Rhynchosia senna var. angustifolia (A.Gray) Grear

Authority

Grear, John W. 1978. A revision of the New World species of Rhynchosia (Leguminosae-Faboideae). Mem. New York Bot. Gard. 31 (1): 1-168.

Family

Fabaceae

Scientific Name

Rhynchosia senna var. angustifolia (A.Gray) Grear

Description

Variety Description - Stems 40-100 cm, prostrate or twining, rarely subascending, puberulous to strigulose or villosulose. Stipules linear or linear-lanceolate, 1.5-3 mm long, 0.5 mm wide. Petioles 5-35(-40) mm, puberulous, strigulose or villosulose. Leaflets ovate to lanceolate-ovate or rhombic-ovate and oblong to oblong-lanceolate or linear-lanceolate, base subcordate or obtuse, 0.2-2 cm wide, puberulous, strigulose or villosulose above and beneath, terminal petiolule 2-10(-20) mm. Inflorescences 6-15 mm, peduncle subobsolete, pedicels 1-6 mm, not exceeding the petiole. Bracts setose or linear, 0.5-1 mm long, 0.5 mm wide. Calyx not exceeding the corolla, 3-5.5 mm, tube 1.5-2.5 mm, lobes subulate, acute or acuminate, 1-3.5 mm, vexillar sinus 0.5-2 mm. Corolla 5-7 mm. Standard widely ovate, puberulous, 5-7 mm long, 3.5-5 mm wide, auricles 0.5 mm. Wings oblong-ovate, glabrous, 5-6 mm long, 1-2 mm wide. Keel blades 5-6.5 mm long, 2-2.5 mm wide, claw 1.5-2 mm. Stamens 4-6 mm. Fruits puberulous or puberulous-hirsutulous intermixed, 1.5-2.5 cm long, 5-6 mm wide, beak 1-2 mm. Seeds 3-4 mm long, 3-4 mm wide.

