Food. This includes plants consumed by humans as major constituents of food preparations, and clearly comprises the most important class economically. Important specialized sources for food data include Plants for Human Consumption (Kunkel 1984), CRC Handbook of Nuts (Duke 1989), some PROSEA volumes (Maesen and Somaatmadja 1989, Verheij and Coronel 1991, Siemonsma and Piluek 1993), the unpublished data for the new Food and Feed Crops of the United States (rev. ed., Markle et al., 1998), and Facciola (1990).

Food additives. Here plants are included that are also consumed, but as minor constituents of food preparations. Particularly useful references for food additives include Tucker (1986) and Tucker and Lawrence (1987) in Herbs, Spices, and Medicinal Plants (Craker and Simon 1986, 1987), Huxley et al. (1992), and Leung and Foster (1996).

Animal food. We recognize only two subclasses: 1) fodder for plant material harvested and fed to domestic animals, and 2) forage for plant material upon which domestic animals feed themselves. Especially valuable sources for these data include Grass Varieties in the United States (Alderson and Sharp 1995), and the previously cited 't Mannetje and Jones (1992), Lazarides and Hince (1993), and Markle et al. (1998).

Bee plants. No subclasses are used for this use class. While numerous species throughout the world are of some importance as honey plants, an exhaustive treatment of these has not been attempted. Many of the most important ones have been identified for this publication. A useful reference consulted for these data was Ortega Sada (1987).

Invertebrate food. Only a small number of plants serving as hosts for beneficial invertebrates, such as silkworms, have been identified. Doubtless we have missed others of some importance.

Materials. This class includes a number of important economic plants, such as those which furnish fiber, timber, gums, resins, and industrial or essential oils. Among the numerous references providing data, the most important were Important Forest Trees of the United States (Little 1978), Encyclopedia of World Timbers (Boutelje 1980), Uphof (1968), Tucker (1986), Tucker and Lawrence (1987), Mabberley (1987, 1997), Dransfield and Manokaran (1993), Soerianegara and Lemmens (1993), Dransfield and Widjaja (1995), and Leung and Foster (1996).

Fuels. Included here with more traditional sources of fuel are rapidly growing plants with potential to provide biomass for electricity generation. The most important references utilized were Firewood Crops: Shrub and Tree Species for Energy Production (National Academy of Sciences 1980) and Energy Plant Species (El Bassam 1998).

Social uses. This minor use class includes few plants of widespread economic importance, although tobacco and those which provide illicit drugs such as opium or cocaine are of considerable economic impact.

Vertebrate poisons. The economic impact of the poisonous plants classified here is largely negative. Although fish poisons provide a very important benefit in primitive cultures, they are seldom traded commercially and have been treated only superficially. Nearly all plants recorded here are toxic to humans and livestock, collectively classed as "mammals" in level 2. Only a few plants toxic to birds, chiefly poultry, have been distinguished, although most plants poisonous to mammals doubtless apply here also. Our list of poisonous plants is certainly not exhaustive, as many plants other than those treated here would certainly be toxic to humans or domestic animals if they were consumed, but this effect has not been properly documented. Our coverage is restricted to those species whose toxic effects have been established in the plant toxicology literature. We have been aided in this effort by D. J. Wagstaff, former plant toxicologist of U.S. Food and Drug Administration. Data are largely derived from Poisonous Plants of the United States (Kingsbury 1964), Poisonous Plants in Britain and their Effects on Animals and Man (Cooper and Johnson 1984), AMA Handbook of Poisonous and Injurious Plants (Lampe and McCann 1985), Plant Poisonings and Mycotoxicoses of Livestock in Southern Africa (Kellerman et al. 1987), Poisonous Plants: Proceedings of the Third International Symposium (Jones et al. 1992), and Lazarides and Hince (1993).

Non-vertebrate poisons. The plants classified here provide materials that serve as organic pesticides.

