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Doctrine of Signatures: An Explanation of Medicinal Plant Discovery or Dissemination of Knowledge?' BRADLEY C. BENNETT

Department of Biological Sciences and Center for Ethnobiology and Natural Products, Florida International University, Miami, FL 33199

DOCTRINE OF SIGNATURES: AN EXPLANATION OF MEDICINAL PLANT DISCOVERY OR DISSEMINATION OF KNOWLEDGE? The Doctrine of Signatures (DOS) is found throughout the world. Most scholars dismiss it as a "primitive" or "prescientific" idea. Despite its long history, the doctrine has had little critical review. A careful evaluation of signatures suggests four things. (1) There is no evidence that morphological plant signatures ever led to the discovery of medicinal properties. Considering DOS in this manner is unproductive and largely untestable. (2) Signatures are post hoc attributions rather than a priori clues to the utility of medicinal plants. (3) It is productive to redefine signatures to include organoleptic properties associated with therapeutic value. Plants with strong odors or bitter tastes, for example, commonly are found in pharmacopoeias. (4) DOS should be considered for what it primarily is—a way of disseminating information. DOS fundamentally is a mnemonic and, therefore, is exceedingly valuable in traditional cultures.

Key Words: Doctrine of signatures, medicinal plant selection, medicinal plants, mnemonic, organoleptic properties, transmission of knowledge, Paracelsus, signatures.

The Doctrine of Signatures (DOS) is a much–maligned theory that purportedly explains how humans discovered medicinal uses of some plants. DOS could be stated as form recapitulates function—physical characteristics of plants reveal their therapeutic value. For example, bloodroot (see Table 1 for binomials, families, and signatures of all cited taxa) contains the orange–red alkaloid sanguinarine (Bennett et al. 1990). According to DOS proponents, the reddish color indicates the plant's efficacy in treating blood ailments. DOS also has a more specific meaning. Some believe that the physical clues to a plants use were given by the Creator. The wild woods were full of creatures whose value was written on each of them in the language of signatures, if the seeker for simples could only manage to decipher the label with which it had been considerately tagged at the creation. (Eggleston 1901) While DOS is often dismissed as primitive superstition, I argue that it is primarily a symbolic

1

Received 18 October 2006; accepted 27 April 2007

Economic Botany, 61(3), 2007, pp. 246-255.

device used to transfer information, especially in preliterate societies. Shepard (2002) suggested a similar idea when he wrote, "I hypothesize that sensory evaluation serves as a mnemonic purpose as well." Historical discussions of DOS are post hoc interpretations by scholars who attempt to explain why a plant is used for a particular ailment. There is little evidence that a priori analyses of morphological signatures led to the discovery of any medicinal plant. Moreover, this application of DOS is largely untestableresearchers were not present when the utility of medicinal plants was discovered. However, physical properties correlated with phytochemical attributes (e.g., strong odors and the presence of monoterpenes, bitter taste and the presence of alkaloids) may have led to the discovery of some healing plants. The Doctrine of Signatures is ubiquitous (Balee 1994). According to Powell (1901), "All American tribes entertain a profound belief in the doctrine of signatures." Read (1927) wrote that DOS is "to be found extensively quoted in the Orient and Occident." DOS is alluded to in classical Greek literature on medicinal plants. Milton

© 2007, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.

Species

Aconitum nape/lies L. Fittonia albivenis (Lindl. ex Deitch) Brummitt Sanguinaria canadensis L. Anchusa officinalis L. (perhaps) Euphrasia officinalis L. Digitalis lanata Ehrhart, Digitalis purpurea L. Panax quinquefolium L. Scorpiurus sp. (perhaps) Columnea tessmannii Mansf. Mandragora officinalis L. Alkanna tinctoria Tausch Columnea ericae Mansf. Portulaca oleracea L. Saxifiaga spp. Rauvoia serpentina (L.) Benth. ex Kurz Aristolochia serpentaria L. Momordica charantia L. Sedum acre L. Juglans regia L. Cynoglossum virginianum L. Salix spp. Columnea crassa C.V. Morton Magnolia spp. Melissa officinalis L.

