subspecies versus variety

[Ellen Dean]

Dear Plant Taxacomers:
My students often ask me about the difference between plant subspecies,
variety, and forma.  I had been of the opinion that variety was supposed to
encompass geographic variation that could lead to speciation but was not
yet complete and forma was interpopulation variation.  Subspecies was used
in the past in a similar way to subgenus and subfamily but now is used
often interchangeably with variety.  Can you recommend any favorite papers
or book chapters on the subject so that I can give my students better
information?  Thank you for your help. Ellen