27
The Plant Kingdom: Seed Plants
Lecture Outline
I.
II.
The seed plants, gymnosperms and flowering plants, bear seeds
A. After fertilization in seed plants, an ovule (the megasporangium) and its
enclosed structures develop a seed
1. These have integuments—layers of sporophyte tissue that encloses the
ovule
B. Gymnosperms bear exposed seeds on cones in almost all species
C. Flowering plants produce seeds within a fruit and are very diverse
D. Both are vascular plants (having xylem and phloem), with an alternation of
generations
1. The sporophyte is the dominant generation
2. The gametophyte is reduced and lives within the sporophyte plant, on
which it is nutritionally dependant
3. All are heterosporous microspores and megaspores
Gymnosperms are the "naked seed" plants
A. Gymnosperms include some of the largest plants (such as the giant sequoia,
which weighs over 5400 metric tons, 6000 tons); the tallest plants (such as the
coastal redwood, grows to nearly 112 m, 364 feet tall) and the oldest trees
(such as a bristlecone pine), about 4900 years old
1. Gymnosperms are polyphyletic and may be classified into 4 phyla
2. The largest phylum is phylum Pinophyta, comprising about 550 species
3. Phylum Ginkgophyta and phylum Cycadophyta are evolutionary
remnants of once successful groups
4. Phylum Gnetophyta is an odd group that has a questionable affinity with
the rest of the gymnosperms
B. Conifers are woody plants that produce their seeds in cones
1. Conifers are the most familiar gymnosperms, including pines, spruces, and
firs
a) Conifers are woody trees or shrubs that produce 2 tissues—wood and
bark (not herbaceous), do not typically lose their leaves (needles) all at
once, and produce resins
(1) Wood—tracheids have long, slender cells with pits
(2) Needles (leaves) form in clusters, some scale-like; most are
evergreen, but some are deciduous
b) Most conifers are monoecious, bearing both male and female cones on
the same tree
c) Conifers dominate in cooler areas of the northern hemisphere and
some more tropical areas of South America, Australia, and Asia
d)
2.
Conifers are used by humans for wood, turpentine, resins, in
landscaping, as Christmas trees, and in pharmaceutical products
Pines (Pinus) represent a typical conifer life cycle
a) The pine tree is the sporophyte, and produces microspores and
megaspores in male and female cones, respectively
b) Male cones have microsporophylls with microsporangia that contain
microsporocytes (= microspore mother cells)
(1) Microsporocytes undergo meiosis to produce haploid
microspores, which develop into the reduced male gametophyte,
the pollen grain
(2) 4 cells, 2 generative cells and tube cells, are involved in
reproduction
(3) Pollen grains are typically carried by wind currents to the female
cones
c) Female cones (= seed cones) are the typical pine cones; larger than the
male cones and found on the upper branches of the pine tree
(1) Female cones have megasporangia (ovules) that contain
megasporocytes (megaspore mother cells)
(2) Megasporocytes undergo meiosis to produce 4 haploid
megaspores, one of which develops into the female gametophyte
(the others degenerate)
(3) The female gametophyte produces an egg in each of the
archegonia
d) The female cone opens at the time of pollen release, and pollen grains
fall between the scales and stick to a drop of fluid produced by the
cone
(1) As the fluid evaporates, pollen is drawn closer to the
megasporangium, and then the scales close
e) Pollen grains grow to the egg within the archegonium via a pollen
tube that grows into the female gametophyte
(1) The generative cell divides and forms a stalk cell and a body cell,
which divides and forms 2 nonmotile sperm cells
(a) Sperm cells are discharged by the egg
(2) One sperm fuses with the egg to form a zygote; and the
germinated pollen grain with the new pine tube is the mature
male gametophyte
(3) The other sperm degenerates
f) The tissue of the female gametophyte becomes nutritive tissue in the
mature seed
(1) A seed coat surrounds the embryo and nutritive tissue
(2) A papery wing develops to allow dispersal by wind, hence no
need for water dispersal
g)
Pollination occurs in the spring, and pollen tubes grow slowly, so that
fertilization occurs several years later
(1) Seed development takes several months and may remain within
the cone for several years before being shed
h)
The sporophyte is dominant; the gametophyte is a very reduced
microscopic structure found within the cone
(1) The female gametophyte produces archegonia, but the male
gametophyte does not even produce antheridia
