In article , Iris Cohen
writes

The older authorities, including IPNI (IK) include Celtis in the Ulmaceae,
which belongs to the order Urticales. When did the above happen, & what is the
rationale?

The APG classification tends towards the lumper side for orders; they've
sunk Urticales (and, IIRC, some other orders) in Rosales. Urticales
survives as the suborder Urticinae.

It seems that the DNA data places Celtis and allies, hence Celtidacee,
as distinct from Ulmaceae, and has Cannabaceae/Cannabinaceae nested
within Celtidaceae; Cannabaceae has priority. Celtidaceae and
Cannabaceae are also united by chromosome number, FWIW.

http://www.inbio.ac.cr/papers/manual.../oct98lit.html

refers to a 1998 paper

Wiegrefe, S. J., K. J. Sytsma & R. D. Guries. 1998. The Ulmaceae, one
family or two? Evidence from chloroplast DNA restriction site mapping.
Pl. Syst. Evol. 210: 249--270

A later paper is

Song, B.-H., X.-Q. Wang, F.-Z. Li & D.-Y. Hong. 2001. Further evidence
for paraphyly of the Celtidaceae from the chloroplast gene matK. Pl.
Syst. Evol. 228: 107–115.

There is more discussion at

http://www.mobot.org/MOBOT/research/...rosalesweb.htm

--
Stewart Robert Hinsley