Senna [K. Bauhin] P. Miller, Gard. Diet., Abr. ed. 4, vol. 3. 1754, sensu mag- nopere ampliat. Irwin & Barneby in Polhill & Raven, 1981, p. 105. Generitypus: "Senna alexandrina sive foliis acutis C[aspar] B[auhin] P[inax 397]" = Senna alexandrina P. Miller, 1768.—Cassia sect. Senna [Tournefort] DeCandolle ex Colladon, Hist. Casses 92. 1816. Cassia subgen. Senna Bentham in Martius, Fl. Bras. 15(2): 96. 1870; Trans. Linn. Soc. London 27: 513, 518. 1871.
Senna sensu Gaertner, Fruct. sem. pl. 2: 312, t. 146, fig. 4. 1791; Link, Handbuch 2: 139. 1831; Roxburgh, Fl. Ind. ed. Carey, 2: 339. 1832.
Cassia sect. Senna sensu Bentham in Bentham & Hooker, Gen. Pl. 1(2): 139. 1865.
General characters of subtribe Cassiinae, with these particular ones: pedicels ebracteolate (except S. peralteana and S. paradictyon); androecium (Fig. 10, 11) zygomorphic, functionally 4-10-merous, the stamens ± graduated in length from ad- to abaxial side of fl, the 3 adaxial stamens (except in Palaeotropic sect. Psilorhegma) staminodal, the (6-)7 fertile anthers basifixed, usually modified into heteromorphic sets of 4 median and (2-)3 abaxial (these lacking in S. hayesiana), all beakless or variably beaked or appendaged at apex, where dehiscent by 2 separate or confluent pores or short slits, the thecae either glabrous or pubescent but not ciliolate along the lateral (closed) sutures; pod either dehiscent or indehiscent, if the former inertly so through both sutures, the valves not elastically coiling, when indehiscent either breaking up into 1-seeded achenelike segments or the seeds released only by rotting of the valves; seed-funicle filiform; seed- coat commonly but not always areolate, never serially pitted.—Trees, shrubs, herbs, some monocarpic, the roots often blackish, lacking bacterial nodules; extrafloral nectaries (Fig. 9), when present, ovoid, globose, claviform or phalloid, excreting nectar from convex surface (plane in petiolar gland of S. kuhlmannii); foliage often foetid; lfts 1-many pairs or lvs rarely (except for transient eophylls) absent or (in Australia) phyllodal; inflorescence of 1-many-fld racemes, these axillary from hornotinous stems or sometimes cauliforous, then either solitary or paniculate; hypanthium solid, shallowly bowl-shaped or slenderly vase-shaped, often not externally differentiated from pedicel; calyx-lobes either equal or strongly graduated, the inner then longer, mostly obtuse; petals (in America always) yellow, rarely white, drying either yellow (brown) or whitish dark-veined, the corolla either zygomorphic or irregular, one abaxial petal then highly modified either as a flag or as an androecial shield, the vexillar petal almost always interior in praefloration; pistil either centric or enantiostylous; ovules 5-266; stigma punctiform or dilated and cupular, usually ciliolate; pod (Fig. 12-14) highly variable in length, texture and compression, primitively piano-compressed, becoming turgid, angulate, terete, or rarely either its valves or its sutures winged lengthwise, the valves sometimes crested or otherwise elevated over seeds, the dry or pulp- filled cavity continuous or commonly transversely septate, the valves commonly continuous, sometimes breaking up into panels or into indehiscent achenelike segments; seeds free or (Madagascan Senna perrieri Ghesqu.) adherent to the endocarp, mostly 1-, less often 2-, exceptionally 4-seriate, either transverse or basipetal in orientation, lying broadside either to the valves or to the interseminal septa, their testa naked or wax-coated (crackled when ripe), smooth or irregularly pitted, areolate or not; 2n = 22, 23, 26, 28.—Spp. ±260, about 4/5’s American, these referred below to 35 series in 6 sections, the whole genus circumtropical but extending into warm temperate, temperate desert-grassland and exceptionally into cool-temperate zones of both hemispheres, almost ubiquitous throughout the Neotropics and as widespread (but less numerous in kinds) in tropical Africa, Madagascar, and Australia, fewer in s.-e. Asia and Pacifica. Many cultivated for ornament.
