FWIW, I'd suggest the mystery plant is probably a sapling
of either Albizia julibissin or Gleditsia triacanthos. Both are
commonly cultivated as ornamentals in temperate areas, and
both can freely volunteer in other plants' pots. [More likely
it's Albizia; IIRC young Gleditsia usually also have some long
once-pinnate leaves not shown in these photos]

"Iris Cohen" wrote in message
...

the New Mexico species will be part of
Acaciella

Oh no! When & how did this happen?

It's required by various recent molecular phylogenetic studies that
clearly show that the main groups of traditional "Acacia" include
large groups that are actually closer kin [in terms of recency of
common ancestry] to species classified in other genera than they are
to the other "Acacia". [I.e., "Acacia" is paraphyletic or
polyphyletic]

As I recall it, the "genus" turned out to include lines
closer to other genera of mimosoid legumes like Albizzia, Inga,
Pithecellobium, Prosopis, etc. than to one another.

Does that include A. farnesiana?

According to below, that particular species apparently belongs to the
traditional "_Acacia_ subgenus _Acacia_", so it should remain in
the genus Acacia even after the genus is redefined and split into smaller
genera. So it stays in, but the phyllode-bearing Australian species are
out.

Some relevant citations and abstracts from a quick search:

TITLE: Overview of the generic status of Acacia (Leguminosae:
Mimosoideae).
AUTHOR, EDITOR, INVENTOR: Maslin,-B-R [Author,-Reprint-Author]; Miller,
-J-T [Author]; Seigler,-D-S [Author]
SOURCE: Australian-Systematic-Botany. 2003; 16(1): 1-18
ABSTRACT: The systematic treatment and circumscription of the group of
plants presently recognised as the genus Acacia has a complex history.
The genus was first described by Philip Miller in 1754 and until 1842,
when George Bentham clearly defined it's generic limits (by restricting
the name Acacia to mimosoid plants having numerous free stamens), a
number of species which are now referable to genera within tribes
Ingeae and Mimoseae had been referred to it. As presently defined,
Acacia is a cosmopolitan genus containing in excess of 1350 species and
together with the monotypic genus Faidherbia Chev. (which occurs in
Africa and the Middle East), comprises tribe Acacieae within subfamily
Mimosoideae. The current classification of Acacia views the genus as
comprising three large subgenera, namely subg. Acacia (c. 161 species,
pantropical), subg. Aculeiferum Vassal (235 species; pantropical) and
subg. Phyllodineae (DC.) Seringe (syn. subg. Heterophyllum Vassal) (960
species, largely confined to Australia). In 1986, Pedley proposed that
these three subgenera be attributed generic rank, namely Acacia,
Senegalia Rafinesque and Racosperma C.Martius, respectively, but this
proposal was not widely adopted. Subsequently, the results of
monographic and floristic works have greatly expanded knowledge, not
only of Acacia, but also of its presumed relatives in tribes Ingeae and
Mimoseae. Cladistic analyses of chloroplast genes have been especially
informative in developing a better understanding of phylogenetic
relationships of the group. The new data clearly show that the genus as
presently defined (i.e. Acacia sens. lat.) is not monophyletic.
Furthermore, five separate monophyletic groups can be recognised within
Acacia sens. lat. and it is recommended that these each be recognised
as a distinct genus. The five genera correspond to those recognised by
Pedley, except that Senegalia sens. lat. is now regarded as comprising
three genera, namely Senegalia sens. str., Acaciella Britton & Rose
(based on Acacia subg. Aculeiferum sect. Filicinae (Benth.) Pedley)
and an undescribed genus based on a group of species related to Acacia
coulteri Benth. Acacia subg. Acacia appears to be located in tribe
Mimoseae. The relationships of subg. Phyllodineae, subg. Aculeiferum
sens. str., sect. Filicinae, the 'Acacia coulteri' group and Faidherbia
are not fully resolved, although in all studies these groups are shown
to be monophyletic. Although it is appropriate that each be recognised
as a distinct genus, the application of the names Acacia and Racosperma
is currently under consideration and it is therefore not appropriate to
use these names until this matter is resolved.