(2) There is no necessity for external water to take the pollen grains to
the female cones, and therefore the conifers are reproductively
totally adapted to life on land
C. Cycads (phylum Cycadophyta) have seed cones and compound leaves
1. Cycads were very abundant during the Triassic period of the Mesozoic era,
which is often called the "Age of Cycads"
2. Only about 140 species of cycads remain today
a) Many species are endangered species, as they are over-collected in the
wild to be sold as ornamental plants
3. Cycads are restricted to the tropics and subtropics
4. The leaves of cycads resemble the leaves of ferns or palms
5. The reproduction is similar to that of pines, except that cycads are
dioecious (seed cones are on female plants and pollen cones are on male
plants)
a) Cycads have retained the primitive feature of flagellated sperm, but do
develop a pollen tube, so water is not needed for fertilization; they are
pollinated by air or insects
b) Pollen grain germinates and grows a pollen tube; sperm cells are
released at the top of the tube and then swim to the egg
D. Phylum Ginkgophyta is represented by a single extant species, Ginkgo biloba
1. Ginkgoes are extinct in the wild; all trees today come from cultivated
specimens from China
2. Ginkgoes are commonly planted in cities, as they are somewhat resistant to
air pollution
3. Ginkgo has deciduous leaves, which are pretty in the fall, is dioecious, and
produces flagellated sperms, but does develop a pollen tube
4. Some cities have prohibitions on planting female trees, as the seeds are
very odorous
5. Ginkgos have been used medicinally for centuries
E. Gnetophytes include three unusual genera
1. Phylum Gnetophyta has 70 species within only 3 genera: Gnetum, Ephedra,
and Welwitschia
2. Gnetophytes have cells in their xylem that are the vessel elements, which
resemble those of angiosperms
a) The cone clusters of some gnetophytes resemble flowers
3. Gnetum is a genus of tropical vines and trees with leaves and other
characteristics similar to flowering plants
4.
5.
III.
Ephedra is a shrub found in dry regions that resembles horsetails
a) Ephedrine, which is used to treat asthma, is derived from Ephedra,
implicated in a few deaths due to overdoses when sold as an
unregulated herbal medicine
Welwitschia is a monospecific genus found in southwestern African deserts
with only two leaves, which shed over time
Flowering plants produce flowers, fruits, and seeds
A. Members of phylum Magnoliophyta are the angiosperms or flowering plants
1. These plants are ubiquitous and very speciose—about 235,000 species are
classified in this phylum
2. Flowering plants are characterized by flowers as the organ of sexual
reproduction with development of the seeds within a fruit, which is a
unique double fertilization
3. Angiosperms have vessel elements in their xylem and sieve-tube members
in their phloem, both of which are very efficient
4. Flowering plants are important as all of our major food plants are
angiosperms
5. Economic botany deals with the use of plants, mostly flowering plants, by
humans for lumber, fibers, medicines, and various other commercial
products
B. Monocots and dicots are the two classes of flowering plants
1. Members of class Liliopsida include plants with parallel leaf venation,
floral parts in 3s or multiples of 3, a fibrous root system, scattered vascular
bundles, and one cotyledon in the seed, with persistent endosperm in the
mature seed
a) Monocots are typically herbaceous
b) Examples of monocots are lilies, grasses, corn, palms, and orchids
2. Members of class Magnoliopsida include plants with netted leaf venation,
floral parts in multiples of 4 or 5, a tap root system, vascular bundles
arranged in a ring, and two cotyledons in the seed, but endosperm is
absent in the mature seed
a) Dicots may be herbaceous or woody
b) Examples of dicots are roses, oak trees, potatoes, and daisies
3. Monocots and dicots may be paraphyletic
C. Flowers are involved in sexual reproduction
1. Flowers consist of sepals, petals, stamens, and carpels arranged in whorls
around a stalk (peduncle); (a flower or flowers are referred to as an
inflorescence)
a) Sepals and petals are sterile modified leaves
b) Stamens and carpels are fertile modified leaves
c) A flower with all four parts is complete, a flower missing any of the
four parts is incomplete
d)
2.
A flower with both stamens and carpels is perfect, a flower having
stamens or carpels is imperfect
Sepals function in protecting the other flower parts while it is a bud
a) The sepals are collectively referred to as the calyx
3.