The syndrome of characters that separates Senna from Cassia sens, restr., and incidentally from Chamaecrista also, has been discussed under the title of genus Cassia. In this place we propose to scrutinize some aspects of its complex internal structure and concurrently to justify, or at least explain, the principles that underly the taxonomy that follows.
Over its global range Senna displays a rich diversity of detail in habitat and habit, in foliage, inflorescence, flower and fruit. The principal variable features are here analyzed from the viewpoint of relative specialization in the belief that such analysis, set against a broad geographical background, yields insights into the recent evolutionary history of the genus.
Life-form: Dwarf tufted herbs, rank annual ones, buried shrubs that take the form of herbs arising from a xylopodium, ephedroid bushes, sarmentose bush- ropes, free-standing shrubs and umbrageous trees are numbered among the sennas. We suppose, a priori, that the common ancestors of subtribe Cassiinae, and presumably of Senna, were primitively woody, mesophytic, macrophyllous, macrothermic and heliophilous. From this base arose the more specialized life-forms: the liana adapted to closed forest, the monocarpic herb to disturbed successional environment, the microphyllous and leafless shrubs to seasonally dry savanna and desert, the herbaceous cryptophytes to winter frost or to fire. Almost all contemporary sennas, by this criterion, are specialized in growth-form, many highly so. We assume that an inherent adaptability has enabled Senna to proliferate into increasingly arid and otherwise hostile environments. This general trend has occurred independently in North America, South America, Africa and Australia, sometimes on parallel lines.
Pubescence: Seven main types of trichome are encountered in Senna: i) simple short, incurved or straight unicellular, whitish or often lutescent, almost universal throughout the genus even if only vestigial along margins of leaf-stalk or sepal; ii) minute dilated-glandiform but not secretory, often colored (red, brown, orange), frequent in groups of species otherwise heterogeneous; iii) fine setiform unicellular, much longer than type i, of curiously erratic occurrence both within groups of related species and within the individual species; iv) 2-several-cellular setiform, excretory at apex, a type rare in this genus (ser. Interglandulosae) that is highly developed in Chamaecrista sect. Absus; v) spiculiform, non-secretory though often discolored, found mostly around the pulvinus and pulvinules of leaves, and within or around the floral hypanthium; vi) capitate-glandular, known only in monotypic ser. Aculeatae; and vii) stellate, unique to monotypic ser. Astroites. While hairs of type iv, vi and vii are obviously specialized the remainder are widespread, without mutual correlation, throughout the genus and may be absent or present in pairs of closely related species, sometimes in individuals of one species. Epidermal prickles, as well as glandular hairs of type vi, are unique to S. aculeata.
Stipules: The stipules of Cassiinae in general have no obvious function. They do not protect the leaf-bud and are ordinarily small, or caducous, or both. Those of Senna range from minute dry subulate structures through oblanceolate (often falcate) herbaceous blades to the broad foliaceous ones of e.g. S. (Bacillares) herzogii and latifolia (in part), S. (Stipulaceae) stipulacea, S. (Coriaceae) corifolia, S. (Coluteoideae) subulata and the Paleotropic Cassia auriculata L. Within a given group, often within a given species, development of foliaceous stipules is often capricious and labile, and has clearly arisen in several mutually remote evolutionary lines. They appear to have been lost entirely in ser. Aphyllae.
Leaves: A leaf composed of perhaps 5-10 pairs of moderately ample leaflets is the model from which it is most easy to derive all leaves of contemporary Senna. Specialization has proceeded primarily in two directions: toward fewer and larger and toward more numerous and smaller leaflets, the reciprocal adjustment between size and number furnishing a more or less constant photosynthetic apparatus. The range of variation in number within species rises and falls with the average number, becoming exactly stabilized only at one, two or rarely three pairs. Where Senna has become adapted to desert or near-desert conditions the photosynthetic function has been transferred, partly or entirely, from the leaflets to green stems (ser. Aphyllae, Armatae, of South and North America respectively) or to phyllodes (in Australian members of sect. Psilorhegma). In some cases (e.g. S. acuruensis, S. pallida sens, lat.) a change of leaflet number is the principal morphological expression of recent geographical divergence. In other groups, especially in the bijugous ser. Bacillares, leaflet number must have been stabilized at an early date, antecedent to the proliferation of the species.
