4  Organization of the taxonomic part (Part A)

The Checklist follows a very traditional taxonomic arrangement: pteridophytes (families 1–12), gymnosperms (families 13–14), monocots (families 15–37, Typhaceae – Poaceae), and dicots (families 38–97, Salicaceae – Asteraceae). Genera are also arranged in some proposed taxonomic order within families and species within genera. This has been chosen instead of an alphabetic order because there often is need for notes on a group of genera and very often for notes on a group of species.
            Taxa that we fully accept are enumerated consecutively with one number for families (e.g, 47 Caryophyllaceae), one for genera (e.g., 4701 Stellaria), one for species (e.g., 470119  Stellaria ruscifolia), and with letters for races (e.g. 470119a Stellaria ruscifolia subsp. aleutica). The enumerated entities should be considered as functional species for, e.g., conservation purposes. Taxa that we do not fully accept are in many cases still included, not to loose information, but informally and without enumeration. For these taxa the information is also reported in a simpler way.
            No special information is given for families. For genera is reported author and place and year of publication. For accepted species, subspecies, and varieties, the information is summarized under five items:
            1  Nomenclature
            2  Types
            3  Chromosome numbers and ploidal levels
            4  Phytogeographical pattern and distribution among countries
            5  Notes

4.1  Nomenclature

The nomenclatural information reported for the genera, species and subspecific entities are: (a) a main name as proposed for the Checklist, (b) its basionym if different from the main name, and (c) relevant synonyms. Names are given with authors and places and years of publication. Authors are abbreviated according to Brummitt & Powell (1992) or, for a few authors absent from that account, on the principles applied therein. Books and comparable works are abbreviated according to Stafleu & Cowan (1976–1988) or the principles applied therein. Periodicals are abbreviated according to Bridgson (2004 Botanico-Periodicum-Huntianum 2nd ed.) or the principles applied therein.
            By main name we mean the priority name that in our opinion best reflects the rank we find appropriate for the taxon in question. This is the name and the rank that we propose for panarctic use.
            By synonyms we mean names that we consider as representing the same taxon. We have not distinguished between names based on the same type and on different types. The synonymy of some arctic plants is extensive as the same plant may have been described several times in different regions, and as the views of their taxonomic level often have varied. A full synonymy is not intended in the Checklist. What we have aimed for is to include the names used as accepted names in the more recent major floristic surveys of the Arctic in general and of the single countries and major regions. We denote these floristic accounts as our 'standard sources'. They include the more recent standard Floras for the major (usually national) areas, and in addition some more general sources. We have also accepted names from recent revisions and monographs and have allowed for some additional names on special justification. We have tried to evaluate each of the names, whether they represent separate taxa or belong in the synonymy of others.
            The applied standard sources (more bibliographic data in the references):

General sources
Hultén, E. 1958. The amphi-atlantic plants.
Hultén, E. 1962, 1971. The circumpolar plants, I–II.
Hultén, E. & Fries, M. 1986. Atlas of North European Vascular Plants I–III.
Löve, Á. & Löve, D. 1975. Cytotaxonomical atlas of the arctic flora.

Europe
Tutin, T.G. et al. (eds.) 1968–1980. Flora Europaea II–V (1. edition).
Tutin, T.G. et al. (eds.) 1993. Flora Europaea I (2. edition).
Jalas, J. et al., Kurtto, A. et al. (eds.) 1972–2004. Atlas florae europaeae 1–14 (from pteridophytes to parts of Rosaceae completed).

Iceland
Kristinsson, H. 1987. A Guide to the Flowering Plants and Ferns of Iceland.

            Note that Löve 1970, 1977, Íslenzk ferdaflóra, is not considered by us as a standard source due to its inclusion of a number of otherwise unsubstantiated records for Iceland and an aberrant nomenclature.

Norway (including Jan Mayen and Svalbard)
Elven, R. & Elvebakk, A. 1996. A catalogue of Svalbard plants, fungi, algae, and cyanobacteria. Part 1. Vascular plants.
Lid, J. & Lid, D.T. 1994. Lid's Norsk flora, 6. edition by R. Elven.
Lid, J. & Lid, D.T. 2005. Lid's Norsk flora, 7. edition by R. Elven et al.
Rønning. O.I. 1996. Svalbards flora, 3. edition.
Jonsell, B. (ed.) 2000–2001. Flora Nordica 1–2 (from pteridophytes to Fumariaceae).

