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GYMNOSPERMS
I. Two major groups of vascular plants:
A. Seed plants - reproduce via seeds
1. Seed = a structure in which the embryo (the young
sporophyte) is shed from the parent plant, enclosed within a resistant coat,
together with a supply of food that aids its establishment
2. The majority of extant plants are seed plants
B. Seedless plants - reproduce via spores
II. Characteristics of seed plants:
A. Megaphylls
B. Heterospory
C. A reduced megagametophyte retained within the megaspore
D. A megaspore retained within a fleshy megasporangium called a nucellus
E. Pollen - a structure which carries the male gamete
to the female gamete
1. Seed plants do not require water for fertilization
III. Evolution of seeds
A. The seed habit arose by 365 million years ago via fusion of
vegetative tissues around the megasporangium
1. This additional protective layer is called an integument
2. The integument has a small opening, the micropyle,
through which fertilization takes place
3. Ovule = an integumented megasporangium
4. Following fertilization the integument will become the seed coat
IV. Classification
A. There are five phyla of extant seed plants:
1. Four of the phyla have naked ovules
borne on modified sporopylls. These are called "gymnosperms"
2. In the remaining phylum (Anthophyta) the ovules are
enclosed within a protective structure called an ovary
PHYLA OF GYMNOSPERMS
I. Coniferophyta - commonly called conifers
A. About 50 genera and 550 species
B. Common members include the pines, hemlocks, spruces, firs, yews,
cypresses, junipers and redwoods
C. Conifers are most common at the higher higher latitudes, towards the
poles
D. Conifers arose by 300 million years ago
E. Conifers include some of the largest organisms on earth, e.g. Coast
redwoods may reach 380 feet tall and giant sequoias may reach diameters of 36
feet. A bristlecone pine, dated at 4,900 years old, is one of the oldest
organisms on earth.
F. Conifers are one of the most economically important groups of plants.
They supply building materials and paper pulp
G. Most conifers are evergreen, but there are a few deciduous species, e.g.
bald cypress and larch
H. All conifers are woody and they have a bifacial
vascular cambium that produces xylem to the inside and phloem to the outside
1. As the tree grows the center xylem becomes lignified
to provide additional support
2. Lignin is a chemical deposited in the secondary walls
a. Heartwood = center, nonfunctional, lignified
secondary xylem
b. Sapwood = outer, functional, non-lignified
secondary xylem
3. With initiation of secondary growth the epidermis is replaced by a periderm
produced by the cork cambium
a. Bark = all tissue external to the vascular cambium.
I. Conifer reproduction (see your text for the Pine life cycle)
1. Megasporangia and microsporangia are borne in separate megastrobili
and microstrobili (cones)
2. Microstrobili (male cones) are relatively small and
they dry up and wither away shortly after shedding their pollen. The
strobili consist of a central axis with pairs of microsporangia on the underside
of microsporophylls. Within the immature microsporangia microsporocytes
(microspore mother cells) undergo meiosis to produce four haploid microspores.
Each microspore develops into a winged pollen grain which consists of:
a. Two prothallial cells
b. One generative cell
c. One tube cell
d. This four celled pollen grain is the immature male gametophyte. The
pollen is shed at this stage and it carried by the wind to an immature
megastrobilus (female cone)
3. During the spring pollination season the megastrobili (female cones)
have their cone scales open. The pollen in the wind is caught and held by a
sticky secretion. As the secretion dries up the pollen is drawn into the micropyle.
The cone scales then grow together
4. The pollen tube begins to digest through the nucellus towards the
developing megagametophyte
5. Twelve months after pollination the generative cell divides to form a
sterile cell and a spermatogenous cell
a. The spermatogenous cell then divides to produce two sperm
cells. At this stage the male gametophyte is mature
6. Megastrobili (female cones) are larger than the
microstrobili. A pair of ovules sit on top of a seed-scale
complex which is subtended by a sterile bract. Each ovule contains a
multicellular nucellus surrounded by a thick integument with an opening (micropyle)
facing inward
7. Each nucellus contains a megasporocyte (megaspore mother cell) which
undergoes meiosis to produce four haploid cells. Three degenerate and the
remaining one develops into the megagametophyte over a six month period.
This takes place up to six months after pollination. Development of the
megagametophyte is therefore a full year behind formation of the pollen
responsible for its fertilization
8. At 13 months after pollination, inside the megagametophyte, a large
number of free nuclear divisions take place, forming 2,000 free nuclei. Then
cell walls form to make solid tissue. This gametophytic tissue
will become the stored food inside the mature seed
9. 15 months after pollination 2 - 3 archegonia, each containing an egg,
form. At this time the pollen tube reaches the archegonia and it discharges
both sperm nuclei into the archegonia. One sperm fertilizes the egg, the
other degenerates
10. As the developing zygote divides to form the embryo four tiers of
cells are produced. These form suspensors which push the
developing embryo deep into the gametophyte tissue (developing stored food).
Simultaneously the integument forms the seed coat
11. The seeds mature and they are shed during the second autumn, when the
cone scales open. Therefore if takes about 2 1/2 years from pollination to
the shedding of seeds
II. Cycadophyta - Commonly called cycads or sago palms
A. About 11 genera and 140 species worldwide
B. They grow up to 50 feet tall
C. Cycads go back at least 250 million years and they were especially
abundant during the Mesozoic, which is often called the age of cycads and
dinosaurs
D. The genus Zamia is native to Florida and widely
planted in California
E. Cycads have secondary growth and their reproductive structures resemble
big pine cones. The cones are produced on separate male and female plants
F. Cycad sperm, up to 0.4 mm long, are perhaps the largest in nature. The
sperm also have up to 70,000 flagella
III. Ginkgophyta - Commonly called Ginkgo or the maidenhair
tree
A. Only one extant species, Ginkgo biloba
B. They are unisexual, deciduous trees up to 100 feet tall. The trees are
common on our campus
C. Ovules are borne in pairs at the end of short stalks. The seeds have a
fleshy, foul smelling covering, giving them the appearance of fruits
D. Microsporophylls are borne in very loose cone-like structures. The sperm
are multiflagellated
IV. Gnetophyta - North American representatives are called
Mormon Tea
A. 3 genera and about 70 species worldwide
B. Like angiosperms they have xylem vessels
C. Ephedra is Mormon Tea, a small shrub of desert regions. It has
small scale like leaves and it superficially resembles Equisetum.
D. Welwitschia (p. 492) is a bizarre looking plant of the fog zones
in the deserts of Namibia (southwest Africa). Most of the plant is buried in
the soil. The above ground portion consists of a massive, woody, concave disk
with two very long, strap shaped persistent leaves. Cones arise on stalks of
disk
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