Paleozoic Herbaceous Lycopsids and the Beginnings of Extant Lycopodium Sens Lat. And Selaginella Sens. Lat

9 Pages·1992·6.4 MB·

by  Barry A Thomas

Number 3 Thomas
Volume 79, 625
1992 Paleozoic Herbaceous Lycopsids
f
modern
megasporangiate fructifications, or isolated mega- copsids, or the evolutionary history of the
sporangiate organs could be assigned to families. herbaceous genera Selaginella sens. lat. and Ly-
copodium
I All other organs are then described as satellite taxa sens, lat.?
assigned either to a family or to an order, as seems Available evidence suggests that the extant her-
An
most appropriate in each case. assumption baceous lycopsids most probably had their origins
(2.)
same Upper
that placing different species in the same genus in the Carboniferous equatorial region
implies phylogenetic relationship rather than mere where the arborescent lycopsids were reaching their
Euramerican
phenetic similarity can influence the conception of zenith. In contrast with the equatorial
Angaran Upper
northern Carboniferous
plant communities, and region, the
distribution patterns, pa-
leogeography. know whether and Permian floras showed increasing endemism.
(3.) It is difficult to
many numbers An-
specimens are from herbaceous or arbores- There were large of lycopsids in the
Meyen
cent lycopsids, and easy make assumptions garan Carboniferous that (1972) described
to
is
it
that are untestable on limited evidence. as a cheerless and monotonous *'brush" of fairly
When further evidence found more short straight sticks and suggested that the scanty
of larger,
is
and
complete, vegetation was associated with lakes rivers.
specimens, or of attached reproductive
There were no herbaceous forms; the lycop-
organs, the possible interpretations must be treated truly
were represented by such genera of shrubby
with care. For example, the Lower Carboniferous sids
European genus £5 emend Thomas plants as Lophiodendron Zalessky, 1936, 7b-
Kidston
A:^a/ia
& Meyen mtWertJronRadczenko emend. Meyen, 1912, An-
(1984b), also described from South Af-
& Meyen, Ursodendron Rad-
(Brown garophloios 1972,
rica Lemoigne, 1977) and (Tho-
Siberia
& Angarodendron
mas emend. Meyen, 1972,
Meyen, czenko
1984b), has generally been re-
&
Thomas
Tunguskadendron
1918,
garded as a taxon of comparatively smaU lycopsids Zalessky,
emend
& Meyen, 1984c, and Eskdalia Kidston Tho-
(Thomas, 1968; Meyen, Thomas Meyen,
1976;
&
mas Meyen, 1984b.
Rowe
1984b). and
(1988a), however, redescribed
renamed
some Simil
specimens of Scutellocladus var-
& Upper tundra vegetation of Gon-
Paleozoic
iabilis Lele Walton (1962) from the Visean the
There were
Drybrook dwanaland (RetaUack, 1980). instead
Sandstone Dean,
of the Forest of
shrubs such as the Argentinian
or
very small trees
Gloucestershire, United Kingdom, as E. variabilis,
described
which
he an
believed be the terminal shoots of
to
98 and the BrazUian Permian
arborescent changelsky et al. (1
lycopsid because branching pat- 1
of )
its
Archan-
Chaloner 1968.
Brasilodendron et
tern and al.,
This
its possession of terminal cones.
Bumhudendron
showed
(1981) that
gelsky et
species al.
underlines the dangers of making gener-
paganzianum had branch structures rather
fertile
alized inferences from comparison of species based
and Chaloner (1968) suggested
than et al.