Medicines. Here we include plants that serve as sources of specific pharmaceutical agents and those that are widely used, mostly in the crude sense, as "folklore" remedies. Only a portion of the former group are approved by the U.S. Food and Drug Administration for use as drugs in the United States. The list of pharmaceutical chemicals is largely derived from Global Importance of Medicinal Plants (Farnsworth and Soejarto, draft manuscript, 1988). Important sources of folklore medicines, i.e., herbal drugs, include Herbs of Commerce (Foster 1992), Herbal Drugs and Phytopharmaceuticals (Bisset 1994), and Leung and Foster (1996). As already mentioned, the numerous plants used locally as medicines have been excluded, as have those plants being tested for potential medicinal benefit. It is difficult to determine the current commercial importance of many medicinal plants, so some important ones may have been missed and others included unnecessarily.

Environmental uses. Many valuable economic plants are classified here, including those used for erosion control, soil improvement (i.e., green manures or cover crops), and agroforestry, but most included here are ornamentals. The ornamentals were probably the most difficult to evaluate as to whether or not to include particular species. We have tried to glean from the sources utilized only the most commercially important because an exhaustive treatment of this group is beyond our scope and is available elsewhere. Hybridization has played a major role in the development of many improved cultivars of ornamentals, and some are of complex hybrid origin. Only those which can adequately be represented by binomial or trinomial botanical nomenclature are included. Many are best treated with cultivar or cultivar-group names, which have not been treated or accepted in this account. Among the numerous references providing data on environmental uses, the most important were The European Garden Flora (Walters et al. 1984, 1986, 1989; Cullen et al. 1995, 1997), Manual of Cultivated Broad-leaved Trees and Shrubs (Krüssmann 1984, 1985, 1986), and some mentioned elsewhere: Bailey Hortorium Staff (1976), Huxley et al. (1992), Encke et al. (1993), and Alderson and Sharp (1994).

Gene sources. In addition to known or potential sources of beneficial genes for specific improvements in crop plants, numerous species can be successfully crossed with crop species to provide fertile progeny and thus must be considered important as genetic resource plants. These primary gene pool members have been included for most crops with the designations "related to" or "progenitor of" preceding the crop name at subclass level. Similarly, those plants that have been hybridized extensively to produce important ornamentals are labelled "for ornamental cultivars" at this level. A few species important as models for research in plant biology are included in this category.

Weeds. This class is, like vertebrate poisons, one of negative economic impact. Included here are both weeds of croplands and natural habitats. Those weeds which are listed in the rules of seed-testing organizations like the Association of Official Seed Analysts and the International Seed Testing Association have been labelled "possible seed contaminant." It is difficult to judge the negative costs of some weeds, but we have attemped to include all the most troublesome ones. In this endeavor, we have been guided by several sources, namely CIBA-GEIGY Weed Tables (Häfliger and Brun-Hool 1968-1975), The World's Worst Weeds (Holm et al. 1977), A Geographical Atlas of World Weeds (Holm et al. 1979), Grass Weeds (Häfliger and Scholz 1980, 1981), Monocot Weeds (Häfliger et al. 1982), Dicot Weeds (Häfliger et al. 1988), Noxious Weeds of Australia (Parsons and Cuthbertson 1992), and World Weeds (Holm et al. 1997) as well as unpublished documents on U.S. federal and state noxious weeds.

Harmful organism host. This class includes plants that serve as alternate hosts for crop pests and plants that serve as test organisms for detecting viral diseases in crops. Useful references include Virus Diseases of Small Fruits (Converse 1987) and the Disease Compendium Series of the American Phytopathological Society.

CITES Appendices I and II. Here we record those plants subject to regulations of the Convention on International Trade in Endangered Species (CITES). For genera having all species covered under Appendix I, we have attempted to include every recognized species, although they are not itemized in the Appendix itself. An additional entry of the form "Paphiopedilum spp." has been added to indicate any other species of genera such as Paphiopedilum not already included. Similarly, entries such as "DIDIEREACEAE spp." imply that all members of this family are covered, in this case under Appendix II, although they have not been enumerated. Species of Appendix II all-inclusive genera have only been itemized for smaller genera. Useful references were CITES Cactaceae Checklist (Hunt 1992) and CITES Orchid Checklist (Roberts et al. 1995).