Common name

aconite akapmas bloodroot bugloss eyebright foxglove ginseng herb scorpius machancha mandrake orchanet puca panga purslane, common saxifrage snakeroot snakewort soroci stonecrop walnut, English wild comfrey willow not cited not cited not c i ted Solanaceae Boraginaceae Gesneriaceae Portulaccaceae Saxifragaceae Apocynaceae Aristolochiaceae Cucurbitaceae Crassulaceae Juglandaceae Boraginaceae Salicaceae Gesneriaceae Magnoliaceae Lamiaceae

Geseneriaceae

Ranunculaceae Acanthaceae Papaveraceae Boraginaceae Orobanchaceae Plantaginaceae Araliaceae Fabaceae

Family

eye like fruit liver—shaped leaves red—orange latex seeds shaped like a viper's head striped petals heart—shaped leaves(?) human—shaped rhizome coiled shoots resemble a scorpion's tail red—tipped leaves human—shaped roots viper—shaped seeds red—tipped leaf stems worm—like breaks rocks as it grows roots coiled, snake—like leaf shape is similar to a snake's head red aril leaf shape convoluted cotyledons fruits cling to skin supple branches red spots on leaves heart—shaped leaves heart—shaped leaves

Signature

ocular problem liver pain blood venomous bites eye problems cardiac problems panacea scorpion bites menstrual bleeding, snakebite panacea snakebite menstrual bleeding, snakebite anti—parasitic kidney stones snake bite snake bite blood tonic kidney stones brain ailments memory rheumatic joints nose bleeding cardiotonic heart weakness

Medical Use

READ 1927; ROWLINGS 1999; SAINE 2002; TYLER 1993). FAMILY DESIGNATIONS FOLLOW ANGIOSPERM PHYLOGENY GROUP (APG). N.A. = NOT APPLICABLE.

1658 [ORIG. 1558] ; DIOSCORIDES 2000 [ORIG. A.D. 651 ; GERARD 1975 [ORIG. 1597]; ToUW 1982; GRIEvE 1971 [oRlc. 19311; MOONEY 1992 [ORIG. 1900];

TABLE 1. EXAMPLES OF THE DOCTRINE OF SIGNATURES (BENNETT ET AL. 2002; CARLSON 1986; COLE 2004 [ORIG. 1656]; CULPEPER 1652; DELLA PORTA

N rL^

C

G1

0

zrn

O H

H

z z

Fri

O J

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(2002 [orig. 1667]) described the Archangel Michael's use of eyebright to restore Adam's vision. Eyebright's distinctively characteristic striped petals are supposedly reminiscent of bloodshot eyes (Tyler 1993). DOS appears in current literature, as well. In Harry Potter and the Chamber of Secrets, Madam Pomfrey revives petrified students with a mandrake root decoction (Rowlings 1999). Mandrake's signature is its human—like roots (Mabberley 1987).

History of DOS through the 19th Century Pliny's Natural History contains the first written record of the "pseudoscientific" Doctrine of Signatures (Encyclopa:adia Britannica 2006a). Dioscorides wrote, "... the Herb Scorpius resembles the tail of the Scorpion, and [emphasis mine] is good against his biting" (Dioscorides 2000 [orig. A.D. 65]). The phrase "and is good against his biting" is very different from "and therefore is used against his biting. Assuming the latter is the common logical fallacy of post hoc ergo propter hoc ("after this, therefore because of this"), which is common in interpretations of signatures. Maycock (1991) claimed, "Galen was resolute in his confidence of an underlying design throughout nature to be demonstrable in accord with a doctrine of signatures." Evidence for Galen's adherence to DOS is weak. He is known best for the concept contraria contrariis curantur ("opposite cures the opposite"), the foundation of allopathic medicine (Cooper 2004). Paracelsus, the greatest DOS proponent, and his followers espoused the position similia similibus curantur ("like cures like"). He believed that God provided signs within plants to indicate their uses: "The soul does not perceive the external or internal physical construction of herbs and roots, but it intuitively perceives their powers and virtues, and recognizes at once their Signatum" (Boericke 2004; Clarke 1923; Keightley 1925). Bohme (1621) reiterated the Paracelsian theory: "The greatest understanding lies in the signatures, wherein man may not only learn to know himself, but also the essence of all essences." Nicholas Culpeper (1652) wrote, "... by the icon or image of every herb, man first found out their virtues." The 17th century botanist William Cole (2004 [orig. 1656]) believed that some plants were given signatures "in order to

