Astragalus lentiginosus

Title

Astragalus lentiginosus

Authors

Rupert C. Barneby

Scientific Name

Astragalus lentiginosus Douglas ex Hook.

Description

289.  Astragalus lentiginosus

Annual or more often biennial or perennial herb, diverse of aspect and extremely variable in stature and in amount, orientation, and distribution of the vesture, the herbage green, cinereous, or silvery-silky, more rarely tomentose; stems (solitary) few to numerous from the crown of a taproot or shortly forking caudex, simple or branched, prostrate, incurved-ascending or erect, mostly 1-10 dm. long; stipules semi- to almost fully amplexicaul-decurrent, free; leaves 1-17 cm. long, with 11-27 (29), or in some early leaves or in some seedling plants only 7-13 leaflets; peduncles much shorter or about as long as the leaves; racemes (3) 5-35 (48)-flowered, subumbellate or loosely racemose, the flowers ascending or spreading in age, the axis not or greatly elongating; pedicels ascending, the fertile ones little or sometimes considerably thickened, persistent in fruit; bracteoles usually 0, or minute when present; calyx 3.8-12.5 mm. long, the tube cylindric, deeply campanulate, or campanulate, the teeth usually much shorter, subulate or triangular, the whole becoming papery, ruptured, marcescent; banner recurved through ±30-50°, 7.4-21.4 mm. long; wings usually a little shorter, the blades narrowly lanceolate to linear-oblong, straight or gently incurved distally, obtuse; keel (shorter or rarely a little longer than the wings) 6-16.4 mm. long, the half- obovate blades incurved through (50) 80-90° to the rounded apex; pod ascending (often humistrate), or spreading and incurved, sessile on the flat or obconic receptacle, readily deciduous when ripe, greatly variable as to length, outline, inflation, and texture, most commonly differentiated into an ovoid or globose, bladdery-inflated, ventrally and often also dorsally open-sulcate body and a deltoid or triangular-acuminate, laterally compressed, more or less incurved (in one var. decurved, in another tubular-filiform) beak, but sometimes lanceolate in outline and then often crescentically or even hamately incurved and little or not inflated, the leathery, papery, or papery-membranous, glabrous or variously pubescent valves inflexed (below the usually unilocular beak) as a complete or rarely incomplete septum, the funicular flange either conspicuous or subobsolete; dehiscence apical, through the gaping beak, after falling; ovules 10-42; seeds brown, olivaceous, orange-brown, brown dotted with purple, or purplish-black, smooth, sparsely pitted, or rarely wrinkled, dull or exceptionally sublustrous, (1.5) 1.8-3.3 (4) mm. long.

The freckled milk-vetch, A. lentiginosus, is hardly a species in any conventional sense of the term. As circumscribed here, it is an excessively polymorphic complex which embraces a range of variation far greater than can appear reasonable at first sight and which becomes, even when accepted on reflection as taxonomically unavoidable, as unwieldy in the mass as it is intricate in internal structure. The foregoing description, which serves merely to list the characters common to the constituent parts of the complex and to give some indication of maximum metric variations, is not much more than an abstract formula.

The incalculable number of populations which go to make up A. lentiginosus are dispersed over a great part of the Columbia and Great Basins, the Colorado Plateau, and the deserts of southeastern California and Arizona, whence they extend a short way into northern Mexico. Most plentiful at middle elevations and almost ubiquitous in the sagebrush climax of the Great Basin proper, the freckled milk-vetch ranges from alpine crests above the limit of trees down to desert dunes at sea level or below it. Some specialized forms have become adapted to the beds of alkaline clay sinks which are moist during the spring months, while others have passed west across the Sierra to establish outposts in the new climatic and floristic provinces of the Great Valley and South Coast Ranges in California. As pointed out in a preliminary study of the group (Barneby, 1945), most of the forms of A. lentiginosus fall rather readily into three categories defined, with often natural-seeming results, in terms of growth-habit, flower-size, petal-color, and raceme-length; and these categories correspond rather closely with sections in the genus Cystium established by Rydberg. The first group, characterized by diffuse or prostrate stems and small, usually whitish flowers arranged in short racemes not elongating in fruit, is typified by A. lentiginosus sens. str. and by the vars. salinus and ineptus, and includes the more specialized and local vars. sierrae, antonius, and albifolius, the last an obligate halophyte. The second group of forms, similar in growth-habit, but characterized by larger, commonly purple flowers, which give rise to pods often of thicker texture, is gathered around a nucleus of vars. diphysus and araneosus and likewise includes comparatively rare and localized or ecologically specialized forms, such as vars. latus and toyabensis or the halophytic var. sesquimetralis. The third group has flowers of moderate or rather large size arranged in much looser racemes, which elongate and early surpass the subtending leaf. They are comparatively coarse plants, with stems either erect or diffuse but most often bushily clumped, commonly purple petals, and thin-textured fruits; the vars. coachellae, yuccanus, australis, and nigricalycis are representative members. These idealized groupings, to which the many important exceptions have not yet been mentioned, are to some degree climatically segregated. The last-mentioned series is proper to desert or near-desert climates at low elevations; the plants are of rapid growth and reach maturity early, sometimes becoming biennial, monocarpic, or even truly annual. Of the groups with compact racemes the first, with small whitish flowers, is mostly northern or becomes montane southward from about the 40th Parallel. The second, with larger, purple flowers, prevails at middle elevations from central Nevada southward. The taxonomy of the complex would be considerably simplified if we could accept these three groups as species, or even as primary subspecific divisions of one. However, even though the majority of freckled milk-vetches can be classified in this way, the resulting arrangement is not natural; moreover, there are many transitional forms which connect the idealized groupings. The var. Fremontii intergrades, on one hand, with var. variabilis of the desert group and, on the other, with var. macrolobus of the northern one. The var. platyphyllidius most closely resembles var. salinus of the northern group, of which it has the white flowers, but the flower- size and leathery pod are proper to the group typified by var. diphysus. The var. idriensis would fall technically into the diphysus group and thereby become far separated from its one very near relative var. sierrae. The same type of modification seems to have occurred several times in different branches of the evolutionary tree, leaving an apparently or perhaps sometimes truly reticulate pattern of relationships. The varieties cannot therefore even be arranged in a sequence that is at the same time logical in terms of the morphology and phylogenetically natural.

