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Distribution of the genus Froelichia.
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Froelichia Moench is a
relatively small genus of 16 species, two
varieties, and five subspecies classified in the subfamily Gomphrenoideae of the Amaranthaceae. The
group represents one of the
geographically most widespread members of
the Gomphrenoideae extending natively from the north central United States to
northern Argentina and Uruguay. It is most abundant and
species-rich
in the dryer regions of the Americas, principally the desert
and semi-desert range lands of the southwestern and south-central
United States and Mexico in North America and the scrublands of central
South America in Brazil, Paraguay, and northern Argentina.
Two
low shrubby species are endemic to the
Galápagos Islands. In all these regions, Froelichia
is most common on sandy substrates where it can
sometimes be the dominant vegetation, though more commonly it is a
minor component of the flora.
The group has generally received
little attention beyond its initial description and inclusion in
various flora treatments however it exhibits a number of interesting
patterns particularly dealing with biogeography and speciation.
Within the Amaranthaceae we know little regarding the
patterns of speciation of individual genera and thus each of these
questions can serve to illustrate patterns within this large and
important family. Some of the particularly interesting
patterns seen in the genus Froelichia
which I am currently pursuing are...
- A large disjunction between North
and South America. One species, F. interrupta
has generally been thought to span this disjunction; however, my study
of herbarium material suggests that this is not the case and that
additional taxa may in fact need to be recognized.
- The potential for repeated
long-range dispersal between North and South America.
Preliminary genetic evidence indicates that F.
xantii, an endemic species of Baja California, may
be most closely related to plants of western South America and not
adjacent Mexico. Further sampling and analysis is necessary
to verify this pattern.
- The introduction and radiation of
taxa in the Galápagos Islands.
- The differentiation between the
two generic sections Dilopha and Hoplotheca.
These two sections differ principally in floral structure and
are for the most part separated geographically with Sec. Dilopha
in eastern South America and Hoplotheca
in Western South America and North America. The divergence between
these groups is not well understood.
- Hybridization. I have
identified putative hybrids among three distinct taxa in southern
Texas. Closer study using molecular markers, chromosome counts, and
embryological development is necessary to fully determine the extent
and characterization of these instances of putative hybridization.
Characteristics
of the genus Froelichia
F. drummondii
Medina Co., Texas, USA
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Froelichia is recognized
most easily by its spikes of small, densely wooly flowers, a
characteristic which has inspired its common names Cottonweed and
Snakecotton in North America. The inflorescence is technically a
terminal synflorescence of opposite lateral spiklets. The central axis
is dominant and terminates in an indeterminate spike identical to the
lateral spiklets. Each spiklet may vary in length from a few
centimeters to one decimeter in length with the spikelets elongating
throughout the flowering season. Flowers are arranged in three or
five-ranked spirals on the spikelet with three-ranked spirals the most
typical. Five-ranked spirals, as shown here in F. drummondii,
are restricted to a few species of the United
States.
Within the Amaranthaceae a fully indurate perianth that surrounds and
is dispersed with the fruit is unique and is another defining character
of the genus. This perianth (often referred to as fruit in many flora
treatments) is characteristically winged on its lateral margins, though
the size and dissection of these wings will vary among species and to a
lesser extent among individuals. These have traditionally
provided some of the most important taxonomic characters for
distinguishing species, especially within the North American species in
which this character is the most variable.
Fruiting
perianth, F. gracilis
F. interrupta var.
alata
Batopilas, Chihuahua, Mexico
 
F. interrupta
F. interrupta
San Luis Potosi, Mexico
Guayas, Ecuador
Froelichia most often frequents open
grasslands, partially shaded forest edges or low scrublands. The annual
species mostly frequent deep sand soils, while the perennial species
will more often frequent rocky hillsides and arroyo bottoms. Froelichia
can also adapt to very nutrient poor gravelly soils associated with
alluvia and railroad ballast, a trait which has facilitated the
introduction of F. gracilis from its native range
in the southern Great Plains into eastern North America by expansion
along railroad right-of-ways. In the Galápagos Islands, Froelichia
will commonly occur in cracks of barren weathered
lava flows sometimes as the only species occurring in those regions.
