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The categories associated with individual plant species, such as Category:Plants described in 2009, include species described in that year. The following guidelines or suggestions are meant to keep these categories consistent.
Note: the guidelines here only apply to organisms covered by the ICN. Different guidelines apply to organisms covered by the ICZN.
Second, "described in year" is meant to indicate that a species was first formally and validly described according to the rules of the International Code of Nomenclature for algae, fungi, and plants (ICN) in that year (thus the earliest possible date is 1753). Use reliable sources to figure out the proper category. If the name has remained unchanged, the International Plant Names Index is a good source. Other databases, such as the World Checklist of Selected Plant FamiliesorThe Plant List are often better for tracking name changes back to the earliest formal description. Note that there may have been a change of rank, e.g. what is now accepted as a species may have been first described at an infraspecific rank. If so, the date of that description is used.
Castilleja attenuata (A.Gray) T.I.Chuang & Heckard. The author citations after this plant name mean that Asa Gray was the first person to describe this species (in 1857 as Orthocarpus attenuatus) and in 1991, T.I.Chuang & Heckard reassessed the species and altered the name (in this case, they moved it to another genus and altered the species gender to match). As a general rule, the species article should be categorized by the earliest formal, valid description, so this species is placed in Category:Plants described in 1857.
Muscari racemosum Mill. was first described by Linnaeus in 1753 as Hyacinthus muscari. When it was transferred to Miller's genus Muscari in 1768, the specific epithet had to change (as the ICN does not allow the same genus name and specific epithet), so there is no "(L.)" in the authority. Nevertheless, the earliest formal description was by Linnaeus, so the article is placed in Category:Plants described in 1753.
Utricularia quinquedentata F.Muell. ex P.Taylor. The "ex" in this author abbreviation indicates that F.Muell. (Ferdinand von Mueller) did not fully or validly publish the species. In this case, he only suggested a name without a description in 1893. Peter Taylor found the reference and published a description based on Mueller's specimens in 1986, so this article is placed in Category:Plants described in 1986.
For reference, the diagram below shows how an article about a plant species fits into the category hierarchy. Parallel hierarchies exist for animal species, fungal species, etc. Start at Category:Plants by year of formal description to explore the plant categories.
Note that the English Wikipedia defines centuries as per MOS:CENTURY; e.g. the 19th century runs from 1801 to 1900. ("Plants described in DECADE" categories were eliminated by consensus in September 2018.)
Not all plant species articles are categorized by year of description, but nevertheless a cumulative plot may be of interest. The rate of description of plant species that are included here shows no signs of diminishing!
It's not possible to reference, and hence directly justify, the addition of categories to an article. So when a year category is present, it is desirable to explain it, at least briefly, in the Taxonomy section, with a suitable secondary source. For example,
A b was first named and described in 1888 by Joe Bloggs.1
A b was first named and described in 1888 by Joe Bloggs, and transferred to the genus CasC d (since C b already existed) by Jane Smith in 1905.2
This ensures that the category "Plants described in 1888" is supported in the text.