Here's the 1986 Pedley paper mentioned above:

TITLE: DERIVATION AND DISPERSAL OF ACACIA LEGUMINOSAE WITH PARTICULAR
REFERENCE TO AUSTRALIA AND THE RECOGNITION OF SENEGALIA AND RACOSPERMA
AUTHOR, EDITOR, INVENTOR: PEDLEY-L [Reprint-author]
SOURCE: Botanical-Journal-of-the-Linnean-Society. 1986; 92(3): 219-254
ABSTRACT: The morphology of seedlings, leaves, flowers and
inflorescences, anatomy of the pod, the occurrence of extra-floral
nectaries, free amino acids of the seeds, flavonoid compounds in
heartwoods, cyanogenic compounds and porate, colporate and extraporate
pollen, and susceptibility to rusts, all indicate that three genera,
Acacia Miller, Senegalia Raf. and Racosperma Martius, should be
recognized. These correspond to currently accepted subgenera of Acacia.
The size of these more narrowly circumscribed genera is in keeping with
the size of genera of other tribes of low diversity in Leguminosae.
Acacia and Senegalia arose independently from the Ingeae, with
Racosperma being derived from Senegalia. Section Filicinae is more
advanced than section Senegalia of Senegalia, and sections Racosperma
and Pulchella, both with at least some species with bipinnate foliage,
are the most advanced of Racosperma, while the other sections
Pleurinervia and Lycopodiifolia have only phyllodinous species.
Long-range dispersal of Racosperma from the Australian region has
occurred, but the broad pattern of distribution is interpreted in
terms of plate tectonics. Racosperma was present in Australia in the
late Cretaceous but did not become widespread until the general drying
of the continent in the Miocene. The flora of SW Australia has been
isolated from the rest of the continent by climatic barriers since the
late Tertiary and has a high proportion of endemic species. Barriers to
plant migration in the east have operated only intermittently and there
is no area comparable in endemism to the southwest.

TITLE: A phylogenetic analysis of the Acacieae and Ingeae (Mimosoideae:
Fabaceae) based on trnK, matK, psbA-trnH, and trnL/trnF sequence data.
AUTHOR, EDITOR, INVENTOR: Miller,-Joseph-T [Author,-Reprint-Author];
Grimes,-James-W [Author]; Murphy,-Daniel-J [Author]; Bayer,-Randall-J
[Author]; Ladiges,-Pauline-Y [Author]
SOURCE: Systematic-Botany. 2003; 28(3): 558-566
ABSTRACT: A phylogenetic analysis of exemplars of Acacieae, Ingeae,
and some Mimoseae, based on trnK, matK, psbA-trnH, and trnL/trnF
sequence data, is presented. The results support other recent studies
in showing that neither the Ingeae nor Acacieae is monophyletic. Some
subgenera of Acacia, specifically subgenera Acacia and Phyllodineae,
are monophyletic, but subg. Acacia is in a basal polychotomy with
various members of Mimoseae and a large clade with the other members
of Ingeae and Acacieae. Acacia subg. Phyllodineae is sister group to
members of the Ingeae. Both the Ingeae and Acacia subg. Aculeiferum
are paraphyletic.

TITLE: Molecular phylogenetics of Acacia subgenera Acacia and
Aculeiferum (Fabaceae: Mimosoideae), based on the chloroplast matK
coding sequence and flanking trnK intron spacer regions.
AUTHOR, EDITOR, INVENTOR: Miller,-Joseph-T [Author,-Reprint-Author];
Bayer,-Randall-J [Author,-Reprint-Author]
SOURCE: Australian-Systematic-Botany. 2003; 16(1): 27-33
ABSTRACT: The genus Acacia is subdivided into the following three
subgenera: subg. Acacia, subg. Aculeiferum and the predominantly
Australian subg. Phyllodineae. Morphological and molecular studies
have suggested that the tribe Acacieae and genus Acacia are artificial
and have a close affinity to the tribe Ingeae. Sequence analysis of
the chloroplast trnK intron, including the matK coding region and
flanking non-coding regions, were undertaken to examine taxon
relationships within Acacia subgenera Acacia and Aculeiferum. Subgenus
Acacia is monophyletic while subgenus Aculeiferum is paraphyletic.
Within the subgenera, major divisions are found based on biogeography,
New World versus African-Asian taxa. These data suggest that characters
such as inflorescence and prickle and/or stipule type are polymorphic
and homoplasious in cladistic analyses within the subgenera.