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set man on the right track," but others were left blank to encourage humans to discover them. Della Porta provided the definitive DOS exposition of the 16th and 17th centuries (Clubb 1965). According to della Porta (1658 [orig. 1558]), "... Bugloss and Orchanet bear seeds like a Viper's head, and these are good to heal their venomous bitings. Likewise Stone—crop and Saxifrage are good to break the stone in a man's bladder." Another follower of Paracelsus, Oswald Croll was equally strong in his espousal of DOS: "In like manner, herbs magically by their signature ... manifest their interiors, concealed in the occult silence of Nature" (Crollius 1669).

History of DOS since the 1900s Five publications cited in the 1947-1996 index to Economic Botany refer to DOS (Kaplan et al. 2001). Vickery (1981) and Touw (1982) cited historical references. Browner (1985) concluded that two plants were "apparently selected" for infertility due to their resemblance to human genitalia. Carlson (1986) wrote, "The closer the resemblance of the [ginseng] root is to the human figure, the greater is it thought of as having associative signature." Hazlet (1986) averred, "The medieval notion that plant morphology can suggest medicinal use of a plant ... was manifest in several Cabecar medicinal plants." More recently Dafni and Lev (2002) described DOS in modern Israel folk medicine. "Heart—shaped" leaves of Melissa officinalis were employed to treat heart weakness (Dafni and Lev 2002).

Critiques of DOS Dodoens (1583) was perhaps the first to dismiss DOS: "... it is so changeable and uncertain that, it seems absolutely unworthy of acceptance" (translation from Arber 1988 [orig. 1912]). Ray (1717 [orig. 1691]) expressed disbelief in DOS. Hahnemann (1825), the founder of homeopathy, ridiculed the theory: "I shall spell the ordinary medical school the humiliation of reminding it of the folly of those ancient physicians who, determining the medicinal powers of crude drugs from their signatures." Thiselton—Dyer (1889), who reviewed DOS in the 16th and 17th centuries, found DOS reference in most medical works and noted that it was "treated with a seriousness characteristic of the

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BENNETT: DOCTRINE OF SIGNATURES

backward state of medical science ...... Read (1927) suggested that the influence of DOS diminished beginning in the 1700s but Arber (1988 [orig. 1912]) argued that it persisted into the 19th century. Modern herbals, pharmacopoeias, textbooks, and peer—reviewed literature are nearly universal in condemning DOS. It is called fanciful, far—fetched, premodern, prescientific, primitive, unreliable, and unscientific (Table 2). Simpson and Ogorzaly (2001) stated that the idea "seems absurd now but received great acclaim when it was proposed. Luckily, it was soon displaced by less subjective and more secular methods of determining a plant's medical efficacy." Tyler (1994) ranked it among his 10 False Tenets of Paraherbalism. Barford and Kvist (1996) concluded that Ecuadorian plants "used according to the Doctrine of Signatures" are less promising pharmacological leads than other medicinal plants. Balee (1994) succinctly summarized the current view, noting that most economic botanists reject DOS. A few scholars are neutral in their descriptions of DOS. The fear of mandrake may have been a "primitive" result of DOS but it probably saved many lives according to Schultes and Hofmann (1980). Davis and Yost (1983) wrote, "Plants with