strobili,
vegetative axes. The type specimens of Esk-
dalia simil
variabilis were assigned to the
originally
sun
& Wahon
monotypic
genus
Scutellocladus Lele
m r
, . , tLower Ci-arbi_ omferous
common world-wide the
(1962), but were Tomiodendron
later included in
become
& seem have restricted to the
Radczenko but later to
Thomas
emend. Meyen, 1972, by
Purdy equatorial belt,
(1983); a genus that has also been described
from &
Alaska Thomas, North Af-
(Spicer 1987),
&
rica
(Mey-
(Lejal-Nicol Massa, 980) and Siberia
1 Hqj^ospory
en,
1976). The
use of either of the widely distrib-
Lycopodites
Lycopodites Brongniart, 1822, or
uted
genera Eskdalia Tomiodendron these
or for
& name been
has
Hutton, 1833, a that
fertUe Lindley is
British Visean shoots could be misinterpreted
presumed herbaceous, leafy lycopsid
as implying given to small,
are
that aU species included within
it
based on vegetative axes
taxa Species
shoots.
of the terminal branches of arborescent ly- ^^^^^
&
Ghosh have
Pal (1990) dis
been included.
*:opsids. the
Scutellocladus, include
redefined to
o
r •
emanatmg from Brongmart
problems s
terminal cussed the
reproductive organs, should not be mis-
whose
Lycopodites for a type species
(1822) use of
^terpreted
in way.
this
confused
&
Hutton's usage of the generic
Lindley
ferring
The
Beginnings Modern Herbaceous Lycopsids
of
name.
been described
What
then can be interpreted from the fossil
onward and some of these are
Upper Devonian
record
the
that might give some indication of the evo-
known
lutionary
herbaceous
relationships within the ly-
626 Annals of the
Garden
Missouri Botanical
Weyl from the Upper Devonian of Belgium an
is isolated
Upper Devonian of Germany has globose sporangia portion of a cone with narrowly triangiJar and
on unspecialized sporophylls and spores that are toothed sporophylls, each bearing a stalked,
slightly
90-120
The
jum in diameter. Other species are merely reniform sporangium. spores are 24O-320
all
vegetative shoots with no indication of their re- jum in diameter and are interpreted as small mega-
Some
productive organs. of these vegetative shoots spores. However, no ligules have been observed,
assigned to Lycopodites are isophyllous, such as These morphological characters, and details of
its
L, pendulas Lesquereux, 1880, from the Upper anatomy and vascular bundles, suggest that B.
Carboniferous of Illinois, U.S.A., L. arheri Ed- famennensis closer to Carboniferous lycopsids
is
New
wards, 1934, from the Jurassic of Zealand than to the early Devonian forms. Unfortunately,
and Lycopodites hannahensis Hsirvis^ 1976, from once again the vegetative organs of the plant are
Wealden.
the English This based on unknown.
last species,
number
a large of erect, dichotomizing vegetative Other herbaceous forms from the Euramerican
stems preserved in a fine-grained sandstone was Upper Carboniferous Coal Measures were very sim-
interpreted by Harris as a slender, upright, reedlike to extant species of Selaginella, Some are
ilar
plant. I have found a similar specimen in fine- known to have been heterosporous, but others are
grained sandstone in the Yorkshire Jurassic. Such known to resemble Selaginella only in morpho-
plants might well have been common on sandy mud logical characters of vegetative shoots. Both iso-
flats on riverbanks and deltas throughout the Me- phyllous and anisophyllous species are known from
Other
sozoic. species of Lycopodites are aniso- the Carboniferous onward.
have
rank of larger leaves The rare anisophyllous shoots either
fairly
and two upper ranks of smaller leaves. The type been placed into Ijco/?o<ff>e5 (as mentioned above),
1906,
included the genus Selaginellites Zeiller,
in
fi
examined by Harris, 1961), of this form. This or even regarded as species of Selaginella sens.
is
character is not restricted in extant species to Se- str. Selaginellites was first used by Zeiller for a
laginella; there is a form of anisophylly occasion- specimen from the Upper Carboniferous of the
ally present in Lycopodium resembling that of Blanzy Coalfield, France that he named Selagi-
Selaginella Lycopodium dichotomously
(e.g., carolinianum nellites suissei. This lycopsid had a
fig-
ured
in Troll, 1937). This clearly has implications branched stem, dimorphic leaves, and a distinctive
and
for the use of anisophylly as an aid to generic bisporangiate cone with apical microsporangia
and
distinction, the problem will be discussed later basal megasporangia. Selaginellites suissei is
in the section on the ^'generic assignment of her- therefore very similar to many living anisophyllous
baceous stro-
lycopsids/' have tetragonous
fossil species of Selaginella that
cathef
Selaginella vogelli Spring, 5.
