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Disclaimer

Although available to the public via the Internet, this materialis targeted to an audience of trained clinical dermatologists.Treatments and techniques described herein should only be done underthe supervision of a physician experienced in their properapplication. Failure to adhere to this guideline will substantiallyincrease the risk of serious adverse consequences, including bodilyinjury or death. 

While this chapter describes the 'dark side' of humaninteraction with plants, it is a mistake to think that thesereactions occur with a majority of plants. Of over 500,000 plantspecies, only about 12,000 have been studied. Within these, some11,000 chemical compounds have been found. Most of these plants andtheir chemicals do not harm people. In fact, only about 500plant-derived chemicals are reported to be toxic via topical orparenteral exposure.¹Finally, potentially harmful plants are often deleterious onlyat particular times during their life cycles.

I. SOME BOTANICALBACKGROUND^2-7

Plant names seem to keep changing. Why are the Umbelliferae alsoknown as the Apiaceae or the Compositae known as Asteraceae? Ispoison ivy a member of the genusRhus or not? The original twokingdoms of Animals and Plants have expanded to five recognizedkingdoms (Animalia, Plantae, Fungi, Protista, and Monera). "PlantDermatitis" now includes reactions to organisms in three of the fivekingdoms. This chapter focuses on responses within the KingdomPlantae.

Systematic botany has two goals; one is to describe the world'sflora by classifying them in a hierarchy of similarity(taxonomy), and the other is to provide referable handles forthese entities so that they may be referred to accurately(nomenclature). Taxonomy utilizes information from numeroussources, including many techniques of analysis and synthesis. Asscientific study progresses, the understanding of naturalrelationships grows, and the categorization of plants continuallychanges. Plant nomenclature, on the other hand, is a quasi-legalscheme that is based upon the "International Code of BotanicalNomenclature" (ICBN) which is maintained by a body called theInternational Association of Plant Taxonomy (IAPT).⁸ The ICBNhas been codifying the procedures for applying referable names toplants since 1867.⁷

Binomial nomenclature

Carl Linnaeus

The general stability of plant nomenclature derives from theseminal works of the 18th century Swedish naturalist, Carl Linnaeus(1707-1778) who introduced the consistent use of "binomialnomenclature"⁶ in a series of works, but most significantly in the two-volumefirst edition ofSpecies plantarum (1753). The nomenclaturalscheme is sometimes called "Linnaean"nomenclature.⁶

Botanical (and zoological) nomenclature identifies every organismby the two names from the last two divisions of the taxonomichierarchy: genus and species. (e.g.Homo sapiens for humanbeings). While the species is the basic unit of systematic botany, itis not precisely defined. While a species is often thought of as agroup of plants that can interbreed, members of some species cannotinterbreed and other species are entirely asexual. In botanicalliterature, the species name is followed by the name of the personwho first published that epithet. In 'Toxicodendron radicans(L.) O. Ktze', '(L.)' refers to Linnaeus who first applied theepithetradicans.'O. Ktze' stands for Otto Kuntze whotransferredradicans into the genusToxicodendron. Thegeneric and specific epithets comprise the 'minor categories ofnomenclature' (Table 1). Suprageneric groupsshow natural relationships among plants and comprise the 'majorcategories of nomenclature'. While genera may be shuffled betweenmajor categories as discoveries dictate, plant names (minorcategories) can remain the same.

Poison ivy, for example

It is occasionally necessary to alter the names of plants. Somename changes occur because a plant was erroneously named to beginwith, e.g., the ICBN holds that the oldest name for a plant is thecorrect name, and occasionally it is shown that a plant received aname that was antedated by another name. Some name changes occurbecause research has shown that what was formerly thought to be anatural group is, in fact, two (or more) groups that are not closelyrelated to each other. As an example, the genusRhus was oncethought to be a cohesive, widespread genus of many species. Commonpoison ivy was traditionally included asRhus radicans or evenRhus toxicodendron.⁹Research in the 1950's and 1960's showed that poison ivy and itsimmediate relatives represented a distinct developmental lineage (or'clade' in the parlance of systematics) and should be treated notwithinRhus but as a distinct genus.^10,11Application of the ICBN showed that the correct generic handle forthe poison ivies and their relatives wasToxicodendron, andthus we now call common poison ivyToxicodendron radicans.'Toxicodendron' literally means 'poisonous tree' and had been used incommon parlance for many centuries. It is noteworthy that while thenomenclatural scheme offers referable names for plants, it alsooffers a clue to what is known of the natural relationships amongplants.

