Nomenclature rules

[shakkai]

I checked the International Code of Botanical Nomenclature
(although the Vienna Code appears not to be online yet, so this is from the St. Louis Code) but this didn't seem to clarify the difference:

CHAPTER I. TAXA AND THEIR RANKS

Article 4

4.1. The secondary ranks of taxa in descending sequence are tribe (tribus) between family and genus, section (sectio) and series (series) between genus and species, and variety (varietas) and form (forma) below species.

So, I found this paper (though I didn't pony up the $12 for the full article - just read the summary!) which states:

"Subspecies and varieties usually defined as requiring some integrity - geographic, ecologic, and/or phylogenetic - beyond the morphological. Despite some attempts to differentiate between subspecies and variety, they are largely equivalent in practice. European taxonomists tend to favor subspecies, whereas their counterparts in the United States usually employ variety. Formae usually are defined as lacking any extramorphological integrity."

-- Current practice in the use of subspecies, variety, and forma in the classification of wild plants, Clement W. Hamilton & Sarah H. Reichard, August 1992


So, it appears that if the range is distinct, or the environment is quite specific - i.e - completely hypothetical - only red flowers in the dry area, only yellow flowers in the wet area - then each is a variety of that species. But if red and yellow are present in the same place, in the same environment, then you have a yellow form and a red form... At least, this is how I interpret it!