Senna pallida

Title

Senna pallida

Authors

Howard S. Irwin, Rupert C. Barneby

Scientific Name

Senna pallida (Vahl) H.S.Irwin & Barneby

Description

177.  Senna pallida (Vahl) Irwin & Barneby, comb. nov. Cassia pallida Vahl, Ecl. Amer. 3: 12. 1807.-Typus infra sub var. pallida indicatur.(?)Cassia biflora Linnaeus, Sp. Pl. 378. 1753, nom. ambig.-"Habitat in Indiis."-Described from a plant grown in Clifford’s garden at Hartekamp = Cassia (No. 12) foliolis octo parium ovalibus emarginatis, glandula acuta inter infima Linnaeus, Hort. Cliff. 159. 1737.-No typus extant either in hb. Cliffort., BM, or at LINN. The protologue (1737 + 1753), strongly suggestive of some member of Interglandulosae, has traditionally been interpreted in the sense of our S. pallida sens, lat., but could apply with equal probability to S. angustisiliqua. lf indeed based on a form of S. pallida, exact application of the name would still be impossible, for var. bahamensis and var. pallida were equally as likely to have reached Europe in early XVIII century.

(?)Cassia tenuissima Linnaeus, Sp. Pl. 378. 1753.-"Habitat in Havana. -Based on a manuscript description of Dr. Houston, for which no corresponding spm survives at BM or LINN.-This obscure name, listed here because equated with a form of Cassia biflora by Vogel (1837, p. 29) and Bentham (1871, p. 543), is indecipherable from the seven words of original description.

Cassia dispar Willdenow, Enum. pl. hort. bot. Berol. 441. 1809, nom. ambig .-"Habitat in America meridionali."-Holotypus, Hb. Willd. 7966, B! consists of sterile branchlets from a plant grown at Berlin. While almost certainly representing S. pallida sens. lat. and probably its var. pallida, precise identification is not possible without flowers.

Precociously flowering shrubs and treelets variable in stature and habit of growth, at anthesis commonly 0.5-5 m, in exceptional forest environment up to 7(-10) m, varying from fully glabrous (the ovary seldom so) to densely pilosulous with pallid or sometimes lutescent, variably spreading or accumbent hairs up to 0.2-1 (-1.3) mm commonly mixed with random minute thickened (but not secretory) trichomes and resinous dots, the lfts greatly variable in number (2-31 pairs), size (0.6-6 cm) and venation, either accrescent upward along rachis or only the proximal somewhat smaller than the remainder, the usually strongly bicolored foliage seasonally dimorphic, the primary lvs often drought-deciduous and replaced by diminished, simpler lvs of axillary brachyblasts, the inflorescence composed of mostly 2 (some rarely 1- or 3-4)-fld racemes arising either solitary from axils of coeval cauline lvs, or from axils of smaller lvs on axillary branchlets, or from highly condensed, leafless or microphyllous brachyblasts, the fls thus all or mostly immersed in foliage or all exserted, then sometimes forming a showy pseudocorymbose or thyrsoid panicle.

Stipules subulate, setiform or linear, less often somewhat expanded when commonly falcate, (1-)1.5-12(-15) x (0.1-)0.2-2 mm, caducous, absent from most fruiting specimens.

Primary cauline lvs (3.5-)4.5-16(-18) cm; petiole (3.5-)5-40 mm; lf-stalk prolonged beyond distal lfts into a setiform deciduous or less often a glandiform, more persistent appendage; petiolar gland always present between or (adnate to rachis and apparently) shortly above first pair of lfts, sessile or more often stipitate, (1-) 1.4-4(-4.3) mm long, the reddish, orange or fuscous head ± laterally compressed, in profile 0.3-1.3(- 1.5) mm wide; less commonly smaller glands at 1-2(-3) succeeding pairs; lfts varying in outline from narrowly oblanceolate, oblong-oblanceolate or oblong-elliptic to broadly obovate, emarginate, obtuse or deltately acute, muticous or conspicuously mucronulate, the margins usually revolute at least near pulvinule, sometimes all around, rarely quite plane; midrib cariniform beneath, giving rise to 4-10(-11) pairs of major camptodrome secondary veins visible or prominulous on lower or both faces (rarely altogether immersed) and sometimes a tertiary or quaternary reticulate venulation, prominulous or defined by color, this developed on lower or both faces.

Peduncles (2-)4-30(-36) mm; axis of racemes 0-1.5(-3) mm; pedicels 8-27(-32) mm, subtended on one side by a sessile or stipitate fusiform gland, in fruit ± abruptly bent above middle; sepals broadly obovate or orbicular very obtuse, greenish, brownish or subpetaloid with usually membranous margins, strongly graduated, the large inner ones (4.5-)5-11(-13) mm, the smaller outermost ± half as long; petals golden yellow, usually puberulent externally especially on claws and major veins, heteromorphic in the manner of ser. Interglandulosae, the 2 larger abaxial ones (13-)14-32(-36) mm; filaments glabrous or puberulent, those of 4 median stamens dilated, sometimes confluent at base, 0.7-2.2 mm, those of 3 abaxial ones filiform, free 2-5.5 mm, the anthers of 4 median stamens 2.86 x 1-1.8 mm, at apex either abruptly truncate or smoothly contracted into a lateral or obliquely terminal, very short beak dehiscent by 2 parallel (very exceptionally confluent) slits, those of 3 abaxial ones 3-8(-9) mm, abruptly contracted into an erect or porrect tubular beak 1-3.5(-3.8) mm opening by a single pore; ovary strigulose, pilosolous, rarely glabrous, the style 0.7-2.2(-2.5) mm, incurved but not dilated distally; ovules mostly 28-50, in var. bahamensis 20-30.