Petiolar glands: We suppose the nectariferous gland to be an archaic feature of Senna which has been suppressed independently in a few advanced evolutionary lines, not closely allied to one another: sect. Senna; some series of sect. Chamaefistula and several of sect. Peiranisia. Alternative hypotheses, that a similar gland has developed independently in different branches of the genus, each from glandless predecessors, or that the glanduliferous and glandless species form mutually exclusive natural groups, are not supported by other data. In some species of sect. Peiranisia ser. Interglandulosae and of sect. Chamaefistula ser. Brachycarpae and Confertae, as in all species of sect. Chamaefistula ser. Coriaceae and Laxiflorae, a nectariferous gland similar to that of the leafstalk occurs at base of or attached to one edge of the pedicels. All of these clavate or phalloid nectaries like the cup- or shield-shaped ones of Chamaecrista, are attractive to ants, which have been supposed to deter predators. The extent to which this symbiotic relationship is of real benefit to the Senna partner is unknown.
Inflorescence: The basic unit of the Senna inflorescence is a raceme axillary to a developed leaf, and an indeterminate sequence of lateral several-flowered racemes is the inflorescence that either prevails or survives in all major groups of species. Reduction and subsequent elimination of distal leaves subtending individual racemes, sometimes accompanied by shortening of the primary axis, produces a terminal exserted thyrse or panicle which, when the elementary racemes are reduced to one or two flowers apiece, may simulate a terminal raceme. The cauliflorous panicle of some Bacillares is structurally identical to that terminating an annotinous branchlet except that it is wholly leafless. Apparently simple cauliflorous racemes in the same series, like those arising from year-old wood of some drought-deciduous members of sect. Peiranisia, are panicles reduced to one elementary racemose axis. These specializations of the inflorescence no doubt correspond to particular biological advantages that may be gained by compression of flowering time and energy into a showy but transient display or by appeal to a particular pollinator. However that may be, a particular inflorescence type has rarely been stabilized within a group of species otherwise defined, and its organization may even be modified in the course of its own maturescence. Flower-number, like leaflet-number, approaches stability only when very low. The scapiform one-flowered raceme of S. pumilio is almost invariable, but those of the prevailingly biflorous Interglandulosae (S. pallida and kindred) are in fact one- to three- or rarely four-flowered.
Calyx: The sepals of Senna are prevailingly graduated from short to longer in an upward spiral and are obtuse at apex. Subequal sepals of ser. Brachycarpae and Tharpia are certainly derived, as are the lanceolate acute one of S. (Bacillares) acutisepala. Venation of the sepals is palmate except in monotypic sect. Paradictyon, which see for discussion, and varies only in strength, prominence and number of primary veins. When proportionately small the sepals may separate long before true anthesis, exposing the immature petals and androecium. Conversely, proportionately ample sepals (as in S. latifolia or S. trolliiflora) permanently nidulate all the inner floral parts. The extremes both occur in the one series Bacillares.
Corolla: Senna displays two basically different types of corolla, one vertically zygomorphic, like that of Cassia, which we consider relatively primitive, the other highly irregular, like that of Chamaecrista, following differentiation of one of the two abaxial petals in opposition to an asymmetrically displaced pistil. In the first case an axis of symmetry runs vertically through each flower, in the second case vertically between two contiguous flowers of a raceme, which mirror each other in the mode called enantiostylous. A third type of corolla, encountered in some Bacillares and elsewhere, is randomly asymmetrical, all petals except the vexillar one being of odd and irregular shapes and sizes. Because this third sort is combined with a centrically (or subcentrically) oriented pistil we regard it as a variation of simple zygomorphy. It seems certain that the asymmetric perianth and pistil which composes the Peiranisia flower is designed to promote outcrossing and evolved in relation to insect pollination, no doubt in more than one evolutionary line. The Malesian Cassia divaricata (cf. De Wit, 1955, p. 242, fig. 2) and the Australian C. odorata, both of which retain the decandrous androecium of sect. Psilorhegma, have the perianth of the American Interglandulosae. The simulation is so exact that De Wit suspected an American origin for C. divaricata and Bentham was deceived by a mislabelled specimen of C. odorata into describing a supposedly Brazilian C. riedelii. Consequently we have no assurance that all the American sennas herein referred to sect. Peiranisia belong to a monophyletic group.
