The distributions of the taxa are shown by county dot maps and the county names are not included in the maps. Figure 1 indicates the county names for each of the 75 counties in the state. Mapping by counties is a convenient way of showing distribution, but of course the larger, natural divisions of the state are more important in a floristic sense. While the natural divisions of the state are not emphasized in the Atlas, it should be obvious to readers that some species are limited to only one or two of the natural divisions while others occur more or less state wide. Figure 2 (note: see also nework-based landform maps) indicates the natural divisions of Arkansas (redrawn from Shepherd et al. 1984). OZ = Ozark, AV = Arkansas Valley, OU = Ouachita Mountains, CP = West Gulf Coastal Plain, MI = Mississippi Alluvial Plain, and CR = Crowley's Ridge. Extensive lists of taxa limited to one or made, but a few examples are: OZ - Cynoglossum officinale, Echium vulgare, Mertensia virginica, Silene ovata, Brickellia grandiflora, Palafoxia callosa var. callosa, etc.; AV - Triodanis leptocarpa, Croptilon hookerianum var. validum, etc.; OU - Amsonia hubrichtii, Solidago ouachitensis, Streptanthus obtusifolilus Cardamine angustata var. ouachitana , Carex ouachitana, etc.; CP - Asclepias obovata, Viburnum nudum, Aster umbellatus, Vaccinium elliottii, etc. ; MI - Amaranthus graecizans, Helenium campestre, etc.; CR - Helianthus microcephalus, Schisandra glabra, etc. Some of these have outlying populations in a county or a few counties outside of their particular natural area. The natural area most likely to provide new species ( and therefore of most interest) appears to be the Ouachita Mountains.
Within each subdivision, the families are listed alphabetically, the genera alphabetically within families, the species alphabetically within genera, and the subspecies and varieties alphabetically within species. The heading at the top of the page indicates the families involved on that page. Generally 12 taxa are covered per Odd-numbered page, and the facing page (unnumbered, but treated as even-numbered) has 12 maps indicating the distribution of the 12 taxa in Arkansas. In a few cases, due to space limitations or the addition of taxa after the page had been typed, 11 or 13 taxa are covered per page. Dots on the maps indicate that I have seen a specimen myself for that county; R's indicate that the taxon has been reported for that county, but that I have not seen voucher material for the report. Most of the R's have come from the dissertations of Tucker (1976) and Taylor (1976) - or from Little (1971, 1977) or Culwell (1987a, 1987b) - and I am confident are highly dependable.
Coverage of the taxa is followed by a list of "Possible Additions" that may occur or be expected in the state but for which I saw no evidence, The probability of these being in Arkansas varies from "very likely" to "perhaps rare". The Possible Additions list is followed by a list of "Excluded Names"; these represent taxa that have been reported by various authors (mostly Demaree 1943) for the state, but which apparently are not in Arkansas. There follows a Summary and the index. The index includes scientific names and common names as well as synonyms.
The coverage of each species (and subspecies or variety) follows the following format:
(2) Synonyms (not underlined). The list of synonyms is not exhaustive.
(3) Comments (in some cases) about the species.
(4) Chromosome number.
(5) Voucher specimen citation.
Chromosome number is listed 2n first, followed by n; the exact source of the chromosome number citation (listed in parentheses in ed. 1 of the Atlas) is not included in this edition (it seemed to be confusing to some readers of the first edition). Chromosome numbers were extracted mostly from Darlington & Wylie (1955), Cave (1958-1960; 1961-1965), Moore (1973, 1974, 1977). Federov (1969), Oruduff (1967 1968), and Coldblatt (1981, 1984). A few numbers were taken from recent literature and from Radford et al, (1968). When two or more chromosome numbers have been reported for a taxon, all are reported and the most commonly reported one is underlined; if no one number predominates in the reports, none are underlined. When the chromosome number of a taxon is in dispute, the most commonly reported number would most likely be the correct number, but of course some taxa have two or more base numbers or ploidy levels, Some earlier reports that are evidently in error have been omitted. If the chromosome number has not been reported (and a surprising number of taxa fit into this category), that fact is indicated by "2n = ?".
In general, formae are not treated in the Atlas. Since they usually differ from typical material in merely a single gene, they are considered of little or no taxonomic significance. Beitel et al. (1981) noted that the practice of designating formae with scientific name is illogical.
The voucher specimens cited are on file at the University of Arkansas at Fayetteville Herbarium (UARK), unless otherwise indicated. Other herbaria are indicated by standard abbreviations (SMU = Southern Methodist University, STAR = Arkansas State University, APCR = Arkansas Tech University, NLU = Northeast Louisiana University; as in Index Herbariorum), or by the name of the institution. Voucher specimens at UARK and at most of the other herbaria have been stamped as such with a rubber stamp.