All in the family

Family names are largely stable, but a few large and widespreadfamilies have alternative names, e.g., Asteraceae (the sunflowerfamily) is alternatively known as Compositae; Lamiaceae (the mintfamily) and Brassicaceae (the mustard family) are known respectivelyas Labiatae and Cruciferae. Their alternative names were used inantiquity, and the ICBN permits their use in formal botany as aconcession to ancient tradition. Four more families complete the listof seven families granted this indulgence by the IAPT: Palmae(Arecaceae, the palm family), Gramineae (Poaceae, the grass family),Leguminosae (Fabaceae, the pea family), and Umbelliferae (Apiaceae,the parsnip family).⁸

Plant taxons

There is not quite a one-to-one correlation with the names ofmajor categories used in the past. For example, a few generationsago, it was common to put all seed plants (i.e., the floweringplants, conifers, cycads, and the ginkgo) into a single divisioncalled 'Spermatophyta'. Botanists now agree that these are disparateentities and are of different lineages. However, at another level,botanical science has shown that the whole collection of vascularplants (i.e., all of the seed plants plus the ferns and fern allies)are allied with the bryophytes (mosses and liverworts) and that thesetwo groups are parallel in their relationships to the many groups ofalgae (seaweeds, etc.), thus the Kingdom Plantae incorporatesvascular plants plus bryophytes, and it implies that the algae aresister-groups.

Table1:The basic nomenclatural scheme for plants using commonpoison ivy as the example. The distinctive ending for each categoryis underlined. Note that the minor categories of nomenclature have nospecial endings.(Author's name) (L.) O. Ktze

Plant Identification

The accurate description of plants rests upon an elaboratetraditional terminology. Contemporary botanical terminology in turnis derived from the scholarly Latin of the medieval university, andit includes a rich vocabulary from classical Greek. The terminologyseems formidable until one has been introduced to it, but the payoffsare evident, for it becomes possible to describe plants succinctlywith less chance of misunderstanding. Unfortunately, a workingknowledge of the terminology is needed to successfully use the plantidentification literature. Plant identification also assumes anunderstanding that plants are highly variable (more so than animals)and that adequate specimens are needed. Therefore, the followingsuggestions and observations are offered:

1) Try to involve a botanist or horticulturist or other plantperson in the identification of plants if there is uncertainty. EveryU.S. state and most Canadian provinces have an established protocolfor routine and rapid plant identification, and usually the serviceisfree. The majority of plants that are submitted to theseagencies are weeds, or plants suspected of being weeds, but a notablenumber are from toxicology laboratories, mostly associated withveterinary medicine. In suburban and rural areas, the county orregional agricultural extension service or farm advisors arecognizant of the procedures for immediate plant identification, andthey have access to regional herbaria (plant information facilities,involving extensive collections of specimens for reference). Urbanareas have similar services, usually accessed through the urbanhorticulturist of the state extension service, or through the localgarden center or botanical garden.

2) Plant identification involves specimens. Herbaceous plants(low-growing plants with fleshy stems as opposed to shrubs whichare low-growing plants with woody stems) should be collected in theirentirety (if possible) with their flowering and/or fruiting materialsintact. Note that "flowering" and "fruiting" are used in theirbotanical senses here. Woody plants should be collected with abouttwo feet of the ends of the branch, with the leaves attached, andwith whatever flowering or fruiting materials may be at hand.Herbaceous and woody plants may be pressed before submitting them foridentification, or they may be put into a plastic bag withnowateradded, and then submitted with data on (a) wherethat plant was collected, and (b) what the habitat was like(cultivated in a garden, wild along a creek bank, in an open field,in higher elevation woodlands, etc.). Identification of plants fromgood specimens will increase the accuracy of the information receivedfrom the botanist.

3) Plant identification from books or on-line schemes must beapproached cautiously, unless the parameters of the books or schemesare well understood. The North American flora north of Mexicoincludes some 15,000 species of vascular plants, of which the vastmajority and the most abundant are flowering plants. At this writing,there is no single source of information on all of these plants(however, a project centered at the Missouri Botanical Garden is atwork on the continental flora). In the United Kingdom, a CD-ROMapplication entitledPoisonous Plants in Britain andIreland combines fast and accurate identification with afriendly interface including photographic images of hundreds ofplants.³ Itincludes 118 plants known to cause contact dermatitis.

There are numerous regional floristic works that account for allthe vascular plants in a region. These works and their equivalentbooks for other regions are written by and for botanists, and theyemploy traditional botanical terminology. However, they do accountfor ALL of the plants growing wild in their respective regions. Booksthat treat weeds or wildflowers or trees and shrubs are inadequatefor general plant identification, for the obvious reason that theycover limited portions of the regional flora. Weed books, forexample, treat only those plants that are able to persist in placesof continual disturbance; wildflower books treat only herbs orsubshrubs that grow outside of cultivation and have showy flowers. Inaddition, weed and wildflower books often derive much of theirinterest from beautiful illustrations, which can be limiting to aperson trying to identify a plant in-hand, for the illustration,particularly a photograph shows the plant only as it occurs at onetime in one place, whereas a written description accounts for thelatitude of variation in a plant.