"Nightingale" schreef in
I have a couple of questions I was hoping someone
could help with....

Given the following two names:

Tillandsia usneoides fo. longissima André, 1889
Tillandsia usneoides var. longissima (André) Mez, 1896

What is the difference between the two? I assume
var. stands for variety, but what does fo. stand
for? Form? Is this a case where the same plant was
reclassified a few years after discovery, or are
they two separate plants?

Thank you for your help,

***
By the look of it, the difference is one of rank only
- "fo." stands for "forma" (preferred abbreviation "f.")
- "var." stands for "varietas"
Ranks may be given in Latin or the language of the publication (varietas is
the Latin, variety is the English) .

Almost surely, both plants will have the same type, and apply to basically
the same group of plants. As is true for any botanical name, the exact group
of plants a name applies to will depend on the taxonomist publishing: it is
not constant, but subject to taxonomic revision. For a well-known example of
this principle: the family name Malvaceae as used a decade ago applied to a
much smaller family than when it is used by the current APG-system (see
http://www.malvaceae.info)

A likely scenario in this case is that Mez decided this plant was different
in only minor aspects from other plants of this species (perhaps it was only
found in the wild in only one spot) and reduced it in rank to that of forma
(English "form").

[This is not 100% guaranteed. Theoretically these might be completely
different plants, and only a careful check of the literature will assure
you. In this case the point is rather academic as Tillandsia usneoides is
quite distinct and such a wild scenario is quite unlikely.]
PvR