(e.g.,
bili
Quansah
by
Baxter
(1971) to a single lycopsid cone from Spring and pervillei Spring:
rifolia 5.
Pennsyl
1988).
mm
anisophyllous
in diameter, suggesting that it might have been There are several other species of
the
from
the fructification of an herbaceous species. The spores Selaginellites that have been described
were of one type and 20-22 Mm have reexamined
all in diameter, Carboniferous Coal Measures.
I
gutbieri
indicating that the cone was either homosporous, the remarkable specimen of Selaginellites
Westphalian
microsporangiate, or a fragment of a heterosporous (Goeppert) Kidston, 1911, from the
was
cone. Baxter, however, D which
stated that Carinostrobus Zwickau Poland,
of the Coalfield,
57,
clearly lacked a Unfortunately, we (1870-1872, pK
ligule. have no refigured by both Schimper
knowledge truly
of the plant that bore the cone. and Hirmer 327). It is
fig. 4) (1927, fig.
Microspores
anisophyllous with terminal cones. \
sev-
from
the Ciratriradites form) were recovered
Heterospory
no
were
megaspores
eral of the cones, although
Heterospory presumed plan«
is to have developed that the
in found. however, possible
quite
It is,
several groups of plants during condition
the late Devonian, had the
only microsporangia similar to
am
for the record of spores shows
fossil a rapid many
increase found some living
in individuals of
number
in the and versicolor
diversity of presumed mega-
phyllous species of Selaginella (e.g., S.
spores during period Hieron-
this
(e.g., Chaloner,
1967). leonensis
Spring, and
S. vogellii Spring S.
A
^Oj
few
heterosporous Devonian
elongata
fructifications prob-
(Quansah,
1988)). Selaginellites
belong
denberg, 1855, however, rather d'ff*"'''"'.j^
is,
describea
who
fi was
reexamined by Halle (1907),
—
Number Thomas 627
3
Volume
79,
Herbaceous Lycopsids
Paleozoic
1992
sporangia as occurring In the axils of ordinary growth. The terminal cones had sporophylls ar-
the
vegetative leaves. This condition found in living ranged in alternating pairs of verticels. It is very
is
such Selaginella denticulata Link similar in morphology to some extant species of
as (L.)
species
where two Selaginella selaginoides except for
and pallidissima Spring, there are (e.g., S. L.),
S.
sizesof sporophylls arranged in four ranks. In these not having a distinctive basal root-bearing organ,
shows very anat-
living species the smaller sporophylls are in the It also a close similarity in stelar
number
same plane as the smaller median vegetative leaves, omy, change in the of protoxylem points,
and
and the larger sporophylls are in the same plane secondary growth in the base of the stem, in
change from centrarch exarc
as the larger lateral vegetative leaves (Quansah, the to steles.
Selagincl
1986). This gives the superficial appearance of Isolated cones are also included in
from
such />o/ans Lundblad, 1948, the
there not being a discrete cone. Kidston (1911) /lYes, as 5.
We
know
Greenland. nothing of their axes,
some specimens from the Trias of
also described
fertile
Westphalian B of the Belgium Coalfield as Seta- and their assignment to this genus is therefore
tenuous.
are
ginellites gutbieri, although his plate figures
Miadesmia membranacea Bertrand (Benson,
labeled as primaevus Goldenberg. There are,
S.
however, the 1908) an isolated cone of a rather different
several characters that distinguish is
might
Belgian cones from The most obvious structure. Its small size suggests that it be-
gutbieri.
