Taxonomy
of Flowering Plants - LECTURE
NOTES
Hugh
D. Wilson
The Pteridophytes
(ferns and their allies)
Features:
1. vascular plants with neither flowers nor seeds
2. spores are produced via meiosis in sporangia
(or sporangial clusters known as sori - photo)
3. spores are usually released from the parent
plant (diploid sporophyte) to produce haploid, free-living
(not dependent on sporophyte, usually photosynthetic) gametophytes
(photo)
3. as opposed to flowering plants, gametophytes usually
develop outside the spore (exosporic)
and
both gamete-producing organs (antheridia, archegonia)
are usually present (see text fig. 8-1, p. 94)
4. gametes (sperm
free-living, motile)
5. embryo/sporophyte - dominant
The term "Pteridophyte" refers to vascular plants with independent
gametophytes and motile sperm that are usually classified into 4 divisions
that comprise nearly 40 families. They include over 9,000 living species
(ca. 365 genera) distributed worldwide, with 893 species (124 genera, 76
hybrids and 176 infraspecific taxa) in North America north of Mexico (Kartesz,
1994). The divisions are also characterized, in part, by the nature of
the sporangia:
EUSPORANGIATE
- a large sporangium developing from several initial cells producing many
spores. These are sometimes fused to form synangia.
LEPTOSPORANGIATE
- small, specialized sporangia developing from a single initial cell producing
a small, definite number (< 128) of spores. These often occur
in a cluster (sorus) that is often covered by a flap of tissue known
as an indusium (photo)
Also relevant is the nature of spores produced: HOMOSPOROUS
- differing from flowering plants in that all spores are the same,
producing bisexual gametophytes vs. HETEROSPOROUS
- similar to flowering plants in that two types of spores are produced,
and these produce two types of gametophytes: MEGASPORES
(develop to form the egg-producing gametophyte or megagameophyte)
and MICROSPORES (develop to form the
sperm-producing gametophyte or microgametophyte); and the nature
of gametophyte development (ENDOSPORIC [within the spore wall as
flowering plants) ] VS. EXOSPORIC
[spore "germination" with plant
development outside the spore]) and the nature of the leaf:
MICROPHYLL - usually small (awn or scale-like) with one vein that
is superficially connected to the stem vascular system vs. MEGAPHYLL
- usually a large leaf (FROND) with reticulate veination that has
direct connection to the stem vascular system and develops by unrolling
CIRCINATE VERNATION.
PSILOTOPHYTA (note Division or Phylum
ending)
PSILOTACEAE (Whisk Fern Family) - two genera
(Psilotum - pan-tropical- and Tmesipteris - Oceania and Australasia)
with about 4-8 species - terrestrial or epiphytic perennials. Only Psilotum
nudum in Texas. A leafless (leaf-like projections = ENATIONS),
rootless (rhizoids) photosynthetic stem with eusporangia (synangia)
and subterranean,
mycotrophic gametophytes.
Either a very primitive remnant of an ancient line, possibly basal to the
vascular plants, or a highly specialized version of the "true" ferns. Currently
a debate. Images from: Australian
National Botanic Gardens, Department
of Botany, University of Wisconsin (Madison), and University
of Hawaii Botany
EQUISETOPHYTA
Equisetaceae (Horse Tail or Scouring Rush Family)
- a single genus - Equisetum - with 35 species, worldwide distribution;
4 species in Texas and 11 in the FNA. Perennial, rhizomatous herbs
of moist places with jointed, ridged, cylindric stems; reduced-scalelike
leaves connate to form a nodal sheath, eusporangia aggregated into
a terminal STROBILI [termed "cones" in Correll
and Johnston (1979)]. Gametophytes photosynthetic - can be unisexual. Easy
to identify, the last of a line that was much more diverse prior to the
rise of the flowering plants. Images
from the Department of Botany, University of Wisconsin (Madison),
the Vascular
Plant Image Gallery, and
University
of Hawaii Botany
LYCOPODIOPHYTA
CLASS Lycopodiopsida (note class ending)
LYCOPODIACEAE (Club Moss or Gound Pine Family)
- commonly treated as two genera Phylloglossum (1- Australia and
New Zealand)) and Lycopodium (350-400 - cosmo - temperate - 4 species
in Texas), but more recently (FNA) Lycopodium "split" into over
10 genera; FNA treats 7 genera (Lycopodium segregates)/27 species
in NA. and 4 species in 3 genera in Texas. Microphyllous, homosporous with
eusporangia in leaf axiles, often clustered a tips of dichotomously branching
stems as STROBILI [STROBILUS]. Gametophytes either above or below ground,
sometimes unisexual. Longest continuous fossil record of any vascular plant,
make up most of the world's coal. Images
from the Department of Botany, University of Wisconsin (Madison),
the Vascular Plant
Image Gallery, and the
University
of Hawaii Botany
CLASS Isoëtopsida
SELAGINELLACEAE - (Spike Moss Family) - single
genus
Selaginella of ca. 700 species (38 in the FNA) most diverse
in tropical and subtropical regions of the world, 12 species in Texas,
some adapted to quite xeric conditions ("resurrection plants"). Superficially
similar to Lycopodium, but often prostrate, creeping with angular
strobili. Distinctive by being heterosporous and endosporic.