In terms of floral specialization the genera of Cassiinae stand at three levels: at the bottom Cassia, with invariably zygomorphic flower; at the top Chamaecrista, in which an asymmetric perianth and displaced pistil, common to all members, must have been stabilized prior to differentiation into section and species; and between them Senna, in which intermediate stages of distortion coexist with exactly zygomorphic and highly asymmetric flowers.
Androecium: It appears probable, a priori, that the functionally decandrous androecium of sect. Psilorhegma (e.g., Cassia nemophila—Fig. 10; see also pod, Fig. 13) is more primitive than and antecedent to that of all other proups of Senna, in which the three adaxial stamens, that opposed to the vexillum and its two immediate antesepalous neighbors, are sterile rudiments, or lacking, or assume a secondary function of flag or eyespot. In the American sennas one can trace progressive differentiation of the seven fertile stamens into two sets of four shorter median and three longer abaxial members, the first set providing food- pollen, which visitor bees extract by vibratory milking, while the second is so oriented as to deposit pollen on the bee’s underside (sect. Chamaefistula) or flanks (sect. Senna) for transport to the stigma of another flower. Developments of this theme are i) degeneration and ultimate loss of the centric abaxial stamen; ii) aggrandizement of the centric abaxial stamen at the expense of its immediate neighbors; iii) loss of all three abaxial stamens (5. hayesiana); and iv) displacement of the two long abaxial antepetalous stamens from a plane parallel to their opposed petal to a plane horizontal to the flower’s vertical axis, the signature of sect. Senna. A converse trend toward uniformity is seen in sect. Chamaefistula ser. Brachycarpae and sect. Peiranisia ser. Isandrae in which the seven fertile stamens, while retaining evident traces of differentiation into two sets and still perceptibly graduated in size from back to front of the flower, resemble each other in orientation of the filament, in size and curvature of the anther and in orientation of its terminal pore or slits.
In part concurrently with and in part independently of differentiation into morphologically and functionally unequal sets, the stamens of Senna have developed specialized orifices for egress of pollen which may be interpreted as promoting its efficient transport. At the simplest each theca of the anther opens by a discrete apical slit looking upward and outward. Later the two slits fuse into one round or U-shaped pore, divided or not by an exiguous septum. The slits of the four median stamens, whether separate or confluent, are shifted by imperceptibly small steps from terminal to a position at first obliquely terminal and finally lateral infraterminal. Perfectly or partly coalescent beaks, especially in the long abaxial anthers, develop by more or less abrupt strangulation of the thecae, and these beaks become variously modified into a truncate tubule, an oblique pollen-cup, or develop a tonguelike appendage produced beyond the true orifice. Anthers evenly tapering to a bluntly rounded or abruptly truncate, biporose apex represent the simplest type in Senna and are presumed to represent that from which the rest arose.
