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The Seed Plants:
Laboratory
Gymnosperms & Angiospserms
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Introduction
Gymnosperms and angiosperms are vascular, sporophyte-dominant plants that produce seeds.
Although these heterosporous plants still reproduce by spores, seeds are the primary means of
reproduction and dispersal. A seed consists of an embryonic sporophyte and food supply
surrounded by a protective coat. Gymnosperms and angiosperms are the most numerous and
widespread plants today because seeds are reproductively superior to spores. A seed contains a
multicellular young plant with embryonic roots, stems, and leaves already formed, whereas a
spore is a single cell. Also, seeds contain an abundant food supply while there are few food
reserves in spores. Finally, seeds have a protective covering, or seed coat, that helps them
survive adverse conditions for a prolonged period of time.
STATION 1: Gymnosperm Survey
Gymnosperms are the “naked seed” plants, producing seeds that are either totally exposed or on
the scales of cones (rather than being enclosed by a fleshy fruit as in the angiosperms).
Gymnosperms are classified taxonomically in the following manner:
Kingdom Plantae
Phylum Coniferophyta - conifers
Phylum Ginkgophyta – ginkgoes
Phylum Cycadophyta - cycads
Phylum Gnetophyta - gnetophytes
Phylum Coniferophyta: Pines
The conifers are the most familiar group of gymnosperms and include pines, firs, and spruces.
Most conifers are cone-bearing, woody evergreens that bear leaves throughout the year. The
leaves of conifers are usually modified into narrow needles with a thick, waxy coat to retain
water. Also, most conifers are monoecious, with male pollen cones and female seed cones borne
on the same plant.
The pine tree has a life cycle typical of conifers. The dominant sporophyte tree bears both male
and female cones. The male cones are very small and occur in clusters near the tips of branches;
male cones are quite fragile and persist only one season as they grow, produce pollen, and
release mature pollen grains into the air. Female cones are larger, and persist for several years
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on the branches of the tree. After several years, winged-seeds are shed from the female cones
and dispersed by the wind.
Male cone w/ pollen
Female cone w/ovules
Phylum Cycadophyta: “Sago Palms”
Cycads are slow-growing, tropical evergreens with palm-like leaves and large seed cones.
These so-called “sago palms” are dioecious, meaning that there are separate sexes with female
plants bearing seed cones and male plants bearing pollen cones. Popular as ornamentals, many
cycads are now endangered due to being over-harvested from the wild.
Sago palm
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Phylum Ginkgophyta: Gingko biloba
The only surviving member of this phylum is Gingko biloba, the oldest species of living trees.
Like the cycads, Gingko biloba is dioecious, with separate male and female trees. The females
bear fleshy seeds that are totally exposed rather than being inside a cone. Unlike evergreens,
gingkoes are deciduous and shed their leaves in the fall. Gingko biloba has gained recognition
in recent years because there is some evidence that it increases blood flow to the brain, possibly
enhancing neurological function.
Ginkgo biloba
Phylum Gnetophyta: Ephedra
Phylum Gnetophyta consists of three diverse genera that share certain characteristics not present
in other gymnosperms. Gnetophytes have the same water-conducting cells (vessel elements)
present in angiosperms, which are more efficient than the water-conducting cells found in other
gymnosperms. Also, some gnetophytes have cones arranged in clusters that give the impression
of flower clusters (there is evidence that flowering plants evolved from a gnetophyte) . Phylum
Gnetophyta includes Ephedra, a plant that produces the stimulant ephedrine.
Ephedra
A. Observe various cones of the genus Pinus.
Contrast the smaller male pollen cones with the larger female cones.
B. Observe specimens of cycads, gingkoes, and gnetophytes.
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Questions:
1. List the characteristics of gymnosperms.
2. List the three components of a seed.
3. Why are seeds reproductively superior to spores?
4. Define:
monoecious dioecious deciduous evergreen –
5. In the space below, list each phylum of gymnosperm, an example and a use of the plant by
humans.
