Taxonomic abbreviations:
The current ICBN, International Code of Botanical Nomenclature (VIENNA CODE) can be found here:
http://ibot.sav.sk/icbn/main.h...
A ranking of taxa and their abbreviations can be found here:
http://ibot.sav.sk/icbn/frames...
Division II, Chapter I, Article 4.2 provides a more complete ranking of taxa.
A list of abbreviations can be found here:
http://ibot.sav.sk/icbn/frames...
International Code of Nomenclature of Cultivated Plants can be found here:
http://www.actahort.org/chroni...
For our purposes here are the taxa and the "recommended" abbreviation (in brackets are other abbreviations found):
genus: gen.
subgenus: subg. [subgen.]
sectio (section): sect.
subsectio (subsection): subsect.
series: ser.
subseries: subser.
species: sp.
subspecies: subsp. [ssp.]
varietas (variety): var. [var]
subvarietas (subvariety): subvar.
forma (form): f. [fo.]
subforma (subform): subf.
The bracketed abbreviations are variations found here and there.