5.
no
although there
differences are that the larger leaves are much long to an herbaceous lycopsid, is
megasporangiate: a single large
more dentate and the sporophylls are approxi- other evidence. It is
each sporangium, which
megaspore occupies in
mately the same as the larger vegetative leaves.
size
by integuments, suggesting an
enclosed
turn
It is therefore very similar to 5. elongata, although is
lepidocarpon
analogy structure with that of
a reexamination of the Belgian material neces- in its
is
1900.
sary before any can be made, Scott,
firm identification
have Car-
also seen other specimens of late
I
from
boniferous herbaceous anisophyllous shoots,
HERBACEOUS
ASSIGNMENT OF
GENERIC
^j,^^
Poland and Germany, have different
that distinctly
LycopSIDS
pQggjL
Such
leaf shapes, leaf orientations and leaf outlines.
encompass
LycopodiM used ho-
generally to
characters are remarkably consistent in living spe- is
vegetative axes, bear-
mosporous lycopsids, or just
(Quansah
Lycopodium.
species of
resemblance to living
so can be used ing a
reasonable they
assume' that
it is to
name
Lycopodites var-
Ahhough usage of the
the
for
distinguishing species of
fossils.
argument any of the
no strong for
there
AnisophyUous forms have been described ies, Is
also
Lycopodium
be included within sens. str.
from Mesozoic and many of these have been fossils to
strata,
taxonomic
seems be a problematic
Lycopodites to
shown
to be heterosporous; examples include Se-
have even been subsequently
some specimens
laginella anasazia Ash, 1972, from the Trias of unit;
even
lycopsids or of
New shown be twigs of larger
1950b, to
Mexico, Lundblad,
Selaginella hallei
When
from daw-
the Rhaetic Sweden and Selaginella
of
/
5ont Watson, Wealden
1969, from
the
&
on
Ghosh, 1990, for a discussion
Pal
There ton (see
Selagi-
are also vegetative shoots such as
he
from Yorkshire Jurassic
the
dichZlVaTZnZskyJ Wklav,~19ll, generic priority),
nella
an.sophyllous structure but
confirmed
from not only its
the and Selaginellites
Jurassic of Siberia
many
be twice as larger lateral
showed there to
nosikovii from Czechoslovakia,
the Cretaceous of
Th fi
be
Lower
nella (= known from the
Selaginellites) are
bett
Carboniferous. These, Selaginellites resi-
called
form-genus ofr st. er•d, e axes and. .to
that of a
mus Rowe Drybrook Sand- tes to
came
(1988b), from the
dehmited by ar parameters
-fu^ial
accept that .s
stone. They and isophyllous, .t
are both herbaceous
"--'^ of Me.ozcnc con.fer
^69) classification
of
-ith impressions
small terminal showing
strobili
i^
shoots
^egaspores
in several sporangia.
about
Isophyllous forms such Selaginella //
as
_ name
another generic
of using in
& the necessity
(Leclercq) 1969, are also
Schlanker Leisman,
;f-
^o-'
^eia^'-
preference to
^nZlZZ
l^nown Euramerican
from Upper Carboniferous
the
ongmally suggested
Zedler (1906)
genus.
bling this
Coal
rare,
Measures, although they are similarly
megaspores
more than four
presence of
t"hat the
This sn^nJoc ^sp^a—rsely r
osppceic;iiecs5 hliaass bueeeenn duecssccriiub^ecdu aas^ j -. v ^^l^_
dM.,ngu.. sh•mg
^ was a char-
megasporang.um
each
Wnched,
determinate in
sprawling and possibly of
Annals the
628 of
Garden
Missouri Botanical
acter of his new genus Selaginellites. Halle (1907) record soon after the isophyllous forms, implying
did not accept that this character was essential and that plants with the Selaginella selaginoides mor-
may
used Selaginellites for all heterosporous fossils. phological form not be ancestral to all Sekg-
Subsequently, Duerden (1929) and Quansah( 1988) meWa-like fossils. It is possible that an ancestral
showed Selaginella can group of both the isophyllous and anisophyllous
that living species of pos-
sess more than four megaspores per sporangium. lycopsids might be eventually recognized in the late
Seward (1910), Harris (1935), Hirmer (1941), Devonian or very early Carboniferous lycopsids.