TITLE: Molecular phylogenetics of Acacia (Fabaceae: Mimosoideae) based
on the chloroplast matK coding sequence and flanking trnK intron spacer
regions
AUTHOR, EDITOR, INVENTOR: Miller,-Joseph-T [Reprint-author]; Bayer,-
Randall-J [Author]SOURCE: American-Journal-of-Botany. 2001; 88(4):
697-705
ABSTRACT: The tribe Acacieae (Fabaceae: Mimosoideae) contains two
genera, the monotypic African Faidherbia and the pantropical Acacia,
which comprise about 1200 species with over 950 confined to Australia.
As currently recognized, the genus Acacia is subdivided into three
subgenera: subg. Acacia, subg. Aculeiferum, and the predominantly
Australian subg. Phyllodineae. Morphological studies have suggested
the tribe Acacieae and genus Acacia are artificial and have a close
affinity to the tribe Ingeae. Based on available data there is no
consensus on whether Acacia should be subdivided. Sequence analysis of
the chloroplast trnK intron, including the matK coding region and
flanking noncoding regions, indicate that neither the tribe Acacieae
nor the genus Acacia are monophyletic. Two subgenera are monophyletic;
section Filicinae of subgenus Aculeiferum does not group with taxa of
the subgenus. Section Filicinae, eight Ingeae genera, and Faidherbia
form a weakly supported paraphyletic grade with respect to subg.
Phyllodineae. Acacia subg. Aculeiferum (s. s.) is sister to the grade.
These data suggest that characters currently used to differentiate
taxa at the tribal, generic, and subgeneric levels are polymorphic
and homoplasious in cladistic analyses.

TITLE: Implications of chloroplast DNA restriction site variation for
systematics of Acacia (Fabaceae: Mimosoideae)
AUTHOR, EDITOR, INVENTOR: Clarke,-H-David [Reprint-author]; Downie,-
Stephen-R [Author]; Seigler,-David-S [Author]
SOURCE: Systematic-Botany. 2000; 25(4): 618-632
ABSTRACT: Comparative restriction site mapping of the chloroplast
genome of 26 accessions of Acacia plus outgroups (Caesalpinia, Albizia,
and Ebenopsis) was carried out to analyze phylogenetic relationships
among the subgenera of Acacia and, in particular, within Acacia
subgenus Acacia. One or more taxa from each of seven New World species-
groups plus two African species of Acacia subgenus Acacia were included
in the analysis to generate hypotheses of the evolution and radiation
of this group. Restriction maps constructed from data from 11
restriction enzymes yielded 212 informative restriction sites out of a
total of 900. Parsimony analysis resulted in a total of 12 most
parsimonious trees of 663 steps each. The strict consensus tree and
bootstrap and decay indices indicate strong support for monophyly of
Acacia subgenus Acacia and provisional support for the paraphyly of
Acacia overall. Moderately to strongly supported clades within Acacia
subgenus Acacia indicate that the mesically adapted A. macracantha
species-group is polyphyletic and may represent lineages with sister
group relationships with both the ant-acacias and the more xerically
adapted A. farnesiana species-group. A group of Caribbean species was
found to be ancestral in Acacia subgenus Acacia and African and South
American species were found to be relatively derived with respect to
North American species, lending support to a Boreotropical, rather than
a Gondwanan, hypothesis of the historical biogeography of Acacia
subgenus Acacia.