Discussion

Rhynchosia texana var angustifolia A.. Gray, Pl. Wright. 1: 44. 1852. Rhynchosia angustifolia Engelmann ex A. Gray, Pl. Wright. 1: 44. 1852, pro syn., non R. angustifolia (Jacquin) de Candolle, 1825. Dolicholus angustifolius (Engelmann ex A. Gray) O. Kuntze, Rev. Gen 3, 2: 60. 1898. Dolicholus texanus (“texensis”) var angustifolius (A. Gray) Vail, Bull. Torrey Club 26: 108. 1899. Rhynchosia texana Torrey & Gray, Fl. N. Am. 1: 687. 1838. Type: “Texas.” Drummond sn, w/o date (holotype, NY!; isotypes, K!, LE!). Dolicholus texanus (Torrey & Gray) Vail, Bull. Torrey Club 26: 108 ( texensis ). 1899. Rhynchosia texana f cordobensis Grisebach, Abh. Böhm. Ges. Wiss. Goet. 19: 74. 1874. Type: ARGENTINA, “en las cercanias de Córdoba,” P G. Lorentz 360, Nov, Dec 1870 (holotype, presumably at GOET, not seen; isotype, SI!). Rhynchosia senna var cordobensis (Grisebach) Burkart in Cabrera: Fl. Prov. Buenos Aires 4(3a): 625. 1967. Rhynchosia texana var microphylla Hassler, Repert. Sp. Nov. 16: 230. 1919. Type: PARAGUAY. “Chaco septentrionalis, Puerto Talavera,” K. Fiebrig 1323, Sep 1907 (lectotype selected, G!; isotypes, F!, G!, K!). Rhynchosia senna var foliolis lanceolatis Ball, Linn. Jour. Bot. 21: 216. 1884. Described from G. Claray 189, K! Type. TEXAS. “Praires near New Braunfels,” F. Lindheimer 412, Jul 1850 (lectotype selected, MO!; isotypes, GH!, K!, MICH!, NY!, PH!). Local Names. Sen del campo (Argentina: Catamarca); tacuruzal (Argentina: Chaco). The characters separating R. senna var angustifolia from var senna are discussed in the treatment of the latter entity. The differences are few and the two show some evidence of possible hybridization in areas of Argentina where their ranges overlap. After examining numerous specimens, including the types, assembled from collections made throughout the New World, one cannot help but be aware of the striking similarity between the two except for the few characters already mentioned which are technical in the highest degree. In spite of these similarities, the two varieties have been treated as distinct species since Torrey & Gray described what is now R. senna var angustifolia as a new species, R. texana, in 1838. This would seem to have a logical explanation. Early taxonomists in the U. S. seldom bothered to investigate the possibility of the same taxon occurring south of the border, especially if there was a large amphitropical disjunction involved, as is the case with var angustifolia as well as many other species and varieties of angiosperms with discontinuous distributions between the western U. S. and northern Mexico and South America. In describing R. texana from Texas as new, Torrey & Gray were obviously not aware that a very similar plant had already been described earlier from Argentina as R. senna. At least no mention was made of this. Considering the reduced opportunities for communication and consultation of type material in the early 1800’s, this is not too surprising. What is surprising is the failure of modern-day taxonomists to consult the type specimens and descriptions of these two taxa and to continue to add to the confusion between them. Bentham was well aware of their similarity and annoted sheets of the type collection by Drummond of R. texana in the Hooker Herbarium at K as “R. senna var texana.” In Martius’ Flora Brasiliensis (1859), however, Bentham decided that they were conspecific and R. texana is listed as a synonym of R. senna. Bentham, however, has elements of R. diversifolia (leaflets orbicular and pods straight) represented in the specific description of R. senna. He points out the disjunction and comments that this species, like R. minima, often has larger flowers on the Brazilian plants than on the North American ones. In describing two forms of R. senna based on leaflet shape collected by Claray at Bahia Blanca, Argentina, Ball (1884) was unaware of the distributional gap involved because he states that it “appears to be widely spread from Texas and Mexico to the boarders of Patagonia. I do not think that R. texana, Torr. & Gr., can be separated from it.” In her revision of Rhynchosia, Vail (1899) preferred to maintain R. texana as a distinct species following correspondence with J. H. Burkill at K concerning South American collections there. I was pleased to discover in the herbarium at NY a letter from Burkill to Vail dated Jan. 2, 1899, which also included a few leaves and a flower from the type collection of R. senna for Vail’s perusal. This segment is definitely what I am calling var senna. Burkill informs Vail that he has examined the collections of R. senna from South America and that there are without doubt two distinct forms as Bentham had already pointed out. He concludes the letter by stating, “Here you have my impressions. I think one may make a variety as Bentham did but one cannot go further.” Even with this sage advice, Vail still would not make a decision, as shown by her following statement: “I have kept them apart here as the latter species [R. senna] is not very well known and will probably need further study to determine its rightful position.” Rhynchosia senna var angustifolia has two distinct leaflet forms. Leaflets on the lower parts of the branches are mostly ovate to lanceolate-ovate or rhombic-ovate while those on the upper, more twining portions of the stems, are linear-oblong or linear-lanceolate. The latter form has been described as a variety of R. senna in South America and as R. texana var angustifolia in North America. Although Burkart maintains R. texana (= R. senna var angustifolia) as distinct from R. senna in Las Leguminosas Argentinas (1943, 1952), he stresses their strong resemblance and suggests that what is being referred to as R. texana may be a variety of R. senna with more twining stems and more narrow leaflets. Before his death in 1975, I had frequently corresponded with Dr. Burkart concerning the relationship between the two taxa in Argentina and the identity of the North American taxon. I eventually met him in Boulder, Colorado, in 1973 and we discussed the matter further. We both agreed that they were indeed probably conspecific, but that this should be tested by biosystematic work of an experimental nature utilizing primarily studies of populations in the field, cytology, chromotography and attempts at hybridization. Although the characters used for separating them are few, I feel that the two should be maintained as separate varieties for the time being. This will serve to emphasize the similarities between them until the genetic relationships can be studied. The new combination Rhynchosia senna var angustifolia is based on R. texana var angustifolia, which was applied to forms of the species with narrow leaflets by Asa Gray in 1852. This early validly published varietal epithet was initally overlooked by me and, as a result, many herbarium sheets have been mistakenly annotated as R. senna var texana for plants formerly known as R. texana. For this reason, all such sheets returned to the herbaria cited herein and bearing my annotation labels as R. senna var texana should be reannotated as R. senna var angustifolia. A dimorphic condition with respect to the flower exists in many collections of var angustifolia from North America. In addition to the normal flower type, a morphologically distinct cleistogamous type of flower is frequently produced along with the normal type. These cleistogamous flowers are almost spherical in shape with a greatly enlarged calyx. The standard is extremely wide and folded over at the edges and the auricles and claw are vestigial. The wings and keel blades are widely ovate. The stamens are extremely short (3-4 mm). The carpel and stamens are completely enclosed by the keel blades which are enclosed in the wings. They, in turn, are covered over by the wide standard which is almost completely enclosed by the enlarged calyx. Dissection of young ovaries reveal well-developed ovules which appear to be maturing normally. This would indicate self-fertility. An unusual leaf variant was collected in 1966 near Langtry, Texas, in Val Verde county (Correll & Rollins 32600). A few unifoliolate entire leaves occur at the base of the plant with the remaining ones further up on the stem showing a transition from a weakly to strongly tri-lobed condition. It would appear that the usual three leaflets composing the leaf have fused in their development, producing a unique, abnormal three-lobed leaf form which is represented by only this one collection and is not of taxonomic significance. Reproductive parts are typical of those of var angustifolia. The discontinuous distribution exhibited by var angustifolia closely parallels that of many other legume taxa, and the disjunction might be explained by a number of factors, all of which are speculative. One explanation is that the taxon was once more widespread and continuous and that the plants in the gap area have since disappeared due to subsequent changes in the environment leading to a lack of suitable habitats. Parallel evolution is considered too unlikely. Long-distance dispersal in the past by migrating birds might account for this type of amphitropical disjunction. The role of birds in seed dispersal of some species of Rhynchosia has already been mentioned, as well as the probable introduction of R. minima into the New World from Africa. If a species can be introduced from one contintent to another, then surely the possibility exists for its introduction from South to North America. Rhynchosia senna var angustifolia seems to exhibit a greater diversity in South America and this would suggest the direction of dispersal. The malvaceous species, Cienfuegosia drummondii, has a disjunct distribution somewhat like that of var angustifolia and the comments of Fryxell (1967) in trying to explain the gap in distribution for that taxon would seem to be equally applicable in this case. The facts tend to argue for a recent long-distance dispersal for the disjunction rather than by an ancient migration. This could probably not have been accomplished during historical times, but would be more likely to have occurred in post-Pleistocene times. Although the plants have no obvious adaptation for long-range dispersal, they do have the following: extremely hard seed coats permitting a degree of dormancy and longevity of the seed, self-compatibility and a high reproductive potential which would suffice to accomplish the disjunction if by chance a single individual was established, and perennial duration of the woody rootstock, which would enhance the reproductive potential.

Distribution

Distribution. Texas, southern Arizona and New Mexico; Mexico and South America (Fig. 18). Occurring mostly on dry limestone, sandy or igneous soils in grasslands, canyons, mountain slopes, roadsides, forest clearings and entering the desert in Sonora; to elevations of 2650 m.

United States of America North America| Mexico North America| South America|