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a strong odor might repel symptoms in a manner reminiscent of the doctrine of signatures." In reference to willow bark's efficacy iri treating rheumatic pains, Kreig (1964) noted that, `At least one of these quaint beliefs had a fragment of truth in it." Fewer scholars interpret DOS in a positive light. In describing the discovery of a digitalis—like substance in a Mexican Magnolia species with cordate leaves, Plotkin (2000) commented, "As ludicrous as it sounds, the doctrine has yielded at least one medicinal compound in wide use until recently." James (n.d.) supported the utility of DOS as a mnemonic device: "In a period where most of the world was still largely illiterate, it is likely that the Doctrine of Signatures was useful as a mnemonic aid for the apprentice who was learning by observation and rote." The late Stephen Jay Gould (2000) provided the most honest assessment of DOS: I question our usual dismissal of this older approach as absurd, mystical, or even prescientific (in any more than a purely chronological sense) ... But how can we blame our forebears for not knowing what later generations would discover? We might as well despise ourselves because our grandchildren will, no doubt, understand the world in a different way.

TABLE 2. CRITIQUES OF THE DOCTRINE OF SIGNATURES. Recent Assessments

Source

"Viewed from a scientific standpoint, their theory and diagnosis are entirely wrong." "pseudo-scientific" "The modern educated man is apt to be cynical in his attitude to remedies based upon such unscientific dogmas." "primitive imagination correlating shapes of organs and shapes of herbs by magical impulse" + "myth" "makes as much sense as believing that walnuts are good for mental illness because their kernels resemble the brain" "no scientific basis to this concept" "far-fetched" "seems absurd now" "unreliable, unscientific and absolutely contrary to the homeopathic methodology" "pre-modern knowledge" "totally debunked" "magical similarity was re-exhumed in an empirical and intuitive manner without any scientific understanding or experimental proof " "fanciful medical theory" "pseudoscientific methods"

Mooney (1992 [orig. 1900]) Arber (1988 [orig. 1912]) Read (1927) Buchanan (1991 [orig. 1938]) Tyler (1994) Levetin and McMahon (1999) Sumner (2000) Simpson and Ogorzaly (2001) Saine (2002) Panese (2003) Reed (2005a) Bellavite et al. (2005) EncyclopEadia Britannica (2006b) EncyclopEadia Britannica (2006b)

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Reinterpreting DOS Both critics and proponents of DOS assume that there is no diffusion of knowledge. According to their arguments, one would have to admit that it is used for continual rediscovery of a plant's medicinal value. In traditional cultures, plant knowledge is effectively passed from one generation to the next through observation and oral tradition. DOS should be reevaluated with respect to (1) its role in the discovery of medicinal plants, (2) post hoc attribution of signatures, (3) the nature of signatures, and (4) its role as a mnemonic. DOS AND DISCOVERY OF MEDICINAL PLANTS

Scholars were not present when healers in traditional cultures first learned of the value of medicinal plants. Therefore, it is impossible to say whether signatures influenced plant selection. If DOS was a ubiquitous method of choosing medicinal plants, as many argue, then plants bearing signatures should be more widely used than those lacking a signature. For example, one would expect many examples of heart—shaped leaves in botanical pharmacopoeias for cardiac disease. Is a plant's heart—shaped leaf a signature indicating the plant's utility in treating cardiac conditions? To address this question, I searched the International Plant Name Index (http://www .ipni.org) for six specific epithets that refer to heart—shaped leaves. The search yielded 2,584 binomials. I then randomly selected 80 binomials (ca. 3% of the total), sampling each epithet proportional to its occurrence (Table 3). For invalid names, I substituted another binomial with the same specific epithet. Next, I searched the print and electronic literature using both accepted names and common synonyms for ethnobotanical usage and pharmacological studies. Consulted literature included the Journal ofEthnopharmacology, Schultes and Raffauf 's The Healing Forest: Medicinal and Toxic Plants of the Northwest Amazonia (1990), Moerman's Native American Ethnobotany

