emarginate, flat leaflets 5-19 mm. long; peduncles stout, erect, 4-11 (14) cm. long, longer or shorter than the leaf; racemes 14-24-flowered, rather densely so at early anthesis, the flowers ascending at a wide angle, the axis elongating, 4-13 cm. long in fruit; bracts lanceolate, submembranous, pallid or purplish, 1.5-4 mm. long; pedicels ascending, at anthesis 1-1.5 mm., in fruit thickened, 2-2.5 mm. long; bracteoles apparently 0; calyx thinly strigulose with black, white, or mixed hairs, 7-9.2 mm. long, the oblique or subsymmetric disc 1.1-1.4 mm. deep, the deeply campanulate tube 5—6.5 mm. long, 2.8—3.8 mm. in diameter, the subulate teeth 1.7—2.7 mm. long, the whole becoming papery, ruptured, marcescent; petals purple, drying violet; banner recurved through ± 50°, broadly rhombic-obovate or -oblanceolate, deeply notched, (14) 16.3-18.2 mm. long, 8-12.4 mm. wide; wings (13.4) 15—16.6 mm. long, the claws (4.5) 5.3—7.3 mm., the narrowly oblanceolate or linear-oblong, obtuse or truncate, slightly incurved or straight blades 9.2-11.1 mm. long, 2.3-2.7 mm. wide; keel 14.4-15.7 mm. long, the claws 5.2—7.2 mm., the broadly half-obovate blades 8.3—10 mm. long, 3.4—3.9 mm. wide, incurved through 60-90° to the rounded apex; anthers 0.6-0.8 mm. long; pod ascending or spreading and incurved-ascending, sessile, oblong-ellipsoid, (1.4) 1.7-2.5 cm. long, (4.5) 5-7 mm. in diameter, straight or gently incurved, broadly rounded and then turbinately contracted at base, narrowed at apex into a stiff subulate cusp 2-4 mm. long, subterete at first, becoming sulcate ventrally, carinate by the thick, 3-ribbed ventral suture, the somewhat fleshy, green or purple-dotted, glabrous or minutely puberulent valves becoming stiffly papery or almost leathery, stramineous, inflexed below the beak as a partial or almost complete septum 1.7 2.5 mm. wide; ovules 26-35; seeds unkown.
Distribution and Ecology - Bluffs, cliff terraces, and gullied badlands, ± 3000—4000 feet, local and apparently rare, known only from valleys and canyons of the Colorado and Virgin Rivers in northern Mohave County, Arizona, and eastern Clark County, Nevada. —Map. No. 72.—April to June.
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Discussion
The Mokiak milk-vetch. a pitfall and a snare to three generations of botanists is still poorly known and perhaps wrongly placed among the Preussiani. Nothing has been learned about its reaction to selenium in the soil: but judged from exterior morphology alone, the species seems closely related to A. Preussii. Around the head of Lake Mead A. mokiacensis and A Preussii var. laxiflorus grow close together, and there is no obvious way of distinguishing the two until the fruits have formed. The fruit of A. Preussii is strictly unilocular, that of A. mokiacensis is fully or almost bilocular, but in A. Crotalariae often and in A. Beathii always there is some indication of a rudimentary (or incipient) septum, so the addition of A. mokiacensis to sect. Preussiani introduces no new morphological feature. The septum is merely better developed in this species.
Until quite recently A. mokiacensis has been known only from Palmer's type-collection (very likely not from Mokiak Pass itself, but from father south along Grand Wash) and from material collected by Jones at Mica Spring in the Virgin Mountains, just west of Grand Wash in Nevada. These two classic collections were early associated by Jones with plants collected by Lemmon gatherings as A. ursinus, but the flowers are too large to fit well with the typus of that imperfectly; know entity, and they are now interpreted as representing an unusual form of A. lentiginosus var. palans. In his Revision, Jones united A. ursinus and A. Wilsonii with A. mokiacensis as one variety of A. lentiginosus, and it seems probable that he arrived at this decision after transferring his concept of genuine A. mokiacensis to the species collected by Lemmon. However that may be, the original A. mokiacensis was lost to view, and even when Rydberg revived it as a species of Tium (Rydberg, 1931, p. 406) it remained encumbered with the Lemmon collections which supplied some of the supposed critical characters. The same error, by now so heavily documented as to become almost holy writ, was perpetuated in my revision of A. lentiginosus (1945, p. 137), and a new element of absurdity introduced in the form of an unnamed (but described) form, A. mokiacensis ß, which, on account of its inflated pod combined with the flower and habit of genuine A. mokiacensis, seemed to link the latter even more firmly to the A. lentiginosus complex. This form is recognized in these pages as an independent A. lentiginosus var. ambiguus, but is perhaps still germane to a discussion of A. mokiacensis because of the similarity in general appearance.
In northern Mohave County, Arizona, and Clark County, Nevada, there are five astragali, A. Preussii var. Preussii and var. laxiflorus, A. mokiacensis, and A. lentiginosus var. palans and var. ambiguus, which occupy similar sites and are so much alike at anthesis (each to at least one, commonly to several of the others) that well-formed fruit is often required for certain identification. The pod of A. Preussii is truly unilocular and persistent; that of A. lentiginosus is fully bilocular and deciduous; that of A. mokiacensis is persistent but provided with a broad septum. The pods of var. laxiflorus and var. ambiguus are bladdery or at least greatly inflated, in contrast to the narrowly ellipsoid pods of var. palans and A. mokiacensis, but the pod of var. Preussii varies from slenderly fusiform to broadly oblong-ovoid. So far as the fruit alone is concerned, A. Preussii var. Preussii and A. lentiginosus var. palans possess all the morphological ingredients which would be needed in theory to create the other three forms. It seems possible that A. mokiacensis at least, and very possibly also var. laxiflorus and var. ambiguus are recombination types derived from a hybrid between A. Preussii and some variety of A. lentiginosus, presumably var. palans, but conceivably var. yuccanus, which extends into the area under discussion from its center of abundance farther south. The hypothesis is in harmony with what is known of the distribution of the species involved and would explain the odd dispersal of A. Preussii var. laxiflorus as an enclave within the range of the widespread var. Preussii. The whole question will deserve closer scrutiny as the canyons of the Colorado River downstream from Peach Springs become better known botanically.
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Objects
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Distribution
Arizona United States of America North America| Nevada United States of America North America|