Pistil: The pistil of Senna, whether aligned along the vertical axis of floral symmetry or turned alternately left and right in opposition to a modified abaxial petal, varies little in form except for length of style and size and orientation of the stigma. The number of ovules, on the other hand, varies enormously, with consequences to the pod discussed under the next heading. Curiously there is only partial correlation between ovule number and capacity of the stigmatic orifice. The massive hollow stigma of S. (Bacillares) quinquangulata, accommodates enough pollen grains to fertilize more than 200 ovules, but the stigmatic orifice of S. (Coluteoideae) pendula, which may have as many as 150 ovules, remains minute and punctiform like that of most pauciovulate species. We suppose that a simple linear style of moderate length, with punctiform stigma looking upward, preserves the archetypical form. A distally incurved style, dilated or not below the stigma, promotes self-fertilization of the flower, whereas the whole architecture of the longistylous flower, especially that of sect. Peiranisia when the long anthers are shielded by one abaxial petal from contact with the laterally displaced pistil, seems designed for out-crossing. A trend away from entomophily to autogamy which is expressed by simultaneous inward curvature, shortening and dilation of the style has reached a climax in the advanced monocarpic S. uniflora. In this species the enlarged stigma is poised in the bud to receive precociously released pollen from anthers directly facing it; and most flowers are, in fact, selffecundated before the small ephemeral corolla expands. Changes in spatial relationships between pistil and androecium, upon which successful pollination is contingent, probably have occurred many times in the evolutionary history of Senna, but once made are conservative and consequently become significant in interpretation of affinities.
Pod: The simplest form of Senna pod is a piano-compressed bean inertly dehiscent along both sutures and enclosing some 10-20 seeds which are laid across a dry cavity and turned broadside to the valves. From such a pod all other types seem to be derived, either by multiplication (less often by loss) of ovules, by elaboration of a pulpy endocarp enveloping the seeds, by pressures brought to bear on the valves by the swelling seeds, by epidermal excrescences of wings or crests arising from either valve or suture or, in a few highly specialized groups, by development of lomentiform constrictions and lines of fracture between seeds. Seeds of American sennas are highly vulnerable to attack from larvae of Bruchidae, the effects of which, in absence of chemical defenses, are mitigated by alternate strategies: production over a long season of many few-seeded pods, some of which are likely to evade the predator; and production of few massive multi ovulate pods which, even when infested, are likely to produce at least a small viable harvest. Reduplication of ovules, by far the commoner (or more successful) of the two strategies, has had profound effects on the ultimate outline and compression of the pod and often also on orientation and shape of the mature seeds, each of which is obliged to adapt itself to a limited space in its chamber of the pod, as the body of a mollusc adapts itself to its shell. A great number of ovules can be accommodated in different ways, either stacked in one row along an elongate and, for economy’s sake, narrow pod; or displaced laterally into two rows along a pod at once half as long but twice as wide; or turned broadside to the interseminal septa which, by closing up the rank of seeds, permits a sacrifice of length to a plump capacious cavity; or exceptionally, as in S. ruiziana, doubly doubled up into four interdigitating ranks, an unexpected compromise between length of pod and a high number of propagules requiring space for growth.
Disregarding epidermal excrescences and lines of fracture between seeds, the diversity of the senna pod can be viewed as the outcome of reciprocal adjustments between ovule-number and potential expansion of valvular tissue. These adjustments, however, have not followed a single unidirectional sequence but appear rather to have occurred at random, independently, and as often as not at the level of specific differentiation. For example, the cavity of the crenately winged pod of S. alata is rhombic in cross section and the seeds become rhomboidally distorted at maturity, compressed vertically to the internally crumpled cotyledons and consequently bearing the areole on the narrower faces of the testa. But in closely related S. reticulata, which has a conventional piano-compressed pod, the seed accommodates itself to the cavity, not vice versa, and the areole is on the broader faces. The gamut of variation in pods of ser. Bacillares, assuming for argument that this otherwise morphologically uniform group is indeed mono- phyletic, is extraordinarily wide, involving ovule-number and deployment of seeds, mode of dehiscence, development of pulpy endocarp, intrusion of a woody false septum from one suture and other minor features. The small ser. Isandrae, defined within sect. Peiranisia by a strikingly individual androecium, exhibits a comparable diversity in ovule-number and in texture and dehiscence of the pod. As an index of affinity the pod has been much overvalued by past students of Cassiinae.
Seed: The graphic display of senna seeds assembled by Lasseigne (1979, figs. 90 incl.) well illustrates their diversity in size and outline, much of which, as discussed above, we attribute to constraints from the walls of the individual locule. The most notable external feature of the seed is the areole, present in all extra-American members of the genus and in the majority of New World species. Because the areole is common to all sections of Senna, and to all its most disparate monotypic series, we regard it as an archaic feature, wholly suppressed only in ser. Stipulaceae, Corymbosae, monotypic Harleyanae and most species of ser. Bacillares, but surviving in a few species of the latter series. The alternative hypothesis, that an unmarked seed survives in a few evolutionary active American groups and that an areole has appeared independently in remotely related ones scattered around the globe, appears less likely.