Pod pendulous or spreading-declined at random angles, the stipe (2.5)3-9 mm, the linear, straight, decurved or somewhat contorted body mostly 5-15(-17, in one var. 17-26) x 2.5-8.5(-10) mm diam, the ultimately brown or nigrescent papery valves raised over each seed as a simple transverse mound or a more or less sharply defined 4-armed pyramid; seeds in broader pods obovate (rarely oblong- obovate), in narrower ones rhombic-paddle-shaped in lateral view, 2.3- 4.8(-6) x (1.5-) 1.7-3.4(-3.7) mm, the testa pale amber or brownish to yellowish- olivaceous, exceptionally castaneous, dull or exceptionally sublustrous, smooth becoming crackled or exceptionally pitted overall, the areole round, elliptic or oblong-elliptic 0.4-2.4(-3.5) x 0.3-1.2 mm.-Fig. 1 (petiolar nectary).

Apart from questions of nomenclature, discussed below, our concept of S. pallida is equivalent to Cassia biflora sensu Bentham (1871, p. 543) with these differences: the specific limits are narrowed by loss of var. rostrata (=S. rostrata + S. aversiflora) and of those Antillean and South American Interglandulosae characterized by U-shaped dehiscence of the median anthers (=S. angustisiliqua and varieties); but augmented in another direction by annexation of Cassia foliolosa and by numerous forms which Bentham a century ago had no opportunity of knowing. The division of the biflorous Interglandulosae into major groups differentiated by anther dehiscence is somewhat artificial and technical, the more so because in some Guatemalan populations of S. pallida var. quiedondilla and again in some individual flowers of var. gaumeri the twin beaks of one or more median anthers are so shortened as to become confluent. Moreover the overall phenetic similarity between S. angustisiliqua and some forms of S. pallida demonstrates an intimate relationship. However, once S. angustisiliqua is admitted to the already forbiddingly polymorphic S. pallida, it becomes logically indefensible to exclude the also close kindred S. polyphylla, S. robiniifolia and S. viciifolia which have the androecium of S. angustisiliqua and hardly more decisive morphological differences than the varieties of S. pallida recognized herein. The Brazilian S. rostrata and S. aversiflora, of which the diagnostic fruits and vesture were unknown to Bentham, are in reality much further removed from the core of S. pallida, both in form and in geographical dispersal, despite their similar androecium, than are S. angustisiliqua and its immediate allies.

Bentham called attention to the wide variation in flower-size in his C. biflora and modern observers (Schery, 1951, p. 74; Isely, 1975, p. 71) have commented on instability of pubescence and noted the need of revision and the probable segregation of infraspecific taxa. The latter prophecy has been fully borne out in our study which is based on herbarium material many times greater than that available to Bentham a century ago, but flower-size and pubescence have proved obstructive rather than helpful in the definition of what appear to be, for less immediately obvious reasons, genuine self-perpetuating entities. The stems and foliage of S. pallida vary from perfectly glabrous to densely pilosulous throughout, but the imperceptibly gradual intergrades in type of vesture, which itself varies independently in density, orientation, length and color, are only very locally correlated with other morphological features of significance and against a wider background emerge as erratic. Thus, following var. pallida across the northern states of Venezuela we find the plants in Carabobo glabrous, in Aragua variably pilosulous, in Distrito Federal, Miranda and Yaracuy randomly pilosulous and subglabrous, and in Nueva Esparta again prevailingly glabrous, as in distant Carabobo or on the nearby island of Grenada. The small range of the exceptionally pilose form (Peiranisia velutina sens, str.) that represents var. pallida on the Cauca-Patia divide in southern Colombia is bracketed by populations of glabrous plants, immediately to the north (Huila) and south (Narino). In our var. bahamensis, endemic to the Bahamas and far eastern Cuba, the prevailing condition is glabrous below the ovary, but our one collection from Atwood Cay, otherwise identical (even to the diagnostic androecium) to those from islands to the north and south, is pilosulous. Pilosulous Cassia trichocraspedon, originally thought to be exceptional in the long-ciliate sutures of the pod, and glabrate C. tortuosa share a broad pod that is foreign to vicariant forms of var. pallida and except for pubescence are essentially identical; we refer these without hesitation to a collective var. trichocraspedon. However visually arresting, pubescence in this group contributes nothing to the definition of useful taxonomic categories.

