"GYMNOSPERMS" |
"Gymnosperms" include all seed plants other than the
angiosperms, or flowering plants. The absence of an enclosing structure is
believed to be primitive, thus the first seed plants were "gymnosperms."
The seeds of "gymnosperms" are borne exposed in open structures, such as
cones or leaves. The derivation of the word "gymnosperm" means naked seeds
and is from the Greek words gymno meaning naked or bare and
sperm meaning seed. Fossil records indicate "gymnosperms"
originated in the Paleozoic Era, during the middle Devonian Period about
390 million years ago, and reached their greatest diversity during the
Mesozoic Era, between 60 to 225 million years ago. Although the
"gymnosperms" were at one time treated as one closely allied group, many
modern workers recognize several major groups, which include the commonly
known conifers, cycads, and ginkgos, and the lesser known gnetopsids
group, extinct progymnosperms, and extinct seed ferns. |
The life cycle of "gymnosperms" depicts a dominant
sporophyte and reduced gametophytes with the female gametophyte retained
on the sporophyte (see illustration of conifer life cycle). The male and
female sex organs may be formed in compact structures on short stem axes
called cones or strobili or on specialized leaves or in flowers. In
"gymnosperms", the sexes are separate, so both male and female sporangia
are produced either on the same plant (monoecism) or on separate plants
(dioecism). Meiosis occurs both in the pollen (male) and in the ovulate
(female) cones or strobili or sporangia on specialized leaves that are
formed on the sporophyte.
The pollen strobili have many sporangia
(microsporangia) in which pollen mother cells (microspore mother cells)
undergo meiosis to produce pollen grains (male gametophytes). The mature
pollen grain (1n) is winged in many "gymnosperms", a feature that
facilitates air transport. Most "gymnosperms" are wind-pollinated except
for cycads, which are beetle-pollinated.
In the ovulate cones,
there are one or more ovules located at the base of each cone scale
(=short shoot). An ovule consists of a megasporangium, is surrounded by a
single covering or integument, and eventually will become a seed. In each
ovule, a megaspore mother cell undergoes meiosis to produce four
megaspores, of which three disintegrate and one forms the female
gametophyte (1n). The female gametophyte contains two or three
groups of cells called archegonia, each of which contains a large egg
nucleus.
Pollination occurs once a genetically compatible
pollen grain lands directly on the ovule. The pollen grain germinates
and grows into the ovule, penetrating the female gametophyte and
eventually fertilizing an egg nucleus. There may be more than one egg
fertilized, but only one zygote develops into an embryo. It may be an
extended time, such as one year or longer, between pollination and
subsequent fertilization. Once a zygote is formed, further development
leads to a seed, which consists of an embryo surrounded usually by
nutritive tissue and a seed coat. The nutritive tissue of "gymnosperm"
seeds is derived from the female gametophyte.
"Gymnosperms" are
a diverse and paraphyletic group of seed plants, and can be characterized
by the following features: exposed seeds formed in cones, pollination
occurring by pollen landing directly on the ovules, generally one
fertilization event in the ovules, and tracheids as the water-conducting
cells (xylem) in the vascular system. The groups of living gymnosperms
are the conifers, cycads, Ginkgo, and Gnetales. These can be identified by
observing habit, leaf type, and position and structure of the reproductive
organs. |
1. Conifers The conifers are the most common and
abundant group of "gymnosperms" alive today. They are woody plants with
scale-like or needle-like leaves. Most of the leaves are evergreen,
although some are deciduous (e.g., larch or Larix). Three common
families are included: Pinaceae, Cupressaceae (now includes the group
formerly recognized as Taxodiaceae), and Taxaceae. Most of the
representatives of these families are monoecious, but a few are dioecious.
The Pinaceae are monoecious, form needle-like leaves, and include the
common pines, firs, larches, and spruces. The Cupressaceae are monoecious
or dioecious, form scale-like or needle-like leaves, and include junipers,
cedars, cypresses, giant sequoia, coast redwood, and dawn redwood. The
Taxaceae are dioecious or monoecious, form needle-like leaves, produce a
fleshy, often brightly colored structure called an aril that surrounds
each seed, and include yews. |
Pinus
strobus (photo credit: W. Judd) |
2. Cycads The cycads are distinctive palm-like
plants that grow in tropical and subtropical regions of the world. They
form thick, slow-growing stems, and large compound leaves that are either
erect and conduplicate, or involute with the main axis emerging erect from
the apical bud and with each leaflet coiled inward, or expanding from a
circinate position. The leaves form crowns at the end of the stem apices.
Cycads are dioecious and produce cones or sporangia borne on open
sporophylls. |
Dioon
edule (photo Credit: W. Judd) |
3. Ginkgo This group includes a single living
species, Ginkgo biloba or maidenhair tree, in the family
Ginkgoaceae. Leaves of Ginkgo are well-represented in the fossil
record and confirm that the plants were abundant 170 million years ago
during the mid-Jurassic Period of the Mesozoic Era. The leaves are
deciduous, alternate, and fan-shaped, and show dichotomously branched
veins. The plants are dioecious with pairs of unprotected ovules formed at
the ends of short shoots and with pollen produced in male sporangia
aggregated to form a cone-like organ. |
Ginkgo
biloba (photo credit: S. Darwin) |
4. Gnetales The gnetopsids contain three highly
distinctive monotypic families (each with one genus): Gnetaceae
(Gnetum), Ephedraceae (Ephedra), and Welwitschiaceae
(Welwitschia). The gnetopsids are non-angiospermous seed plants.
Some recent studies suggest that, among extant or living plants, the
gnetopsids are the closest relatives of angiosperms or flowering plants.
The vessels in the wood of gnetopsids are derived from tracheids with
circular bordered bits, in contrast to the vessels of angiosperms, which
are derived from scalariform tracheids. The tropical and subtropical
species of Gnetum are vines or small shrubs and have broad leaves
resembling those of angiosperms. Ephedra occurs in dry areas of the
southwestern United States and northern Mexico and are scrubby bushes with
reduced and scale-like leaves; double fertilization has been documented to
occur in this genus. Welwitschia occurs naturally in deserts of
South-West Africa and exhibits a short, wide stem bearing two wide leaves
that grow perennially from a basal meristem. |
Ephedra
viridis (photo credit: C. Campbell)
Welwitschia mirabilis (photo credit: M. F. Denton)
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