Chaloner (1967), and Rowe (1988a) used Sela- However, it is also possible that these plants are
much
ginellites for all fossil shoots that were closer to less closely related than has been previously
Selaginella than any other genus. In contrast, thought.
Townrow
Darrah Lundblad
(1938), (1950b),
&
Schlanker Leisman Watson
(1968), (1969),
Evidence for the Generic Assignment of
FossiL
(1969), and Ash (1972) used Selaginella for fossil
The Extant Species of the Selaginellaceae
shoots because they could not demonstrate any
Wildenow
real morphological differences from the extant spe- Early classifications
cies of that genus. The problem of using generic Sprengel, 1827) placed Selaginella within the ge-
names presented
of extant plants for specimens a nus Ijco/?oG?mm, though Spring (1850)
fossil
is
major one encountered throughout palaeobotany the major monograph of the genus Selaginella as
Even most work-
though
(e.g., Collinson, 1986). In the case of the herba- presently understood.
it is
on
and agreed
ceous anisophyllous axes, further complications can ers have generally understood trie
there
arise during attempts to include species of parameters delimiting the genus Selaginella,
fossil
many taxonomically
Selaginella in the described subgenera, or to in- have been attempts to express
morpholo-
them
corporate into the various genera suggested groups of species that have distinctive
genus by var-
for splitting Selaginella. This problem be The taxonoraic divisions of the
will dis- gies.
number subgenera or
cussed in the following section. ious authors differ in the of
Fossil plants are usually incomplete fragments genera that they accept.
A
of whole organisms, which pose even more prob- study of African spe< :ginella (Q
approxi
lems for generic assignment. For example, Lund- sah, 1986) and a preliminary review of
(Quansah
blad (1948) assigned a small detached heterospo-
be
genus should
rous cone to Selaginellites with no knowledge of Thomas, 1985) suggest that the
those
subgenera:
vegetative morphology, even though the cone was divided into at least two genera or
anisophyllous
broader than those of living species of Selagi- with isophyllous leaves and those with
all
may recent studies,
nella. This approach be the simplest way of leaves. This supported by several
is
&
Jermy
when Crabbe
dealing with the situation but, the are including Jermy (1967),
fossils et
al.
& Tryon
Tryon
merely and
vegetative axes, taxonomic decisions have (1976), Alston et (1981),
al.
two su -
made
to be without knowledge of their reproductive (1981), whose authors have recognized
with-
Stachygynandrum.
organs
and
genera, Selaginella
Baker
from
how
It is interesting to note the use of these in the genus. The two groups originate
sub-
two
generic names can vary, even within a pub- (1883, 1887) The other
single classification.
Walto
lication. For example, Lundblad (1950a) named
-
.^^„^...«^ ..^-
^ .
, j
Heterostachyh
an isolated cone from the Rhaetic of Sweden Se- Alston (1938), Homostachys and
le^^
laginellites hallei sp. nov. but referred the asso- are best recognized at a lower hierarchical
anisophylio"
ciated vegetative, anisophyllous axes to Lycopodi- within the group containing all the
tes scanicus Nathorst ex Halle, Later, Lundblad
species.
sub
(1950b) found specimens proposed five
fertile sharing the char- Jermy (1986) however,
has,
of
subgenera
acters of both these species and named them The two
Se- genera within Selaginella.
e
comb charac
t
several
^ anisophyllous species differ in
name
to use the cone rather than
taxonomically including cone morphologies. It is
their ^^^^^f|^
could
Selaginellites
rules to note that Carboniferous
^_
'^
ority (cf. Greuter et 1988), subgenera; 5. g^^
al., included both of these
in
Many co
systematists have Bower sporophylls,
followed
(1935) with uniform and tetrastichous
its
arguing Stachyg)^
in that Selaginella
selaginoides subgenus
L. a be included Jermy's
is in
Carbonifero^
beca
drum the
Beauv.) Baker, while
(P.