A marked feature of S. pallida is seasonal dimorphism of the foliage, perhaps potentially present in all forms of the species but becoming especially evident in regions subject to annual dry seasons such as the Sonoran Desert and Yucatan Peninsula. Here a given plant may produce in wet months solitary two-flowered racemes from axils of ample primary stem leaves and, during drought, when those primary leaves have been shed, continue to flower from brachyblasts, either leafless or clad in much diminished and often much simplified leaves. The drought phases of varieties which, in days of prosperity, are obviously different in number or venation of the leaflets, or in their pods and seeds, are disconcertingly similar.

As for flower-size, it seems impossible to derive any useful criteria from this source. The seemingly random irregularity of the corolla is difficult to measure exactly, and we have found that flowers from two stems in one collection, from two collections from one site, and from sympatric collections taken at different seasons of the year, may span most of the range of variation in the whole series Interglandulosae. It is true that the flower of var. bahamensis is consistently on the small side of the norm and that of var. macdougalliana near or at its upper limit, but no discontinuities of dimension were found between any two closely allied forms. The same can be said of the anthers and their filaments.

The characters found most useful as taxonomic markers within S. pallida are emphasized in the varietal key following. Most of them are derived from aspects of the foliage: the condition of the seta, the length of the petiole in relation to the interfoliolar segments of the leafstalk, the number, size, accrescence, and in particular the venulation of the leaflets. These are reinforced in some cases by features of the pod or seed, rarely of the always inconveniently caducous stipules, and in almost all cases by considerations of range, indispensable for identification of incomplete specimens and those representing the inhibited drought-phase of their sort. The ranges of most varieties are mutually exclusive, but the differences, especially in southern Mexico and Central America, are as often altitudinal as not. In all cases where our knowledge is reasonably detailed we find that the varieties adapted to cool uplands are different from those in tierra caliente at the same latitude. In Pacific Mexico var. pallida and var. geminiflora are sympatric in the lowlands; we suspect that they are ecologically separated, but not totally for intermediate forms are frequent. Exceptionally two or three varieties of var. pallida occur in one place. At the same season in different years, near Lachao (municipio of Juquila) in the Sierra Madre of southern Oaxaca MacDougall found populations of var. geminiflora (with about 5 pairs of large, upwardly accrescent, intricately venulose leaflets), var. macdougalliana (similar except for smooth leaflets and glandiform seta), and var. triquetripes (with 11-15 pairs of small, non-accrescent leaflets, a normal petiolar seta and dilated rachis). These specimens, taken out of context of S. pallida sens, lat., might pardonably be attributed to distinct species. Unfortunately we have no information on micro-habitats or possibly critical differentiation by soil, exposure or elevation. All have the same flower adapted to pollination by bumblebees and they flower simultaneously at midwinter. A combined genetic-ecological study of this and similar situations in highland Guatemala would surely be rewarding.

For more than two centuries the epithet constantly applied to the present species is the descriptive biflora, which dates from Species Plantarum (1753) but is based on a plant seen by Linnaeus in the conservatory of George Clifford at Hartekamp in 1736. With the passage of years Cassia biflora became stabilized in the literature, but at the same time it acquired by accretion an increasingly vague and comprehensive meaning. The broad concept of a polymorphic C. biflora (excluding var. rostrata) developed by Bentham is coextensive with our Senna pallida and S. angustisiliqua combined, including such of the now recognizable infraspecific taxa as had reached Europe at that time. Britton & Rose (1930) disassembled a number of the component elements of C. biflora sensu Bentham into independent species of Peiranisia, but their analysis was incomplete and the residue left in the synonymy of P. biflora was still a disorderly mixture of S. pallida (several vars.) and S. angustisiliqua. Precise identification of Cassia biflora L. is not possible, because no typus was preserved. The pro- tologue (Linnaeus, 1753) and the antecedent description in Hortus Cliffortianus (1738) are the only evidence to be considered. The description is a full one and clearly indicates a Senna of ser. Interglandulosae with two-flowered racemes but the critical features that distinguish S. angustisiliqua from S. pallida or, assuming for sake of argument that it was indeed our S. pallida, that would determine the particular variety of it, are not mentioned. Linnaeus attributed C. biflora to the Indies, meaning the West Indies (and not India, as Britton & Rose, 1930, p. 260, erroneously supposed), but this might indicate either Jamaica, or the Bahamas, or Hispaniola, or Cuba and consequently a different taxon. We mention under S. pallida var. bahamensis the supposition of N. E. Brown that Linnaean C. biflora came from the Bahamas, but there is no trace of hard evidence that such was the case.

The identity of C. tenuissima L., which was at first doubtfully (Colladon, 1816) and later by many authors without qualification referred to the synonymy of C. biflora, is equally mystifying. It was based by Linnaeus on a manuscript phrase- name of William Houston, but no corresponding specimen can be found in either the Linnaean collections or among Philip Miller’s at British Museum, and the perfunctory protologue defies interpretation.

Precise application of these two Linnaean names, based on convincing typifications, has eluded us. We regard them as insolubly ambiguous, and regretfully fall back for an epithet on the next earliest valid binomial, C. pallida Vahl.