Botanical Dermatology

Thomas W. McGovern, MAJ, MC
Theodore M. Barkley, PhD

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Disclaimer

Although available to the public via the Internet, this material is targeted to an audience of trained clinical dermatologists. Treatments and techniques described herein should only be done under the supervision of a physician experienced in their proper application. Failure to adhere to this guideline will substantially increase the risk of serious adverse consequences, including bodily injury or death. 

While this chapter describes the 'dark side' of human interaction with plants, it is a mistake to think that these reactions occur with a majority of plants. Of over 500,000 plant species, only about 12,000 have been studied. Within these, some 11,000 chemical compounds have been found. Most of these plants and their chemicals do not harm people. In fact, only about 500 plant-derived chemicals are reported to be toxic via topical or parenteral exposure.¹ Finally, potentially harmful plants are often deleterious only at particular times during their life cycles.

I. SOME BOTANICAL BACKGROUND^2-7

Plant names seem to keep changing. Why are the Umbelliferae also known as the Apiaceae or the Compositae known as Asteraceae? Is poison ivy a member of the genus Rhus or not? The original two kingdoms of Animals and Plants have expanded to five recognized kingdoms (Animalia, Plantae, Fungi, Protista, and Monera). "Plant Dermatitis" now includes reactions to organisms in three of the five kingdoms. This chapter focuses on responses within the Kingdom Plantae.

Systematic botany has two goals; one is to describe the world's flora by classifying them in a hierarchy of similarity (taxonomy), and the other is to provide referable handles for these entities so that they may be referred to accurately (nomenclature). Taxonomy utilizes information from numerous sources, including many techniques of analysis and synthesis. As scientific study progresses, the understanding of natural relationships grows, and the categorization of plants continually changes. Plant nomenclature, on the other hand, is a quasi-legal scheme that is based upon the "International Code of Botanical Nomenclature" (ICBN) which is maintained by a body called the International Association of Plant Taxonomy (IAPT) .⁸ The ICBN has been codifying the procedures for applying referable names to plants since 1867.⁷

Hyperlink to the International Association of Plant Taxonomy (IAPT)-->

Binomial nomenclature

Carl Linnaeus

The general stability of plant nomenclature derives from the seminal works of the 18th century Swedish naturalist, Carl Linnaeus (1707-1778) who introduced the consistent use of "binomial nomenclature"⁶ in a series of works, but most significantly in the two-volume first edition of Species plantarum (1753). The nomenclatural scheme is sometimes called "Linnaean" nomenclature.⁶

Botanical (and zoological) nomenclature identifies every organism by the two names from the last two divisions of the taxonomic hierarchy: genus and species. (e.g. Homo sapiens for human beings). While the species is the basic unit of systematic botany, it is not precisely defined. While a species is often thought of as a group of plants that can interbreed, members of some species cannot interbreed and other species are entirely asexual. In botanical literature, the species name is followed by the name of the person who first published that epithet. In 'Toxicodendron radicans (L.) O. Ktze', '(L.)' refers to Linnaeus who first applied the epithet radicans. 'O. Ktze' stands for Otto Kuntze who transferred radicans into the genus Toxicodendron. The generic and specific epithets comprise the 'minor categories of nomenclature' (Table 1). Suprageneric groups show natural relationships among plants and comprise the 'major categories of nomenclature'. While genera may be shuffled between major categories as discoveries dictate, plant names (minor categories) can remain the same.

Poison ivy, for example

It is occasionally necessary to alter the names of plants. Some name changes occur because a plant was erroneously named to begin with, e.g., the ICBN holds that the oldest name for a plant is the correct name, and occasionally it is shown that a plant received a name that was antedated by another name. Some name changes occur because research has shown that what was formerly thought to be a natural group is, in fact, two (or more) groups that are not closely related to each other. As an example, the genus Rhus was once thought to be a cohesive, widespread genus of many species. Common poison ivy was traditionally included as Rhus radicans or even Rhus toxicodendron.⁹ Research in the 1950's and 1960's showed that poison ivy and its immediate relatives represented a distinct developmental lineage (or 'clade' in the parlance of systematics) and should be treated not within Rhus but as a distinct genus.^{10 , 11} Application of the ICBN showed that the correct generic handle for the poison ivies and their relatives was Toxicodendron, and thus we now call common poison ivy Toxicodendron radicans. 'Toxicodendron' literally means 'poisonous tree' and had been used in common parlance for many centuries. It is noteworthy that while the nomenclatural scheme offers referable names for plants, it also offers a clue to what is known of the natural relationships among plants.