m
J^r-
included
roots be
Selaginellites elongata could
^
Work
phyllous heterosporous forms appeared Baker.
in the my's subgenus Heterostachys
fossil
p
Number
3 Thomas
Volume
79, 629
1992 Paleozoic Herbaceous Lycopsids
may
progress on the Carboniferous species give Brongniart, A. 1822. Sur la classification et la distri-
bution des vegetauxfossiles en general, sur ceux
et
further evidence in support of Jermy's idea of
des terrains de sediment superieur en particulier.
creating five subgenera.
Mem. 203-348.
Mus. Nat.
Hist. 8:
It is, at least, clear that both isophyllous and Brown, & Y. Lemoigne. 1977. Presence du genre
J.
known Dwyka
anisophyllous Selaginella-hke plants are Eshdalia a la base des Series dans I'Etat
d'Orange datation du debut du grand cycle
from the Carboniferous onwards and some et gla-
that of
en Afrique du Sud. Compt. Rend.
ciaire postsilurian
these have been thought even to be generically
1509-1511,
Hebd. Seances Acad. 284:
Sci.
from The
bdistinguishable extant Selaginella,
or- W. 437-802
Chaloner, G. 196?! Lycophyta. Pp. in
I
&
igin of isophyllous and anisophyllous lycopsids at E. Boureau, S. Jovest-Ast, 0. A. Hoeg W. G,
437-
approximately the same point geological time Chaloner (editors), Traite de Paleobotanique, II:
in
—
802.
can be taken as additional evidence for regarding
&
K. U. Leistikow A. Hill. 1968. Brasi-
,
the two groups of plants as taxonomically distinct.
pedroanum
lodendron gen. nov. and 5. (Carruthers)
Indeed, the recognizable two
division into different comb, Permian lycopod from Rev.
nov., a Brasil.
117-136.
morphologies, which has existed for approximately Palaeobot. Palynol, 28:
ME.
Use modern names
300 Collinson, 1986. of generic
million years, strengthens considerably the
u &
^^ Pp. 91-101. R. A. Spiccr B.
plant
r makmg ,i j. fossils. tfi
;a»rrag,uirmre.on„tf , . . - ,
for the division at generic rather
^ ^^^^^^ Systematic and Taxonomic Ap-
^^^^^^J^^^
than
subgeneric
level.
proaches Palaeobotany. Syst. Assoc. Special Vol.
in
31.
&
new
Crabbe, A. A. C. Jermy. 1976. Six species
J.
LrrERATURE
Cited
of Selaginella from Tropical South America. Fern
AmoN, & 255-262.
A. H. Jermy M. Rankin. 1981. Gaz. 11:
G., A. C.
J.
A
Taxonomy
The genus Selaginella Tropical South America. Dahlen, M. a. 1988. of Selaginella.
in
BuU. Mus. 223-330. study of characters, techniques and classification in
Brit, (Nat. Hist.) Bot. 9:
M.& 277-
Anderson, H.M.Anderson. 1985. Palaeoflora the Hong Kong species. Bot. J. Linn. Soc. 98:
J.
of southern Prodromus South African 302.
Africa, of
W. A
DaRRAH, 1938. remarkable Selaginrlln
Megafloras. Devonian Lower Cretaceous. Balke- C. fossil
to
Mus.
ma, Rotterdam. with preserved female gametophytes. Leafl.
t.
& 113-136.
Archangelsky, S., C. L. Azcuy R. H. Wagner. 1981. 6:
megaspore number
Three dwarf lycophytes from the Carboniferous of Duerden, H. 1929. Variation in in
451-459.
Ann. Bot. 43:
Argentina. 1-35. Selaginella.
Scripta Geol. 64:
& Two new
Benedetto. 1985.
Ash, Edwards, D. L.