Phylum
Example
Use by humans
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STATION 2: Introduction to Angiosperms
Angiosperms, commonly referred to as the flowering plants, are currently the most abundant,
diverse and widespread of all terrestrial plants. Members of this group develop a flower for
sexual reproduction, producing seeds enclosed within a fruit. Also unique among angiosperms
is the process of double fertilization. Flowering plants are sporophyte dominant, with the
large, diploid sporophyte being independent of the microscopic gametophytes.
Within Phylum Anthophyta, flowering plants are classified into two clades based upon the
number of cotyledons, or embryonic seed leaves, present in the seed. The following table
summarizes the major differences between the clades of flowering plants: monocots and
eudicots.
Kingdom Plantae
Phylum Anthophyta (2 clades: monocots and eudicots)
Characteristic
Embryo
Leaf veins
Roots
Floral parts
Monocots
Eudicots
One cotyledon
Two cotyledons
Parallel
Reticulated (net-like)
Fibrous roots
Taproot (large) present
Multiples of three Multiples of four or five
The sporophyte flower usually consists of four whorls of floral organs borne on the expanded
end of a called the receptacle. The lowest and outermost structures, called sepals, are often
leaflike and serve to protect the flower when it is a bud. Usually sepals are green; however, they
may also be brightly colored and resemble petals. Collectively, the sepals of a flower may be
referred to as the calyx. The petals lie within the sepal whorl and are typically brightly colored
to attract pollinators. The collective term for the petal whorl is the corolla. The stamens, male
reproductive floral parts, lie within the petal whorl. Each stamen consists of a slender filament
with a pollen-producing anther at the tip. The female counterpart is the pistil. The pistils are
the innermost floral structures, each composed of three parts: the stigma, style and ovary. The
stigma is the expanded upper portion that secretes a sticky substance to trap and hold the pollen
grains. The style is the stalk that extends from the stigma to the ovary, the swollen hollow basal
region which will contain the ovules. Ovules will develop into seeds following pollination. The
surrounding ovary tissue forms the fruit which encloses the seeds.
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A. Examine the floral specimens provided. Note the variation in size, shape and location of
floral parts between flowers.
B. Using the flower model available, label all floral parts.
1) Identify all four floral whorls, the sepals, petals, stamens and pistil.
2) Examine the stamen and associated reproductive structures, including the filament,
anther, pollen grains, pollen tube, tube nucleus and sperm nuclei.
3) Examine the pistil and associated reproductive structures, including the stigma, style,
ovary, ovule, integuments, micropyle, embryo sac, synergids, antipodals, polar nuclei and
egg.
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Questions:
1. List the characteristics of angiosperms.
2. What is the function of the petals?
Of the sepals?
3. Name the region of the pistil where pollen grains land.
4. Where are pollen grain produced?
5. Fill in the following chart:
Characteristic
Monocots
Eudicots
Embryo
Leaf veins
Roots
Floral parts
6. List the two parts of a stamen.
7. List the three parts of a pistil.
8. What tissue becomes the fruit? What is the function of this fruit tissue (how does it aid the
plant in completing its life cycle)?
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STATION 3: Angiosperm Reproduction
As is observed in all plants, the life cycle of angiosperms involves the alternation of a
multicellular haploid gametophyte with a multicellular diploid sporophyte. The sporophyte
produces spores by meiosis, each of which develops into a gametophyte. The sporophyte is the
dominant generation in the angiosperms and is represented by any mature plant with flowers.
The gametophyte lies within the sporophyte flower and is reduced to a pollen grain (that will
produce two sperm nuclei) or an embryo sac (that will produce an egg) within an ovule.
Development of the Microgametophyte (Pollen Grain)
The development of the microgametophyte (or pollen grain) occurs within pollen sacs of the
anther. Microspore mother cells (microsporocytes) located in the pollen sacs undergo meiosis
to produce four haploid microspores. Each microspore divides mitotically to produce an
immature pollen grain containing two nuclei, the tube nucleus and the generative nucleus. The
pollen grain is considered mature once the generative nucleus divides to form two sperm cells.