Russia
Tolmachev, A.I. / Yurtsev, B.A. et al. 1960–1987. Flora arctica URSS I–X.
Fedorov, A.A. (ed.) 1974–. Flora partis europaeae URSS. I–, and its continuation in Flora europaeae orientalis. Volumes appearing after Flora arctica URSS are taken into account.
Tolmachev, A.I. 1974–. Flora regionis boreali-orientals territoriae europaeae URSS I–. Volumes appearing after Flora arctica URSS are taken into account.
Sekretareva, N. 1999. The vascular plants of the Russian Arctic and adjacent territories.
Several editors. 1988–1997. Flora Sibiri 1–14.
Charkevicz, S.S. (ed.) 1985–1996. Plantae vasculares Orientis Extremi Sovietici 1–8.

North America (including Greenland)
Flora of North America 1993–. Volumes 1–5, 19–23, 25–26.

Alaska
Hultén, E. 1968a. Flora of Alaska and neighboring territories.
Welsh, S.L. 1974. Anderson's Flora of Alaska and adjacent parts of Canada. – This work applies subspecific categories differently from the Checklist. Species names applied are included among synonyms but not varietal names.

Canada
Porsild, A.E. 1965. Illustrated flora of the Canadian Arctic Archipelago, 2. edition.
Porsild, A.E. & Cody, W.J. 1980. Vascular plants of Continental Northwest Territories, Canada.
Cody, W.J. 1996 (and 2000). Flora of the Yukon Territory.
Aiken, S.G. et al. 1996–. Flora of the Canadian Arctic Archipelago.
            In addition we have used but not excerpted systematically recent local floras and flora drafts for northern parts of Manitoba, Ontario, Quebec, and Labrador – Newfoundland.
            Note that Scoggan, H.J. 1978–1979, The Flora of Canada I–IV, is not included among our standard sources.

Greenland
Böcher, T.W. et al. 1978. Grønlands flora, 3. edition.
Feilberg, J. 1984. A phytogeographical study of South Greenland. Vascular plants. – Meddel. Grønl., Biosci. 15: 1–70.
Bay, C. 1992. A phytogeographical study of the vascular plants of northern Greenland – north of 74° northern latitude. – Meddel. Grønl., Biosci. 36: 1–102.
Fredskild, B. 1996. A phytogeographical study of West Greenland between 62°20'N and 74°N. – Meddel. Grønl., Biosci. 45: 1–157.

4.2  Types

The type defines the meaning of a name. The nomenclatural type of a name of a species or subspecific entity is in most cases either: (1) The single item, almost always a herbarium voucher or an illustration, referred to by the original author of the name (the 'holotype'); (2) an item selected later among those referred to or certainly inspected by the original author (a 'lectotype'); (3) or an item selected independently of the original description but to confirm an established usage or specify the meaning of a name (a 'neotype' or rarely an 'epitype' when the original material may be ambiguous). There are also some other possibilities, all regulated by the International Code of Botanical Nomenclature (ICBN, currently the Vienna Code, McNeill et al. 2006).
            Information about the types connected to the names (accepted names, basionyms, synonyms) has been included to some extent, but there has been no systematic search for type information. Type identification and typification is a major aspect of the production of a Flora but not of a Checklist. Where we do not have type information available, we have mostly included some reference to from where the plant in question (or rather the name) was described, especially in cases where its geographical origin may be of importance for the interpretation of the meaning of a name.