In the foregoing section we have summarized our thoughts, developed in greater detail in the taxonomic text that follows, on the gross morphological diversity of Senna and what we can more or less safely infer from it concerning the evolutionary significance of particular character states. A probable sequence of modification of a particular feature is not hard to perceive within the context of a section or series otherwise defined; but synthesis of all these sequences into an orderly model of progressive specialization is not possible. The species of Senna existing in our times present a mosaic of relatively primitive and relatively advanced characters which must have developed at different periods and at different rates of change, in the manner called by Takhtajan (1980, p. 232) heterobathmic. Selection seems to have operated asynchronously on the plant body, the floral mechanism and the fruit, and the data are often contradictory. No sennas primitive at all levels exist today. Difficulties of interpretation arise in distinguishing homology from analogy, and the possibility of evolutionary reversal is troubling. It is hard to reconcile, for example, the ostensibly primitive decandrous androecium of sect. Psilorhegma with the contemporary range of the section in Australia, Pacifica and Indonesia. lf the decandrous androecium were in fact a reversal toward symmetry it would be easier to explain the perplexing overall similarity between the Malesian Cassia (Psilorhegma) divaricata and the American Senna (Peiranisia) pallida, particularly noted by Bentham (1871, p. 554) and De Wit (1955, p. 242).
Most of the rare, narrowly endemic or ecologically insulated American sennas are obviously recent derivatives of more widely dispersed progenitors and most of the relatively few broken-up areas of dispersal in the New World are as easily explained by aggressive discontinuous colonization as by relictual persistence in favorable corners of a formerly continuous range. It is otherwise, however, with the world distribution of sect. Senna, and especially of ser. Pictae, which contains apparently close kindred endemic to Brazil (S. martiana), Africa (S. didymobotrya) and Australia (Cassia venusta F. Muell.). The characters by which these spatially dissevered species are perceived as kindred are not found in the fruit but in the peculiar androecium of sect. Senna coincident with a common facies; the pods, isolated from the plant, are of a generalized type common to diverse groups within the genus. The androecium of sect. Senna, which is specialized and idiosyncratic in high degree, must have evolved prior to differentiation of its species and to establishment of a circumtropical dispersal. It must also be a conservative and stable organ which has survived the genetic upheavals incident to speciation. Differentiation of the androecium is implicated in the primary generic differentiation of subtribe Cassiinae and apparently has subsequently continued an orderly though not necessarily unidirectional progress of specialization. By contrast the pod and seeds have undergone profound changes unaccompanied by perceptible alteration in the flower (cf. S. alata and S. reticulata), a situation that students of Leguminosae will recognize as familiar.
The sectional synopsis of Cassia subgen. Senna presented in Bentham’s monograph (1871, p. 513) is based primarily on carpology. On close acquaintance, however, the reader finds that the pod was not in reality of paramount concern in Bentham’s system which is based rather on intuitive sorting of multiple similarities. Where inconvenient (e.g. ser. Confertae), the pod was ignored. Although we have shifted primary emphasis from the fruit to the organization of the flower, our classification is based foursquare on that of Bentham’s subgenus Senna. Except for minor details, relating to species incompletely known a century ago, our infrageneric groups are, unless enriched by taxa described since 1871, those formulated by Bentham or adapted by him from Colladon and Vogel. The nomenclature, following transfer from Cassia to Senna, is of course different, and reevaluation of the so-called basal petiolar gland of sect. Oncolobium (=ser. Basiglandulosae) of the basipetal seeds of sect. Prososperma (=ser. Trigonel- loideae) has induced us to demote these sections of subgen. Senna to serial status in sect. Chamaefistula. The sections are presented in an order that reflects our views on evolutionary specialization of the perianth and androecium and, we believe, a rational recapitulation of the phylogeny.