View the prepared slide of the anther.
Compare with the illustration below, noting the pollen sac and pollen grains.
Anther x.s.
Development of the Megagametophyte (Embryo Sac)
Each ovary contains a cavity that contains one to many ovules depending upon the plant species.
Penetrating through the integuments, the outermost layers of the ovule, is a small opening, the
micropyle, through which the pollen tube later grows following pollination.
The development of the megagametophyte (or embryo sac) occurs within the ovule of the
ovary. The megaspore mother cells (megasporocytes) located within each ovule undergo
meiosis to produce four haploid cells. Three of these cells usually disintegrate without further
development, leaving one remaining megaspore to produce the embryo sac (female
gametophyte). After three successive mitotic divisions of the megaspore, a mature embryo sac
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consists of seven cells with eight haploid nuclei (three antipodal cells, two synergid cells, one
egg cell and two polar nuclei).
View the prepared slide of embryo sac.
Compare with the illustration below, noting the ovule, integuments, micropyle, synergids,
antipodals, egg and polar nuclei
Embryo sac within ovule containing 8 nuclei
Pollination and Fertilization
Pollen released from the pollen sacs may be carried by wind, water and animals (insects, bats,
humans and birds) to the stigma of the same or neighboring flower. The transfer of pollen from
the anther to the stigma is pollination. Once pollination has occurred, the pollen grain
germinates with the tube nucleus forming a pollen tube. As the tube develops down the style
and into the ovary, the tube nucleus remains in the tip of the pollen tube to govern its
development. The generative nucleus moves into the pollen tube and divides to form two cells,
the sperm.
View the prepared slide of the pollen grains and developing pollen tubes.
Compare with the illustration below, noting the pollen sac and pollen grains. The two
sperm will be visible within the tube.
Pollination & germination of pollen grains
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The pollen tube enters the ovule through the micropyle. As the tube nucleus dies, the two sperm
cells enter the embryo sac. One sperm unites with the egg, resulting in a diploid zygote. The
other sperm fuses with the polar nuclei in the central cell to form the triploid (3n) endosperm, a
nutritive tissue that supports the growing embryo. This process that results in a diploid zygote
and a triploid endosperm is called double fertilization and is characteristic of all angiosperms.
View the prepared slide the embryo with endosperm.
Compare with the illustration below, identifying the embryo, endosperm & integuments.
Developing embryo with surrounding endosperm tissue
Embryo Development and Seed Formation
After fertilization, the sepals, petals, stamen, stigma, and style dry up and fall off. The zygote
undergoes a series of mitotic divisions forming a multicellular embryo located at the base of the
embryo sac near the micropyle. Simultaneously, the endosperm undergoes a series of divisions
filling the space around the embryo.
The ovule becomes the seed, consisting of a dormant sporophyte embryo and packaged food
supply (stored in either the cotyledons or endosperm). The integuments grow and harden to form
a tough, outer covering of the seed known as the seed coat. The ovary usually enlarges and
becomes the fruit. However, in some plants, the fruit is formed from other plant tissues in
addition to the ovary. For example, the fleshy portion of an apple is composed of both the ovary
and receptacle.
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Questions:
1. Name the two nuclei found in a pollen grain. What does each produce?
2. List the two separate fertilization events that occur in flowering plants.
3. Following fertilization, the ovule develops into the ____________, the integuments form the
____________________ and the ovary matures into the ____________________.
4. What is the function of the micropyle?
5. Which cells undergo meiosis in the pollen sacs?
6. Which cells undergo meiosis in the ovules?
7. Describe two reasons why Angiosperms are more successful as a group than Gymnosperms.
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Label the following on the life cycle of a flowering plant below:
stamen, pistil, ovule, anther, pollen grain, pollen tube, embryo sac, integument, micropyle, seed
coat, seed, new sporophyte plant, meiosis, double fertilization, megaspores, microspore,
endosperm (3n), zygote (2n)
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