Images
from the Department of Botany, University of Wisconsin (Madison), the
Vascular Plant
Image Gallery, and the
University
of Hawaii Botany.
ISOËTACEAE - (Quillwort Family) - a single
genus (Isoëtes [150-cosmo-24 in FNA and 2 in Texas] with one
possible generic segregate (Stylites [1 species - Andes]). Largest
microphylls of the Pteridophytes; perennial, often aquatic plants that
have a tufted, grass-like aspect, typically occur in moist situations.
Heterosporous
(megaspores important in species identifications) and gametophyte development
is endosporic. See the Vascular
Plant Image Gallery and the
Department of Botany, University of Wisconsin (Madison)
POLYPODIOPHYTA
"True" ferns, the others are "allies". Diverse and
the subject of much taxonomic confusion - lumping vs. splitting. About
12,000 species worldwide. Major elements:
Subclass Ophioglossidae (note subclass
ending)
Ophioglossaceae - 5 genera (three monotypic and
two - Ophioglossum (Adder's tongue - 40, 5 in Texas) and Botrychium
(Grape Fern - 50-60, 30 in FNA and 2 in Texas) common. Eusporangiate,
homosporous
with a strong leaf dimorphism (sterile (Sporophore) vs. fertile
(Trophophore)) with eusporangia in "fertile spikes" and underground,
micotrophic gametophytes. Images from the
Department of Botany, University of Wisconsin (Madison) and the
Vascular Plant Image
Gallery.
Subclass Polypodiidae
Osmundaceae (Royal Fern Family) - 3 genera (2
Old World) with Osmunda (10) having 2 species in Texas. Often placed
as intermediate between eusporangiate and leptosproangiate ferns
- a "cusp" taxon. Usually with striking leaf dimorphism (sporangia bearing
vs. photosynthetic) - species are often good indicators of acid (often
bogs) habitats. Images from
the
Department of Botany, University of Wisconsin (Madison) and the Vascular
Plant Image Gallery.
"Other" Ferns: the largest and most diverse group - leptosporangiate,
homosporous, variously classified, often (as in Correll and Johnston, 1977)
put together as the Polypodiaceae (50 genera worldwide, 22 genera
with 71 species in Texas - flora of Robertson county lists 9 genera with
10 species). Fern leaf (frond) terminology: PINNA [primary
division of a pinnate leaf or leaflet], PINNULE [primary division of a
PINNA], STIPE [= petiole].. Hatch, et al. (1990) Vascular Plants of Texas
also include the Schizaeaceae (Curly Grass Family - Anemia, Lygodium),
and the Parkeriaceae (Water Fern Family - Ceratopteris).
Texas ferns listed by the Biota of North America Program "split" genera
of the Polypodiaceae (s. lat.) into 7 additional families,
a perspective followed by the FNA treatment. Images available from the
Department of Botany, Univeristy of Wisconsin (Madison) include:
Subclass Marsileidae
Marsileaceae - Water Clover Family. Sori
within hard bean or pea-shaped bodies knows as SPOROCARPS.
Heterosporous (sori of both types in the sporocarp) and, as with the
flowering plants,
endosporic. Spore germination occurs after
rupture of the sporocarp wall. Subsequent hydration produces a gelatinous
structure that carries the sori and spores into the water. 3 genera,
ca. 50 species worldwide, 2 of these with 7 species in the FNA, two genera
(Marsilea [4 species] and Pilularia [1 species]) in Texas.
Images from the Vascular
Plant Image Gallery and the
Department of Botany, University of Wisconsin (Madison).
Subclass Salviniidae
Salviniaceae and Azollaceae - Floating
Ferns - [Azolla treated as Salviniaceae in Correll and Johnston
(1979) - 2 genera, ca. 17 species worldwide, 1 Salvinia and 2 Azolla
species in Texas) - heterosporous, endosporic and - showing a further elaboration
of leaf dimorphism, sporangia born in "containers" made up of specialized
leaves known as SPOROCARPS. Very distinctive ferns. Images from
the Department of Botany, University of Wisconsin: Azollaceae
and Salviniaceae
A good portion of the image references
used here tap fine image resources for non-flowering vascular plants:
the Plant
Systematics Collection, developed and maintained by the Department
of Botany at the University of Wisconsin, and the Non-Flowering
Plant Family Access Page from the Department
of Botany at the University of Hawaii (Dr.
Gerry Carr).
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