Monographs Details: Psorothamnus
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1977. Daleae Imagines, an illustrated revision of Errazurizia Philippi, Psorothamnus Rydberg, Marine Liebmann, and Dalea Lucanus emen. Barneby, including all species of Leguminosae tribe Amorpheae Borissova ever referred to Dalea. Mem. New York Bot. Gard. 27: 1-892.
Scientific Name:Psorothamnus
Synonyms:Dalea emoryi A.Gray
Description:Genus Description - Shrubs and subshrubs, some (especially of sect. Psorothamnus) only softly woody, one arborescent when adult, one herbaceous from long horizontally creeping stolons; pubescence usually copious, the hairs not tarnishing when dry, those of stems sometimes retrorse, those of foliage antrorse or spreading, or the leaves, rarely the whole plant, glabrate; lenslike glands present nearly throughout, especially abundant on stems, lower leaf-surface, raceme-axis, calyx-tube, and pod, some of them often high-convex or prickle-shaped; branches often lignescent in age, or the raceme-axis spinose either before or after anthesis, if before anthesis the flowers appearing lateral to spines; leaves mostly imparipinnate, the 2-6(8) pairs of leaflets jointed, but either sessile or petio- lulate, or more rarely confluent with rachis or (§ Xylodalea) the distal ones with one another, in few spp. some or all leaves 3-foliolate or reduced to a terminal caducous leaflet; flowers racemose or spicate, the pedicels bracteolate or charged with a pair of sessile glands, sometimes both; calyx-tube campanulate, the teeth unequal, the ventral pair broader and nearly always longer than the rest, sometimes partly united behind banner, the larger ones ± net-veined, rarely 1-nerved; petals all inserted on hypanthium rim, deciduous or rarely marcescent, wholly or partly pink, purple, violet, or vivid blue, glabrous or externally pubescent, sometimes gland-tipped; banner short- clawed, the claw linear or cuneate; keel usually as long or longer (rarely shorter) than banner, its blades narrowly overlapping by their external margins and adherent, enclosing the androecium; androecium 10-merous, the filaments united ± 1/2 (2/3) their length, alternate 5 often shorter, the connective gland-tipped or not so; ovules 2, collateral, or (§ Capnodendron) 3-7; seed solitary or (§§ Winnemucca and Capnodendron) 2, when 2 one disposed obliquely above the other; n = 10. — Spp. 9, highly modified xerophytes, of the Sonoran, Chihuahuan, and Mohave deserts and of desert basins within the Colorado Plateau and Intermountain United States (Nevada, s. Utah, n. and w. Arizona, s.-e. California, and Rio Grande valley in New Mexico) and n.-w. Mexico (n.-w. Coahuila, n. Chihuahua, n.-e. Sonora, and almost throughout extra- tropical desert Baja California).


The genus Psorothamnus, here expanded to embrace Psorodendron Rydb. (cf. sect. Xylodalea, infra), is related on the on$ hand to Errazurizia and on the other to Marina, and only through the latter, therefore more remotely, to Dalea as here received. It belongs with Errazurizia, Marina, Eysenhardtia, and Amorpha in the group of Amorpheae with ten pairs of chromosomes, and resembles all of these, except Marina, in having petals inserted directly on the hypanthium rim. It resembles Marina, but not others mentioned, in having an apparently normal papilionaceous keel, the two leaves of which are folded around and conceal the androecium at anthesis. The sect. Xylodalea, with its large, little compressed, exserted pod and single massive seed, makes a close approach to Errazurizia; the sect. Psorothamnus leans toward Marina in its smaller, compressed pod. However the ovule of Marina is solitary, those of Psorothamnus, like other Amorpheae, paired or exceptionally (Ps. spinosus) more numerous, and Marina has the inner petals elevated on a column continuous with the androecium, appearing epistemonous. It is this character which links Marina to Dalea sens. lat. and separates both from Psorothamnus.

The genus Psorothamnus as here redefined embraces not only all of Psorodendron Rydb. but also Parosela thompsonae Vail (misplaced by Rydberg in Parosela subgen. Trichopodium) and Dalea whitingi Kearn. & Peeb., described long after Rydberg’s monograph and now interpreted as only varietally distinct from the last. The dichotomy stressed by Rydberg (1919, and again in 1928, p. 198, in clave) between a genus Psorodendron characterized by a pedicellate flower and exserted (i.e., large, errazurizia-like) pod and a genus Psorothamnus different in its sessile flower and included (more compressed and dalea-like) pod cannot be followed literally. The flower of Psorothamnus polydenius (Torr.) Rydb. is often shortly pedicelled, while the fruit of Psorodendron kingii (Wats.) Rydb., though pedicelled, does not surpass the calyx-teeth. Moreover Psorothamnus thompsonae (Vail) Welsh & Atwood, with a compressed included pod combined with the long pedicel of Psorodendron but the peculiar retrorse stem-pubescence found otherwise only among original members of Psorothamnus sens, str., sits squarely between the two. It should be mentioned that W. F. Mahler, working primarily from the palynological angle, arrived independently at the same inclusive generic concept.

Morphological features not or only rarely duplicated elsewhere in tribe Amorpheae but seen in one or more (but not all together in any one) species of Psorothamnus are both numerous and, in some instances, suggestive of intergeneric relationships: creeping rhizomes (Ps. kingii); leaves all (Ps. spinosus), most (Ps. scoparius), or some (Ps. emoryi) reduced to the terminal leaflet; stem-pubescence retrorse (sect. Psorothamnus except Ps. emoryi); leaflets decurrent on the rachis (Ps. arborescens, Ps. emoryi, but both in part only); axis of the raceme (sects. Capnodendron and Winnemucca) or of sterile branchlets immediately preceding it (Ps. thompsonae) spinose; pedicels bibracteolate (in all but sect. Psorothamnus, where bracteoles are represented, as in all species of Marina, by sessile glands); petals marcescent (Ps. thompsonae); more than one seed maturing from a pair of collateral ovules (Ps. kingii)', more than 2 ovules (Ps. spinosus always, Ps. thompsonae occasionally). The large pod and massive seed of sect. Xylodalea are closely matched in Errazurizia, of which the one Chilean species has, unlike the North American, keel-petals lightly adherent by the overlapping margins, at least in bud. The small compressed pod of sect. Psorothamnus hardly differs from that of some Marina. However these odd or aberrant features of individual Psorothamnus species are, as implied in the list, poorly correlated, and contribute little to clarification of cross-relationships within the framework of the genus. Note, for example, how the characters of the inflorescence and the incidence of a glandular connective cross over, as it were cancelling each other out.

A notable feature of Psorothamnus is the high percentage of taxonomically isolated types. Despite a constitution, at the generic level, adapted to extreme desert conditions, the genus has developed no monocarpic species, as has occurred in Dalea and Marina under comparable climatic stresses. The generally woody character of its members, their mutually remote taxonomic relationships, and the connection via sect. Xylodalea with the apparently relict genus Errazurizia (and thence with Eysenhardtia) mark Psorothamnus as probably ancient and primitive relative to the two epistemonous genera of Amorpheae. It is, however, evidently closer kin to Marina than to Dalea itself.