The petals are typically brightly colored to attract animal pollinators
a) The petals are collectively referred to as the corolla
4. The stamens each have a filament (stalk) and an anther at the end where
microspores develop into pollen grains
a) Each pollen grain has two sperm
5. The carpel or carpels are the female reproductive organs, located in the
center of the flower
a) Carpels produce ovules, which ultimately may develop into seeds
b) The female portion of the flower is sometimes referred to as a pistil
(1) The pistil may consist of a single carpel (simple pistil) or several
fused carpels (compound pistil)
c) Each pistil has a stigma at the distal end, where the pollen grain lands,
a style (neck), and an ovary that contains the ovule(s)
(1) Each ovule contains a female gametophyte that produces one egg
and two polar nuclei
(2) After fertilization, the ovule develops into a seed
D. The life cycle of flowering plants includes double fertilization
1. Flowering plants have a dominant sporophyte generation, and the
gametophyte is reduced to just a few cells
2. Ovules contain a megasporocyte that undergoes meiosis to produce 4
haploid megaspores
a) Only one megaspore develops, divides mitotically, and develops into
the female gametophyte (= embryo sac)
b) The female gametophyte consists of only 8 cells, 3 of which will
participate in fertilization
3. The anther contains microsporocytes that undergo meiosis to produce 4
haploid microspores
a) Each of the microspores develops into the male gametophyte, which is
the pollen grain
4. Pollination involves transfer of the pollen grain to the style of the flower
a) If the pollen grain lands on a style of the same species, it grows a
pollen tube down through the stigma to the ovaries
b) Both sperm enter the embryo sac
5. Fertilization involves both sperm, referred to as double fertilization
a) One sperm fertilizes the egg, resulting in a diploid zygote
b) The other sperm fuses with the two haploid polar nuclei, forming a
triploid cell
(1) The triploid cell develops into endosperm, which is nutritive
material for the embryo
(2) Endosperm is persistent in monocot seeds
(3) Endosperm is absorbed into the cotyledons in dicot seeds
c) Double fertilization is also reported in Ephedrya and Genetum species
E.
Seeds and fruits develop after fertilization
1. Fruits protect the seeds from desiccation and aid in dispersal of seeds by
wind, water, or animals
F. Flowering plants have many adaptations that account for their success
1. Flowers attract a variety of animals, which ensure cross fertilization
2. The development of seeds that contain significant amounts of nutritive
material for the embryo are an important development in the flowering
plants
3. Flowering plants have efficient conduction systems and leaves well
adapted for photosynthesis
4. Abscission of leaves during cold or dry periods allowed the radiation of
flowering plants into harsh habitats
5. The adaptability of the sporophyte generation to new or changing habitats
is remarkable
IV.
Studying how flowers evolved provides insight into the evolutionary process
A. The four organs of a flower evolved from leaves
B. Evidence for this comes from the arrangement of vascular tissue in flowers
and leafy stems
C. Sepals, stamens, and carpals evolved from leaves
D. Petals are not the equivalent of leaves
E. Petals developed from stamens that were sterile leaflets
1. Cultivated plants such as roses and camellias bred more stamens into
petals
F. During 130 million years of angiosperm evolution, the flower structure has
diversified
1. Fusion of structures is seen in some flowers
2. In some, more complexity is seen; in others, greater simplicity
G. The Progymnosperms (now all extinct) evolved from seedless vascular plants
1. The progymnosperms had megaphylls (leaves with branching veins) and
woody tissue (secondary xylem), similar to extant plants to modern
gymnosperms, but reproduced by spores
2. Several progymnosperm fossils have structures intermediate between
seedless and seed plants
3. In 1999, Archaeopteris (the earliest known tree) was discovered
H. The progymnosperms probably gave rise to conifers and 150 seed ferns
independently
1. The seed ferns probably gave rise to the cycads and perhaps the ginkgos
2. Fossils from 200 million years ago show that other progymnosperms were
well established 160-100 million years ago
3. The ancestry of the gnetophytes is unclear, but they may be more closely
related to the angiosperms than to the gymnosperms
I.
Flowering plants probably descended from gymnosperms
1. Plant fossils resembling flowering plants have been dated to the middle of
the Jurassic period
2. The first animal pollinators were probably the beetles
3. The angiosperm line is believed to be monophyletic
J. Angiosperms had diversified and begun to replace the gymnosperms as the
dominant plants by 90 million years ago (late Cretaceous); the oldest
angiosperms existed 125-145 million years ago
1. Many species developed by changes in chromosome number, sympatric
speciation
2. Based on the fossil record, dicots evolved first, and monocots originated
from ancient dicots
3. It is thought that monocots are monophyletic and dicots are paraphyletic
Research and Discussion Topics

Describe our dependence on monocots in agriculture. List monocots that are
typically used as human or livestock food.

Linnaeus called the origin of the angiosperms an "abominable mystery." Discuss the
fossil evidence accumulated so far on the origin of this group. Do you think that
angiosperms would have become the dominant land plants if the climate of the
continents remained similar to that of the Mesozoic era?

How has man’s role in the evolution of plants been evident since the end of the Ice
Ages? How has agriculture changed the face of the planet? How have we
unintentionally favored the growth of "weeds"? How and why have we transported
plants across the globe?