S. 1972. Late Triassic plants from the Chinle J.
herbaceous lycopods from the Devonian
formation in northeastern Arizona. Palaeontology 15: species of
comments on taphonomy.
424-429. Venezuela with their
of
599-618.
Baker, A Palaeontology 28:
G. 1883. genus Selaginella.
J. synopsis of the
New
Edwards, W. N. 1934. Jurassic plants from Zea-
141-145,
Bot. 21: 1-5, 42-46, 80-84, 97-100,
J.
81-
210-213, 240-244, 332-336. land. Ann. Mag. Nat. Hist. Ser. 10, vol. 13:
A
-. 109.
1887. Handbook Syn-
of the Fern- Allies.
Nouveaux du
M. 1974. specimens
Fairon-DemarET,
Orders
opsis of the Genera and Species of the Natural
Bonamo, M.
genre Leclerqia Banks, H. P., P. et
Equisetaceae, Rhi-
Lycopodiaceae, Selaginellaceae,
Queemland
1972 du Givetian du
& Grierson, D. (?)
Zocarpeae. George London. J.
Sons,
Bell
Roy. Nat. Belgique 50:
Banks, H. P. 1944. A new Devonian lycopod genus (Australie). Bull Inst. Sci.
1-4.
from New 31:
south eastern York. Amer. Bot.
J.
—
A new lycophyte cone from the Upper
1977.
649-659.
.
51-
162B:
Devonian of Belgium. Palaeontographica
1968. The early history of land plants. Pp.
•
73-107
and En- 63.
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A
DEVELOPMENTAL PROBLEMS
SELAGINELLA
IN
AN
(SELAGINELLACEAE)
IN
CONTEXT
EVOLUTIONARY
Abstract
This success
The antiquity and diversity of Selaginella species attest to the evolutionary success of the genus.
Recent
may be attributed, in part, to certain morphological and anatomical features that characterize Selaginella.
and marginal
marginal warty
developmental studies of anisophylly, monoplastidy, the ligule, the rhizophore, cells,
concernmg
Unresolved questions
teeth of the leaves, as well as aspects of the heterosporous cycle are discussed.
life
and
rhizophore, a
vegetative features include the function of the morphological interpretation of the so-called
ligule,
^
Among concerning reproduc ive
possible role of distinctive leaf ornamentations In water economy. unresolved questions
anec
mechanisms as they
morphology are the basis of heterospory, control of sporangial development, spore dispersal
genus belagine
inbreeding and outcrossing, and other of the reproductive process. concluded that the
details It is
numerous
offers opportunities for future research.
my
From conten-
the standpoint of both development and Selaginella are from obvious. It is
far
may be foun
number
evolution, the genus Selaginella raises a tion that, at least in part, the reasons
of intriguing questions. In regard the following unique
this
amso-
quotation from Bierhorst (1971) seems appropri- and reproductive features. These include
foliar
certain
ate: presence of
phylly, the the
ligule,
™
(rhizo-
root
epidermal ornamentations, the aerial
f c J c J
Ihe lami.i, y bel1 ag•melIlI aceae mclIudes bel7agi•neirlfa and
, v i i andj certa* m aspe*c^^it^^ of the
monoplastidy,
P^ore),
kknnoowwnn
sev^eerraall vveerryy cclloosseellvy rreellaatteedd ffoossssiill ffoorrmmss wwhhiicchh aarree
em-
be
should
from Lower Carboniferous and more recent heterosporous Hfe cycle (Fig. 1). It
strata.
may
features
Selaginella is probably one of the oldest of all extant phasized that although each of these
genera of vascular plants, second only to Lycopodium, example, the
^^ f^^^^ -^ groups (for
^^j^^^. j^^^^
Despite great antiquity, which might lead one to
its v j n Spladnella),
^^g^'i ^ • r^^^^^ ^• ^ ^r^^^^.^^ ^' ""^^ ^o^c ^in
eexxppeecctt ssppeecciiaalliizzaattiioonn aanndd rreellaattiivvee ggeenneettiicc ssttaaggnnaattiioonn,, '' !Lne
unique to
that
the combination of features is
700 much
there are approximately extant species and it is
paper to
purpose of this
evidence for recent speciation. Selaginella. the
It is
certam
out
and point
discuss these features to
The may lead to tur
key elements of this statement are the an- troversies and questions, which
some older
tiquity and diversity of the genus and the proba- and Although
discussion future studies.
e
been inclu
have
bility that Selaginella is presently undergoing ac- references from the literature
tive speciation. Years ago, the prominent inly
W.