(n.d.), Duke's Phytochemical and Ethnobotanical Databases (n.d.), U.S. National Library of Medicine's PubMed, and Prelude Medicinal Plants Database for Africa (Baerts—Lehmann and Lehmann n.d.). Of the 80 species, 21 are used medicinally. Three of the species are employed in a way that may relate to cardiac medicine. Alchornea cordifolia

TABLE 3. OCCURRENCE OF EPITHETS RELATING TO HEART-SHAPED LEAVES IN THE INTERNATIONAL PLANT NAME INDEX. Epithet

Occurrences

cordata cordatum cordatus cordifolia cordifolium cordifolius Total

980 303 115 824 243 119 2,584

(Schumach. & Thonn. in C.F. Schumacher) Mull. Arg., which has red fruits and cordate leaves, is a smooth muscle relaxant and a diuretic (Ogungbamila and Samuelsson 1990). Antigonon leptopus Hook. & Arn, has antithrombin activity (Chistokhodova et al. 2002). Sida cordifolia L., effective in treating brachycardia, is the only species that clearly has cardiac value (Medeiros et al. 2006). At most, 3 of the 80 species (or less than 4%) are utilized in manner consistent with DOS. These data clearly refute any a priori value of heart—shaped leaves as signs for cardiac activity. A second set of 80 randomly selected species, without respect to specific epithets was analyzed similarly. Of this data set, 17 species were employed in traditional medicine. The proportion of medicinal plants in this sample and the heart—shaped leaf sample do not differ statistically (Fishers Exact test, two—tailed probability P=0.578). In other words, a plant with a distinct sign is no more likely to be used than an arbitrarily selected one. POST HOC ATTRIBUTION

A second problem encountered when examining DOS is that signatures were applied after plants had been incorporated into pharmacopoeias. Read (1927) commented, ".. . in some cases the signatures of drugs were observed after their real use had been discovered." Reed (2005a) would concur: "It is likely that many valuable herbs were in use before the doctrine and that the organ—plant match was made later to accommodate and validate the doctrine." Consider the signature of willow bark cited by Kreig (1964) and many others. In a review of anti—inflammatory drugs, Vane and Botting (1998) provided the following historical account of its discovery.

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On June 2, 1763, Edward Stone read a report to the Royal Society on the use of willow bark in fever. He had accidentally tasted it, and was surprised by its extraordinary bitterness, which reminded him of the taste of cinchona bark (containing quinine), then being used to treat malaria. He believed in the 'doctrine of signatures', which dictated that cures for diseases would be found in the same locations where the malady occurs. Since the `willow delights in a moist and wet soil, where agues chiefly abound', he gathered a pound of willow bark, dried it over baker's oven for 3 months, then ground it to a powder. Rev. Stone "accidentally" tasted the bark. He did not select willow because of a priori morphological cues. The parson may have been motivated by the willow's growth in wet soil and the belief that "remedies lie not far from their causes." However, many other species occupy the same environment. Stone was objective in his approach. "I have no other motives for publishing this valuable specific, than it may have a fair and full trial in all its variety and circumstances and situations" (in Vane and Botting 1998). Morphological cues are insufficient to explain his selection. Mooney (1992 [orig. 1900]) denigrates traditional Cherokee medicines writing, "... the doctor puts into the decoction intended as a vermifuge some of the red fleshy stalks of the common purslane ... because these stalks somewhat resemble worms and consequently must have some occult influence over worms." Dafni and Lev (2002) also reveal the potential for predispositions to influence findings: "When the answer was suspected by us to be related to the Doctrine of Signatures, the next questions was `Why this plant is used for this specific purpose?"' Hazlett (1986), in reference to the red spots on Columnea crassa leaves, says, "This `sign' was interpreted as an indication that the plant is effective in stopping nosebleeds." Here, the passive voice makes it unclear as to who interpreted the signature. Browner (1985) asserted that two species are "are apparently selected" based on DOS. Saine (2002) criticized second–hand assessments, noting "At best, these post hoc associations make studying the materia medica quaint and colorful, but should never be confused with a priori postulations used for prescribing accordingly to the doctrine of signatures."