All in the family

Family names are largely stable, but a few large and widespread families have alternative names, e.g., Asteraceae (the sunflower family) is alternatively known as Compositae; Lamiaceae (the mint family) and Brassicaceae (the mustard family) are known respectively as Labiatae and Cruciferae. Their alternative names were used in antiquity, and the ICBN permits their use in formal botany as a concession to ancient tradition. Four more families complete the list of 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 of major categories used in the past. For example, a few generations ago, it was common to put all seed plants (i.e., the flowering plants, conifers, cycads, and the ginkgo) into a single division called 'Spermatophyta'. Botanists now agree that these are disparate entities and are of different lineages. However, at another level, botanical science has shown that the whole collection of vascular plants (i.e., all of the seed plants plus the ferns and fern allies) are allied with the bryophytes (mosses and liverworts) and that these two groups are parallel in their relationships to the many groups of algae (seaweeds, etc.), thus the Kingdom Plantae incorporates vascular plants plus bryophytes, and it implies that the algae are sister-groups.

Table 1: The basic nomenclatural scheme for plants using common poison ivy as the example. The distinctive ending for each category is underlined. Note that the minor categories of nomenclature have no special endings. (Author's name) (L.) O. Ktze

Plant Identification

The accurate description of plants rests upon an elaborate traditional terminology. Contemporary botanical terminology in turn is derived from the scholarly Latin of the medieval university, and it includes a rich vocabulary from classical Greek. The terminology seems formidable until one has been introduced to it, but the payoffs are evident, for it becomes possible to describe plants succinctly with less chance of misunderstanding. Unfortunately, a working knowledge of the terminology is needed to successfully use the plant identification literature. Plant identification also assumes an understanding that plants are highly variable (more so than animals) and that adequate specimens are needed. Therefore, the following suggestions and observations are offered:

1) Try to involve a botanist or horticulturist or other plant person in the identification of plants if there is uncertainty. Every U.S. state and most Canadian provinces have an established protocol for routine and rapid plant identification, and usually the service is free. The majority of plants that are submitted to these agencies are weeds, or plants suspected of being weeds, but a notable number are from toxicology laboratories, mostly associated with veterinary medicine. In suburban and rural areas, the county or regional agricultural extension service or farm advisors are cognizant of the procedures for immediate plant identification, and they have access to regional herbaria (plant information facilities, involving extensive collections of specimens for reference). Urban areas have similar services, usually accessed through the urban horticulturist of the state extension service, or through the local garden center or botanical garden.

2) Plant identification involves specimens. Herbaceous plants (low-growing plants with fleshy stems as opposed to shrubs which are low-growing plants with woody stems) should be collected in their entirety (if possible) with their flowering and/or fruiting materials intact. Note that "flowering" and "fruiting" are used in their botanical senses here. Woody plants should be collected with about two feet of the ends of the branch, with the leaves attached, and with whatever flowering or fruiting materials may be at hand. Herbaceous and woody plants may be pressed before submitting them for identification, or they may be put into a plastic bag with no water added, and then submitted with data on (a) where that 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 from good specimens will increase the accuracy of the information received from the botanist.

3) Plant identification from books or on-line schemes must be approached cautiously, unless the parameters of the books or schemes are well understood. The North American flora north of Mexico includes some 15,000 species of vascular plants, of which the vast majority 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 at work on the continental flora). In the United Kingdom, a CD-ROM application entitled Poisonous Plants in Britain and Ireland combines fast and accurate identification with a friendly interface including photographic images of hundreds of plants. ³ It includes 118 plants known to cause contact dermatitis.

There are numerous regional floristic works that account for all the vascular plants in a region. These works and their equivalent books for other regions are written by and for botanists, and they employ traditional botanical terminology. However, they do account for ALL of the plants growing wild in their respective regions. Books that treat weeds or wildflowers or trees and shrubs are inadequate for general plant identification, for the obvious reason that they cover limited portions of the regional flora. Weed books, for example, treat only those plants that are able to persist in places of continual disturbance; wildflower books treat only herbs or subshrubs that grow outside of cultivation and have showy flowers. In addition, weed and wildflower books often derive much of their interest from beautiful illustrations, which can be limiting to a person trying to identify a plant in-hand, for the illustration, particularly a photograph shows the plant only as it occurs at one time in one place, whereas a written description accounts for the latitude of variation in a plant.