Outside of its native range Froelichia
has been naturalized in Queensland, Australia (F. floridana
& F. gracilis) likely from agricultural
seed contaminants and in Japan (F. gracilis)
although these reports are very rare.
 
F. drummondii
F. gracilis
Medina Co., Texas, USA
In railroad station
Portsmouth, Ohio, USA
Species list
and synonomy for Froelichia
Accepted names are in bold italic font.
Froelichia section
Hoplotheca (Nuttal)
Moquin-Tandon
Froelichia
arizonica Thornber ex Standley
Froelichia chacoensis Chodat
Froelichia
drummondii Moquin-Tandon
Froelichia floridana var. drummondii
(Moquin-Tandon) Uline & Bray
Froelichia floridana (Nuttall)
Moquin-Tandon
Oplotheca floridana Nuttall
Gomphrena floridana Sprengel
Gomphrena exaltata Delile ex
Moquin-Tandon
Froelichia floridana ß.
pallescens Moquin-Tandon
Froelichia floridana var. campestris
(Small) Fernald
Froelichia gracilis (Hooker) Moquin-Tandon
Oplotheca gracilis Hooker
Froelichia braunii Standley
Froelichia texana (A. Braun)
Small
Hoplotheca texana A. Braun
Froelichia
interrupta (L.) Moquin-Tandon
Gomphrena interrupta L.
Celosia procumbens Jacquin
Gomphrena spicata Lamarck
Froelichia lanata Moench
Lophocarpus procumbens Link
Oplotheca interrupta (L.)
Martius
Everion interrupta Rafinesque
Ninanga interrupta Rafinesque
Froelichia moritziana
Klotzsch ex. Seubert
Froelichia tomentosa var. angustialata
Suessenguth
Froelichia tomentosa forma collaris
Suessenguth
Froelichia tomentosa forma nigra
Suessenguth
Froelichia interrupta
var. alata (S. Watson) R. A.
McCauley
Froelichia alata S.Watson
Froelichia interrupta var. colimensis
R. A. McCauley
Froelichia
juncea B.L. Robinson & Greenman
Froelichia juncea subsp. alata
J.T. Howell
Froelichia nudicaulis subsp.
longispicata J.T. Howell
Froelichia nudicaulis Hook.
f. var. longispicata Christophersen
Froelichia
latifolia R. A. McCauley
Froelichia
nudicaulis Hooker, J.D.
Froelichia nudicaulis subsp. curta
J.T. Howell
Froelichia nudicaulis subsp. lanigera
(Andersson) Eliasson
Froelichia lanigera Andersson
Froelichia lanata Andersson
Froelichia scoparia Robins
Froelichia lanigera subsp. scoparia
J.T. Howell
Froelichia paraguayensis
Chodat
Froelichia texana J.
M. Coulter & E. Fisher
Froelichia interrupta var. cordata
Uline & Bray
Froelichia tomentosa
(Martius) Moquin-Tandon
Oplotheca tomentosa Martius
Froelichia tomentosa var. angustialata
Suessenguth
Froelichia xantii R.A. McCauley
Froelichia
section Dilopha
Moquin-Tandon
Froelichia humboldtiana
(Röm. et Schult.) Seubert
Gomphrena humboldtiana
Röm. et Schult.
Gomphrena lanata Humboldt
Froelichia lanata (H.B.K.)
Moquin-Tandon
Oplotheca lanata Martius
Froelichia procera (Seubert) Pedersen
Froelichia lanata var.
ß procera Seubert
Froelichia lanata fo. albiflora
Chodat
Froelichia lanata fo. roseiflora
Chodat
Froelichia lanata var. paraguayensis
Chodat
Froelichia lanata var. laciniata
Suessenguth
Froelichia
sericea (Hoffmannsegg ex Röm. et Schult.)
Moquin-Tandon
Oplotheca sericea Martius
Gomphrena sericea Hoffmannsegge
ex Röm. et Schult.
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