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[orig. 1912]). A signature, like beauty, is in the eye of the beholder. In 1628, Guy de la Brosse noted that it was easy to imagine any 'resemblance between a plant and an animal that happened to be convenient (cited in Arber 1988 [orig. 1912]). Ray (1717 [orig. 1691]) considered signatures to be "rather fancied by men than designed by Nature." Buchanan (1991 [orig. 1938]) cited foxglove's heart–shaped leaves, as a classic example of DOS. However, the leaves of foxglove are not heart–shaped. Cardiac glycosides are obtained from two species of foxglove. One has lanceolate leaves; foliage of the other is broadly ovate (Bailey et al. 1976). It is as difficult to find a heart signature in Digitalis leaves as it is to visualize human sex organs in the examples cited by Browner (1985). A second way to reevaluate DOS is to reconsider the nature of signatures. The coiled roots of snakeroot for snakebite (Levetin and McMahon 1999), the clinging fruits of wild comfrey for memory (Reed 2005b), and the eye–like seed of aconite for ocular problem (Noth 1998) fit the classical morphological definition. Signatures, however, need not be morphological. A common feature of medicinal pharmacopoeias is a large number of plants with strong odors (e.g. Ankli et al. 1999; Leonti et al. 2002; Shepard 2002). If there were laws of ethnobotany, one would be this: Plants with potent odors and strong tastes are employed in medicine and ritual. These sensory cues are correlated with the presence of bioactive compounds. It is no coincidence that herbs and spices possess antimicrobial activity (e.g., Billing and Sherman 1998; Sherman and Billing 1999). Expanding signatures to include olfactory or gustatory clues is entirely sensible. The human nose functions in the field the same way that a gas chromatograph functions in the lab—both are effective in detecting volatile compounds. Debus (1973) noted, "The chemist's [Paracelsus's] own laboratory procedures seemed to offer the proper key to nature's hidden secrets." Davis and Yost's (1983) suggestion that plants with a strong odor might repel symptoms echoes a similar view. Etkin (1988) argued that the selection of some red plant makes sense due to the presence of antimicrobial and haemostatic red quinones. ROLE AS A MNEMONIC

NATURE OF SIGNATURES

The application of signatures is not only post hoc, but also elastic and inconsistent (Arber 1988

Whether DOS ever led to the discovery of medicinal plants is not testable. Leonti et al. (2002) acknowledged the epistemological problems of

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assigning an elucidation role to the organoleptic properties of medicines. Moreover, many plants that lack signatures are used for the same purpose as those that have signatures. The Shuar employ at least nine species to stanch bleeding (Bennett at al. 2002). With its red—splotched leaves, machancha is the only one of them that bears a sign that could be interpreted as blood. Many species with obvious "signatures" are not employed medicinally. Association with a signature makes it easier to remember a plant and to transmit knowledge about its use. One would expect signatures to converge with plants that truly are efficacious. The Gnobe in Panama use the bright red seeds of soroci as a blood tonic (Bennett, unpub. field notes from June 1999, July 2000, June 2001), as do healers on Andros in the Bahamas (Bennett, unpub. field notes from March 2003). Chaturvedi (2005) demonstrated that extracts from the plant normalized blood glucose level, reduced triglyceride and LDL levels, and increased HDL level. Other species bearing red fruits or seeds are not included in the pharmacopoeias. Though it is unlikely that signatures led to discovery of their medicinal values, many plants that bear signatures are efficacious. Mooney's (1992 [orig. 1900]) dismissal of purslane was premature. The plant is effective in controlling intestinal parasite loads and has gastroprotective activity (Karimi et al. 2004; Quinlan et al. 2002). Resemblance to worms aided in transmission of knowledge about the plant's use. Eyebright eye drops effectively and safely treat conjunctivitis (Stoss et al. 2000). Eyebright was likely used for ocular ailments because it worked and its signature facilitated transmission of the knowledge of its use. DOS utility as a memory aid was first proffered in 1938 by Buchanan (1991 [orig. 1938]) and later mentioned by Etkin (1988), James (n.d.), and Leonti et al. (2002). In his discussion of the transmission of medicinal plant knowledge by the Matisgenka and Yora of Peru, Shepard (2002) supported this role: "In these and other orally transmitted systems of thought, mnemonic cues may be essential to the viability of knowledge transmission. Plants that are both efficacious and easy to remember are more likely to be maintained in the pharmacopoeia of non—literate societies through time." The role of mnemonic in cultural transmission is well established. Physicians, burdened with vast

