Taxonomy
of Flowering Plants - LECTURE
NOTES
Hugh
D. Wilson
The Gymnosperms
Distinctive features (within the context of other vascular plants):
1. similar to angiosperms in that an ovule is produced, but this
is not enclosed by tissue (such as the angiosperm carpel) - 'gymnosperm'
= 'naked seed'.
2. Pollen grain (with microgametophyte within) directly contacts the
ovule to fertilize the egg - in angiosperms the microgametophyte
makes direct contact with the ovule after its emergence from the pollen
grain, BUT angiosperm pollen does not make direct contact with the
ovule.
3. no double fertilization to produce endosperm (although double
fertilization has been documented in Ephedra) - nutritive tissue
for the embryo in gymnosperms is haploid, megagametophytic.
4. absence of vessel elements (exception = Gnetophyta)
5. absence of companion cells in the phloem.
6. presence of a many-celled or many-nucleate megagametophyte
7. presence of archegonia (as in pteridophytes, except
in genera - Gnetum and Welwitschia)
8. mostly woody plants
As with the dicot subclass, Hamameliidae, the gymnosperms are a
small, diverse group (72 genera in fewer than 11 families, ca. 750 species)
that forms, however, a dominant element of the flora in many large
parts of the planet's surface. These plants are also economically
important, mostly as a timber resource, and poorly understood in terms
of biological relationship.
Your text treats this group as a single unit (Division or phyllum Pinophyta)
with three primary subunits treated as subdivisions. The treatment
offered here, which follows that of the Flora of North America (FNA), elevates
these three elements to division rank and moves Ginkgo biloba from
subdivision Pinicae to a monotypic division.
Cycadophyta
Palm or fern-like plants, no vessels, motile sperm, usually dioecious,
seeds born on MEGASPOROPHYLLS, also MICROSPOROPHYLLS, both usually born
in STROBILI. None native to Texas, but several cultivated. An archaic group
(Triassic to present) now represented by 3 families: Cycad images are available
from the University
of Wisconsin (Madison) Department of Botany, and the Department
of Botany at the University of Hawaii. See also: The
Cycad Pages from the Royal
Botanic Gardens Sydney, Australia and the Gymnosperm
Database.
Ginkgophyta
Trichopityaceae (extinct) and Ginkgoaceae
with at least six extinct genera and a single extant species (Ginkgo
biloba) which is native to China and cultivated in urban areas (resists
pollution) throughout the world. Apparently identical fossils that are
200 million years old (Mabberley, 1993) but now known only from cultivation
and probably saved from extinction via cultivation - broad leaves, motile
sperm. Ginkgo biloba images
are available from the University of Wisconsin (Madison) Department of
Botany, and information
on
the species can be obtained from the Gymnosperm
database.
Pinophyta
This division includes most of our extant gymnosperms in 6 families with
52 genera and ca. 614 species. We have two genera of the Pinaceae in Texas
(Pinus (8) and Pseudotsuga (1)). The North American
flora (at least that portion covered by the FNA) includes 6 genera and
66 species. Conifer images
are available from the University of Wisconsin (Madison) Department of
Botany; a detailed (species descriptions) overview of conifers
of the Pacific Northwest is provided by Oregon State University;
and a nice presentation of the ancient
Bristle Cone Pine has been created by Leonard Miller.
order Taxales
Taxaceae
(Yews) - five genera, with 17-20 species in mostly the northern hemisphere
- ovule surounded by green or red fleshy aril (an appendage or outgrowth
from the hilum or funiculus that forms a baccate (berry-like
but not a berry) pulpy or fleshy seed covering. See images
of Taxus
cuspidata from the University of Wisconsin.
order Pinales
Pinaceae
- 10 genera, ca. 200 species mostly in the northern hemisphere with
6 genera (Pinus - pine, Larix - larch, Picea - spruce,
Tsuga - hemlock, Abies - fir, and Pseudotsuga
- Douglas-fir)
and 66 species in the North American Flora. See Fig. 60. (gateway)
Cupressaceae
(including the Taxodiaceae) - 25-30 genera, ca. 110-130 species
with 30 species in the FNA and 3 genera (Taxodium,
Cupressus,
and Juniperus) with 11 species in Texas. Includes
Sequoia sempervirens
(coast Redwood) and Sequoiadendron giganteum (Sierra Redwood), the
tallest and most massive living organisms. See our local Taxodium.
Araucariaceae
- (Norfolk Island Pine or Araucaria Family) - 2 genera, 39 species - Once
(Jurassic and Cretaceous periods) of worldwide distribution, now restricted
to the Southern Hemisphere. "Living fossil" (The Wollemi
Pine - New South Wales, Australia) discovered in 1994, probably
a new genus - only ca. 20 plants known. See See images from the Department
of Botany at the University of Hawaii
Podocarpaceae
- 13-17 genera with ca. 170 species - elements form major forests in some
parts of the family range - Southern Hemisphere. See images from
the Department
of Botany at the University of Hawaii, and our
local podocarp.
Gnetophyta
3 orders - each with a single famly that includes a single genus - with
ca. 90 species - unusual plants that show, relative to other gymnosperm
groups, structural similarities to the flowering plants.
order Ephedrales - shrubs with jointed stems and reduced
leaves, vessels, and double fertilization (no endosperm produced) - Ephedra
- ca. 60 species of the Northern Hemisphere and South America, 12 species
in the FNA and 6 species in Texas. (gateway)
order Welwitschiales - single species (W. mirabilis)
of Namibia (s.w. Africa - 0 to 10 cm of rain per year) - vessels, two leaves,
dioecious, cone-like strobili. See images
from the University of Wisconsin (Madison) Department of Botany and the
Vascular
Plant Image Gallery (gateway)
order Gnetales - single pantropical genus with ca. 30 species.
Mostly rainforest lianas (woody vines that occupy the forest canopy), with
broad, dicot-like, opposite leaves. See images from the University
of Wisconsin (Madison) Department of Botany, and the Department
of Botany at the University of Hawaii. (gateway)
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Plant Evolution, or the Pteridophytes