By an altogether different criterion, the varieties of A. lentiginosus might be divided into two groups of another order. The majority of them, as defined in these pages, consist of an indefinite number of populations which are associated together around an artificially conceived norm or nucleus for taxonomic purposes. The descriptions have been drawn up to cover not only the norm itself but as many of the peripheral populations as possible, and many of these are in one character or another transitional toward a second, usually vicariant variety of the same type. The varieties of this nature form a sort of jigsaw puzzle, of which each piece can be considered as a separate entity but actually hooks on, at least on one side and often on several sides, to adjoining pieces of an uninterrupted fabric. The freckled milk-vetches native to valley or low desert habitats, where the maximum dispersal is delimited less by physiographic barriers than by climatic or edaphic ones, are nearly all of this sort. The second category of varieties consists of those which are highly and more or less sharply localized, either by adaptation to a specialized habitat (as the halophytic vars. albifolius and sesquime- tralis) or by physical isolation (as shown by the insular-montane endemic vars. sierrae, an- tonius, and latus). Generally, the members of this second group do not differ from their nearest relative to any greater degree than do the idealized nuclei of those which form the first. There is, however, a sharply definite, even though extremely narrow morphological discontinuity between them corresponding with the discontinuity in dispersal. It would be convenient if we could distinguish between these two sorts of varieties in nomenclatural terms, but no means of doing so has been devised. A few of the more sharply differentiated forms of A. lentiginosus have been accepted in modern floristic works as species apart, but none of them possess more than a single truly differential charcter (e.g., the yellowish petals of var. nigricalycis, the filiform pod-beak of var. kernensis, etc.) separating them from the central core of A. lentiginosus. Their removal from the complex does somewhat reduce its unwieldy mass, but in the process they assume an importance and distinction which are not rightly theirs. However untidy or impractical the inclusive concept of A. lentiginosus may appear, it is one which is forced upon the unhappy taxonomist by the logic of facts. Perhaps only a botanist long familiar with the situation as it unfolds in the field can appreciate the complexity of the dilemma.

The remarks up to this point apply principally to the forms of A. lentiginosus in which the pod is moderately to greatly inflated, with a globose or ovoid body contracted into a triangular beak. Although the pod varies enormously, both within and between populations, not only in size but also in texture and curvature, it is in all these forms of the same basic design, recognizable at a glance as belonging to the freckled milk-vetch. In a small group of forms of which vars. palans, Wilsonii, and borreganus are typical, the pod is scarcely or not at all inflated, becoming linear-lanceolate in profile; and to the novice meeting these varieties for the first time, they seem to stand well outside the legitimate orbit of A. lentiginosus. Rydberg, in fact, went so far as to refer them to a separate genus; but as experience and material accumulate, the genuine affinities first perceived by Jones, make themselves felt through those individual plants which bear very slightly, then appreciably, then markedly swollen fruits without any perceptible accompanying alteration in the plant-body or in detail of the flower. All of the uninflated types (with the exception of var. maricopae, more recently discovered) were described upon discovery as species apart from A. lentiginosus; it would certainly simplify the taxonomy somewhat, if we could maintain them as such. However a classification which can be defended only for its convenience makes no very strong appeal to the imagination.

The following account of A. lentiginosus follows in most details the preliminary revision (Barneby, 1945). Persons who are interested in some historical aspects of the species and synonyms merely listed but not otherwise mentioned in the following pages, are urged to look back into this dated and in some respects naive paper. Several forms of A. lentiginosus are known to be locoweeds, but they are not readily eaten by stock except in years of unusual drought and are not abundant in the principal summer grazing lands of the western mountains. The following key to the varieties requires material with fruit sufficiently formed to show the size and texture at maturity and at least enough fragments of flowers to demonstrate the length of the keel and the color of the petals. Without one or more of these essentials, a given specimen may often be named successfully by means of the maps and descriptions. The high percentage of plants transitional between two varieties makes it impossible to construct a really efficient key; which would have to be extended and refined in order to deal with a countless throng of individual variants.