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indications, counter indications, and cures use a simple mnemonic to remind them of anticholinergic poisonings: red as a beet, hot as a hare, blind as a bat, dry as a bone, and mad as a hatter. Berlin (1992) found that "... many folk generic names are metaphorically descriptive of some features of the plant or animal species to which they are applied, this to serving as a useful semantic aid to memory." In his discussion of memory and culture, Donald (1997) described "various forms of visual imagination as a means of understanding and retaining quite complex memories." While not explicit in his argument, plant signatures would seemingly qualify as visual imaginations. My fieldwork with the highland Quechua of Peru, the Shuar and Quichua of Amazonian Ecuador, the Chachi of coastal Ecuador, the Gnobe of Panama, and the Seminoles of southern Florida has revealed many examples of plants with apparent signatures. A Quichua healer, Luis Cerda, showed me a red—tipped puca panga leaf. The plant was employed to treat menstrual bleeding and used for the treatment of snakebites. It would have been tempting to ascribe the use of this gesneriad to the DOS, but Luis never said that it was used because the leaf was red. Several years later, I collected a congeneric species on the opposite side of the Andes. Horacio Lopez, a Chachi Shaman provided the same uses, but his gesneriad lacked the red signature. The Shuar, with a pharmacopoeia exceeding 200 species, treat liver ailments with a decoction made from liver—shaped leaves of akapmas (Bennett et al. 2002). The association between the leaf's shape and its use is a way of remembering and transmitting knowledge of the plant's utility. Buchanan (1991 [orig. 1938]) wrote, `Actually it [DOS] is the mnemonic distillate of what must have been considerable experience." The doctrine of signatures is primarily a way of remembering and transmitting plant knowledge, not a means of discovery. Shepard (2002) strongly defends DOS when he writes, "Western scientists should reflect on their own habits before dismissing such mnemonic devices as irrational or pre—scientific." DOS is a meaningful device useful in disseminating knowledge of medicinal plants.

Conclusions The Doctrine of Signatures is found throughout the world. It was cited by the classical Greek writers and was used frequently in herbal medicine

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during the Renaissance. With few exceptions, scholars dismiss DOS as a "primitive" or "prescientific" idea. Despite its long history, the doctrine has had little critical review. A careful evaluation of signatures suggests four things. (1) As classically defined by morphology, signatures seldom if ever led to the discovery of medicinal plants. Considering DOS in this manner is unproductive and largely untestable. (2) Most signatures are post hoc appellations rather than a priori clues. Seeing a particular signature often requires a vivid imagination. (3) A broader concept of signatures that includes organoleptic properties associated with therapeutic value is productive. Plants with strong odors and bitter tastes, even those with no history of medicinal use, warrant pharmacological investigation as these properties are associated with the presence of potentially bioactive compounds. (4) DOS should be considered for what it primarily is-a way of disseminating information. Fundamentally, the DOS is a mnemonic and therefore it is exceedingly valuable in traditional cultures. A fifth point warrants investigation. Moerman (2002, and pers. comm.) suggests that signatures may enhance the "placebo effect" of medicines. Those species with characteristics resembling the disease being treated may be perceived to be more effective by patients.

Acknowledgments I thank Paul Cox for encouraging me to develop this topic for publication and Daniel Moerman for his insightful comments on earlier manuscript drafts. This paper is contribution 128 in Florida International University's "Tropical Biology Publication" series.

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