4.3  Chromosome numbers and ploidal levels

Chromosome numbers and ploidal levels are often important for interpretation of arctic plants, due to the documented high levels of polyploidy in arctic floras. We have therefore, after some initial discussions, decided to summarize much of the chromosome count reports, especially from northern areas, for the entities included in the Checklist. Most of the reports are referred after secondary sources (see below) but for some of the more important reports we have gone back to the primary sources. We also try to roughly indicate where the counted material was collected.
            Initially in the Checklist process (1998) it was decided to use Löve & Löve (1975, Cytotaxonomical Atlas of the Arctic Flora) as a baseline and only to search for and enter counts made and published later. This decision has been revised as Löve & Löve (1975) has proved to be an unreliable baseline. The Löves often changed the information in several ways from what was originally published. They omitted almost all counts that did not fit with their concept that each species – as a rule – should have one and only one chromosome number. This concerns hundreds of the species included by Löve & Löve (1975). In some cases they also 'corrected' (i.e., changed) the numbers from what was originally reported, to fit better with their concepts. Especially where the original counts were reported as a range, the Löves 'adjusted' this to a mean number fitting the prevalent known counts. The most serious objection to the Löves' Atlas is, however, that they transferred numerous counts from the taxon to which they were originally assigned to other taxa, quite certainly without ever checking vouchers for the counts. In addition, the counts made by the Löves themselves often deviate from all other counts assigned to the same taxon, and there is very rarely vouchers available that can be inspected for the Löve counts. All this means that Löve & Löve (1975) gave a much more regular impression of the chromosome number variation in arctic plants than the reality as expressed by the original counts. If a baseline is to be used, Fedorov (1969) is the more reliable compilation. We have therefore omitted Löve & Löve (1975) as a baseline and made numerous notes in the text where there are discrepancies or where they obviously have faked their data or compilations. This is also the policy of, e.g., Flora Nordica (Jonsell 2000, 2001) where counts by the Löves have been decided omitted even if these authors are the major source of, e.g., Icelandic counts.
            Our compilation is then mainly based on Fedorov (1969) for earlier counts and on the regularly published Regnum Vegetabile and Missouri Botanic Garden chromosome number lists (1965–2003), the lists published regularly in Taxon (from 1964 onwards), and the subsequent lists in IOPB Newsletters (1987 onwards). There is an overlap in time between Fedorov (1969) and the regular chromosome number lists that makes the survey comparatively comprehensive up to 2000. The records after this time are rather sketchy, which is deplorable as chromosome numbers still are a major source of biosystematic information and especially of polyploid evolution, in spite of all recently developed molecular markers.
            In the last years, flow cytometry has emerged as a rapid and inexpensive method for deciding ploidal levels, but not for deciding the exact chromosome number. Flow cytometry has been applied on several arctic plants. In the Checklist, reports of flow cytometry results are given as ploidal level.
            In the Checklist, the numbers are always reported as a somatic (mitotic) number ('2n ='), even if the original counts were made on meiotic cells. This is done for practical reasons, also because some of the secondary sources enter only somatic numbers. It may, however, be erroneous for some plants, especially for agamic plants with unreduced gametes. Numbers reported by several independent sources, or where the voucher specimens have been checked, are marked in bold. The chromosome voucher herbarium in St. Petersburg (LE) and the herbarium and database of the University of Alaska Fairbanks (ALA) are especially recommended for those who want to check vouchers of arctic chromosome counts. Some deviating counts (assumed aneuploids, and the results of endoploidy or occasional polyploidy within populations) are included as notes. We have also referred to reports and numbers that we do not include, and why.
            Generally, the Checklist includes counts made in the Arctic and in adjacent boreal areas, which means that counts are included from all of Alaska, most of Canada (and sometimes northern USA), Greenland, Iceland (even if the Löves' counts are often dubious), the other Nordic countries, and northern and central parts of Russia (European Russia, Siberia and Russian Far East). For taxa where few counts are known, all counts (also more southern, temperate ones) are usually included. For others there are notes that there are additional, more southern counts.
            The sources of the counts are reported by author, year of publication, and the areas of origin where we easily have found this information. Very little checking has been done against the original publications. Note that many literary sources for chromosome counts are not included in the References. The majority of these sources are listed by Fedorov (1969), Löve & Löve (1975), or in the lists in Regnum Vegetabile, the Missouri Botanic Garden compilations, Taxon, or the IOBP Newsletters.