TITLE: Phylogenetic analysis of Australian Acacia (Leguminosae:
Mimosoideae) by using sequence variations of an intron and two
intergenic spacers of chloroplast DNA
AUTHOR, EDITOR, INVENTOR: Murphy,-Daniel-J [Author]; Udovicic,-Frank
[Author]; Ladiges,-Pauline-Y [Reprint-author]
SOURCE: Australian-Systematic-Botany. 2000; 13(5): 745-754
ABSTRACT: Three regions of chloroplast DNA are assessed for their
utility for phylogenetic analysis of Acacia subgenus Phyllodineae:
psbA-trnH intergenic spacer, the trnL intron and the trnL-trnF
intergenic spacer. There are large differences in the lengths of the
psbA-trnH spacer (155-440 bp) and trnL-trnF intergenic spacer (101-422
bp) regions, and large multi-residue indels were coded as multistate
characters. Overall information content in these regions is relatively
low, but the total evidence tree has 12 nodes resolved, five with
jackknife support. By using Parkia timoriana as the outgroup, Acacia
subgenus Acacia (A. farnesiana) is basal and Acacia subgenus
Aculeiferum (A. senegal) is the sister taxon to subgenus Phyllodineae.
Although based on a small sample size, within subgenus Phyllodineae,
the results of this study have shown that section Alatae is not
monophyletic, section Lycopodiifoliae is monophyletic and Botrycephalae
is related to members of section Phyllodineae with racemose
inflorescences.

TITLE: A plastid DNA phylogeny of the genus Acacia Miller (Acacieae,
Leguminoseae)
AUTHOR, EDITOR, INVENTOR: Robinson,-Julian [Reprint-author]; Harris,
-Stephen-A [Author]
SOURCE: Botanical-Journal-of-the-Linnean-Society. 2000; 132(3): 195-222
ABSTRACT: Past classifications of the tribe Acacieae Rchb. are outlined
and the confusion concerning the relationships of the three subgenera
of Acacia Mill. are highlighted. A plastid DNA analysis of Acacieae
shows that the genus Acacia is not monophyletic. Furthermore subgenera
Acacia Vassal and Aculeiferum Vassal are sister taxa and neither appear
closely related to subgenus Phyllodineae (DC.) Ser. Subgenera Acacia
and Aculeiferum form a clade that is basal to a well-supported clade
consisting of tribe Ingeae Benth. taxa, Faidherbia albida (Del.) A.
Chev. and subgenus Phyllodineae. The series of relationships suggested
by the cpDNA data contradicts previous investigations of the tribe.
Possible explanations of this conflict are explored, and the taxonomic
implications of the plastid DNA data set are considered.

TITLE: Phylogenetic analysis of Acacia (Mimosaceae) as revealed from
chloroplast RFLP data
AUTHOR, EDITOR, INVENTOR: Bukhari,-Y-M [Reprint-author]; Koivu,-K
[Author]; Tigerstedt,-P-M-A [Author]
SOURCE: Theoretical-and-Applied-Genetics. 1999; 98(2): 291-298
ABSTRACT: Chloroplast DNA of 22 species of Acacia (Tourn.) Miller was
digested with ten restriction endonucleases, Southern-blotted and
probed with cloned fragments covering the chloroplast genome of tobacco
(Nicotiana tabacum L.). Phyletic and phenetic analyses of the resulting
176 polymorphic bands recorded among the 22 species were performed. The
phylogram was reconstructed using heuristic search and Wagner parsimony.
The resulting most parsimonious consensus phylogram displayed three
major phyletic lineages, consistent with the previously established
three subgenera of Acacia. The 10 species of subgenus Acacia and the 6
species of subgenus Heterophyllum formed two monophyletic sister clades.
The 5 species of subgenus Aculeiferum studied and Acacia albida (Syn.
Faidherbia albida) grouped together and were basal to the clades of
subgenera Acacia and Heterophyllum. The phylogram indicated that
subgenus Heterophyllum diverged earlier from subgenus Aculeiferum than
did subgenus Acacia; however, the phenogram indicated the reverse. The
study indicated that A. nilotica and A. farnesiana are sister species,
though A. nilotica is Afro-Asiatic and A. farnesiana is American. The
phenogram separated the three subgenera in agreement with the phylogram,
but the two dendrograms differed regarding the topologies of the
species and the distance of evolution between subgenera Acacia and
Heterophyllum.

cheers