Wardlaw
morphologist C. (1925) also alluded of the past 30 years.
to the evolutionary status of Selaginella:
Morphology
Vegetative
The
genus represented by a large number of
is
have
Selaginella
many of
species, of which are polymorphic. They have Certain vegetative features
a
wide geographical and some
distribution, in floras they
intrieued
vx
sW oi«viv.*x».o
dominate ground rhizophore
the vegetation. Further they
^ot and so-called
exarnpTeTtheTigule the"
^* «**«
v>^»-«i*i^raxj'^ t**^ ixo^^^«^ *-
adaptation to all degrees ol xerophily and hygrophily. ' r __:'iT.onrr to
are ta
two that
are enigmatic structures
would appear from
It these facts that the genus
is
despite
Yet,
students morphology.
successfully retaining its position in the midst of the of plant
a
mterpre
more
highly differentiated Phanerogams. merous morphological
studies, the
many
ot
and of
evolutionary significance
With
remain
etative features of Selaginella ^ u
^
_
body r
apparent
for the evolutionary success exhibited by An plant
examination vegetative
of the
The
author thanks M.
Spring
for preparing
' J. the
illustrations in Figure
1. ,,.o ^043
^Department 0626^-^
of Ecology and
Evolutionary
Biology, Connecticut
University
of Connecticut, Storrs,
U.b.A.
Ann. Missouri Bot. Card.
632-647.
79:
1992-
Number
Volume 3 Webster
79,
633
1992
Evolution of Selaginella
I
•--••7^
I
t
I
Bl.
Figure A. Anisophylly.- B. Ligule. Ligule, in section
Diagram of Selaginella.
1. of structural features
= — -D
= = Marginal warty and marginal
t tip, n neck, b base, glossopodiuni, s sheath C. cells teeth.
g
—
Monoplastidy. Heterospory.
E, Aerial root (rhizophore). F.
a basic simplicity of design, which would seem to 1983a, b) investigated anisophylly in 5. martensii.
many
"lake the by the Unlike plants that are anisophyllous, Sclag-
evolutionary success exhibited
Heterophyllum) no
genus that much more A ex- inella (sect. exhibits transi-
interesting. critical
«niination of certain key vegetative features may tional leaf forms, but rather shows two distinct leaf
provide types borne in pairs along the stem. At each node
insights apparent contradiction.
into
this
and one
one smaller dorsal leaf larger ventral or
r
an
leaf are initiated as opposite pair (Fig.
lateral
^PHYLLY
Leaf dimorphism in Selaginella generally
is
Y),
considered to be related to efficient trapping of
Perhaps
the most obvious vegetative feature
shady habitats with low light mtensity.
found Ught
in
in most Se/a^me/Za anisophylly.
species of
is
certam
noted that anatomical
Dengler (1980) fea-
According
term aniso-
to Dengler (1983a), the
are related to orientation of the shoot to light.
tures
Phylly
refers form related to
to differences in leaf
Most notably, the abaxial (aligular) epidermis of
the most
and
transectional symmetry shoot
of the
and the adax.al (hgular) epidermis
often occurs from shaded the dorsal leaf
shoots
in plagiotropic
consist of nearly isod.ametric
ventral leaf
situations upper of the
i„ which the foliage leaves on the
eh
each containing a single large oroplast.
»«e
of the stem are smaller than those on the lower cells,
These surfaces are directed toward l.^ht. he shad-
«ide. I
In Dengler (1980,
several detaUed
studies,