4.4  Phytogeographical pattern and distribution among countries

A more detailed distribution in biogeographic regions and bioclimatic zones is reported in Part B. In Part A is included only a short-cut reference to the general distributional pattern and the nations where the taxa are reported as arctic, by the following abbreviations: ICE (Iceland), NOR (mainland Norway and Norwegian arctic islands), RUS (European Russia east to and including Polar Ural and Novaya Zemlya), SIB (Siberia from the eastern margin of Polar Ural east to Kolyma), RFE (Russian Far East), ALA (Alaska), CAN (Canada), and GRL (Greenland). There are two reasons for this inclusion of nations: partly that some of the biogeographic regions in Part B cover two nations (Ic = Iceland and Norwegian Jan Mayen, SF = Norwegian Svalbard and Russian Franz Joseph Land, FN = NE mainland Norway and NW mainland Russia, AN = N Alaska and Yukon Terr., and EP = NE Canadian islands and N Greenland), partly that many of the notes concern geography. Nations with borderline occurrence only are marked in parentheses as '(CAN)'. Introduced occurrences are marked with '*' as 'CAN*' if stable, or with '**' as 'CAN**' if casual. Casual occurrences are only reported for taxa that are stable in at least one arctic region.
            The general distribution patterns are reported by use of the following concepts, mostly in combinations:
            American: More or less throughout North America from the Pacific to the Atlantic, and from temperate at least north to boreal.
            N American: As 'American' but confined to the northern (boreal and / or arctic) parts.
            NE American: In NE temperate, boreal, and / or arctic parts of North America, including Greenland.
            NW American: In NW parts of North America, reaching well outside the Beringian parts into at least W–NW Canada.
            N/W American: In N (arctic) and W (Cordilleran / Pacific) parts of North America.
            American Atlantic: In coastal / oceanic regions of E North America and often S Greenland.
            American Beringian: Confined to the (mega‑)Beringian parts of NW North America, meinly Alaska, Yukon, but also the assumedly little or not glaciated parts of the Canadian coast east of Mackenzie R. and parts of the western Canadian Arctic Archipelago.
            American Pacific: In coastal / oceanic parts of W North America south of the Beringian areas.
            Amphi-Atlantic: On both sides of the North Atlantic, in more or less coastal NE North America and / or Greenland, and in more or less coastal continental NW Europe and / or Iceland and Svalbard. (W) denotes main occurrence in NE North America and / or Greenland; (E) main occurrence in NW Europe (including Iceland).
            Amphi-Beringian: On both sides of the North Pacificm Bering Sea, and / or Chukchi Sea; in NE Asia mainly east of Kolyma – Verkhoyansk Mts, in NW North America mainly west of Mackenzie R.; south to S Alaska, Aleutian Is, and Kamtchatka. (E) denotes main occurrence in East (American) Beringia; (W) main occurrence in West (Asian) Beringia.
            Amphi-Pacific: More or less temperate coastal on both sides of the Pacific, south of the Beringian regions. For (E) and (W), see amphi-Beringian.
            Amphi-Pacific / Beringian: On both sides of the Pacific and the Chukchi Sea, both in the Beringian parts and further south.
            C Asian: In S Siberia and C Asia, north of Himalaya.
            N Asian: In N parts of Siberia. May include NW Siberia and N Russian Far East.
            N/C Asian: In N and S parts of Siberia and in C Asia. May include NW Siberia and Russian Far East.
            NE Asian: In temperate, boreal, and / or arctic E-NE Siberia (Yakutia) and Russian Far East, often also the Baikal areas, N Mongolia, and sometimes N China, Korea and Japan.
            Asian Beringian: Confined to the Beringian parts of NE Asia, mainly Chukotka, the Magadan area, the Koryak area, and Kamtchatka.
            Asian Pacific: In coastal / oceanic parts of E Asia south of the Beringian areas.
            Circumboreal: More or less continuous throughout the boreal and othen temperate parts of Eurasia and North America (sometimes including S Greenland).
            Circumboreal / polar: A combined circumboreal and circumpolar pattern, from boreal / temperate to arctic.
            Circumpolar: More or less continuous throughout the arctic and often northernmost boreal parts of Eurasia and North America (including Greenland).
            Circumpolar – alpine: Circumpolar and with occurrences in at least one temperate mountain range, south of the boreal zone.
            Cordilleran: In W North American mountains, sometimes north to Yukon and Alaska.
            Eurasian: Throughout Europe and N and C Asia, sometimes also more southern parts of Asia and N Africa.
            N/C Eurasian: More or less throughout temperate, boreal, and / or arctic Europe, N Siberia, and S Siberia / C Asia, mostly also NE Asia.
            European: In both N and C Europe, sometimes also S Europe. The Urals, Yugorskiy Peninsula, Vaigach and Novaya Zemlya are considered as parts of Europe.
            C European: In Europe north of the Mediterranean parts and south of the Nordic area and N Russia.
            C/S European: In Europe from Germany and Poland southwards, including Caucasus and sometimes Turkey.
            N European: In northern temperate, boreal and / or arctic Europe, in Iceland, N Scandinavia, Svalbard and / or NW Russia (Murman area), sometimes also east of the White Sea.
            NE European: In northern temperate, boreal and / or arctic NE European Russia, mainly or only east of the White Sea.
            European Atlantic: In coastal / oceanic regions of W Europe, including Iceland.
            European – W Siberian: In Europe and also temperate W Siberia, sometimes reaching C Asia.
            Greenlandic: Endemic to or strongly disjunct in Greenland. Occurence in Greenland is otherwise included in American, N American, NE American, or American Atlantic.
            NW Siberian: In NW parts of Siberia, not east of Taimyr – Putorana.

4.5  Notes

In the opinion of the Editors, the 'Notes' are the most important parts of the Checklist besides the choice of accepted name and taxonomic status. The notes are mostly summaries of comments made by the PAF collaborators and by external reviewers. Comments not assigned to a specific person are normally the responsibility of the Editor-in-chief (Elven).
            The notes indicate taxonomic and nomenclatural problems, prospects of future work, and also cases where we have not reached agreement on treatments. Alternative treatments to those chosen for the Checklist are discussed in these notes. Comments to the previously circulated drafts that have been made obsolete by decisions in their favour have, however, been omitted.