The Evolution of Seed Plants

1
The Evolution of Seed Plants
2
The Evolution of Seed Plants

• The Seed Plants
• The Gymnosperms Naked Seeds
• The Angiosperms Flowering Plants

3
The Seed Plants

• The seed plants are the most recent group to
  appear in the evolution of the tracheophytes.
• The seed plants include the gymnosperms (such as
  pines and cycads) and the angiosperms (flowering
  plants).
• There are four living phyla of gymnosperms and
  one phyla of angiosperms.

4
The Seed Plants

• In seed plants, the gametophyte generation is
  reduced, with the haploid gametophyte being
  attached to and nutritionally dependent on the
  diploid sporophyte.

5
The Seed Plants

• The seed plants are heterosporous.
• Separate megasporangia and microsporangia are
  formed on structures that are grouped on short
  axes.
• The megaspores are not shed, but develop into
  female gametophytes within the megasporangia.
• Only one of the meiotic products in the
  megasporangium survives.
• This surviving haploid nucleus produces a
  multicellular female gametophyte that is retained
  within the megasporangium, where it matures and
  is eventually fertilized.

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The Seed Plants

• Male gametophytes are called pollen grains and
  are formed by the division of microspores
  produced meiotically within the microsporangia.
• Pollen grains produce a slender pollen tube that
  elongates and digests its way through the
  sporophyte tissue toward the female gametophyte.
• When the pollen tube reaches the female
  gametophyte, two sperm are released and
  fertilization occurs.
• The resulting diploid zygote divides until an
  embryonic stage is reached, then growth is
  temporarily halted. The end product of this stage
  is a seed.

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Figure 30.3 Pollen Grains
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The Seed Plants

• A seed may contain tissues from three
  generations.
• The seed coat and megasporangium develop from
  tissues of the diploid sporophyte parent.
• Within the megasporangium is the haploid female
  gametophyte tissue of the next generation.
• The center of the seed package contains the third
  generation, in the form of the embryo of the new
  diploid sporophyte.
• The possession of seeds is a major reason for the
  enormous evolutionary success of seed plants.

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The Gymnosperms Naked Seeds

• The gymnosperms do not produce flowers.
  Gymnosperm means naked-seeded, meaning their
  ovules and seeds are not protected by flower or
  fruit tissue.
• There are four clades of living gymnosperms
  today.
• Phylum Cycadophyta, the cycads
• Phylum Ginkgophyta has a single species, Ginkgo
  biloba.
• Phylum Gnetophyta
• Phylum Pinophyta, the conifers

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Figure 30.4 Diversity among the Gymnosperms
(Part 1)
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Figure 30.4 Diversity among the Gymnosperms
(Part 2)
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Figure 30.4 Diversity among the Gymnosperms
(Part 3)
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Figure 30.4 Diversity among the Gymnosperms
(Part 4)
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The Gymnosperms Naked Seeds

• All gymnosperms exhibit secondary growththeir
  stems and roots grow larger in diameter.
• All gymnosperms but the Gnetophyta have only
  tracheids as water-conducting and support cells
  in their xylem.
• Wood is secondary xylem produced by gymnosperms.

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The Gymnosperms Naked Seeds

• Fir, cedar, spruce, and pine all belong to the
  phylum Pinophyta, the conifers or cone-bearers.
• A cone is a modified stem bearing a tight cluster
  of scales specialized for reproduction.
• A strobilus is a conelike cluster of scales that
  are modified leaves inserted on an axis.
• Megaspores are produced in seed cones, and
  microspores are produced in the much smaller
  pollen strobili.

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Figure 30.5 Cones and Strobili (Part 1)
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Figure 30.5 Cones and Strobili (Part 2)
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The Gymnosperms Naked Seeds

• The ovule consists of the integument, the
  megasporangium inside it, and the tissue
  attaching it to the maternal sporophyte.
• Pollen grains enter through a small opening in
  the integument at the tip of the ovule called the
  micropyle.
• The ovules are borne on the upper surfaces of the
  cone scales.
• About half of the conifer species have fruitlike
  tissues associated with their seeds that may be
  eaten by animals, which in turn disperse the
  seeds in their feces, but they are not true
  fruits.

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Figure 30.6 The Life Cycle of a Pine Tree
20
The Angiosperms Flowering Plants

• The phylum Angiospermae consists of over 257,000
  species of flowering plants.
• Angiosperm means enclosed seed.
• The angiosperms represent the extreme of an
  evolutionary trend in the tracheophytesthe
  sporophyte generation becomes larger and more
  independent of the gametophyte, while the
  gametophyte becomes smaller and more dependent on
  the sporophyte.

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The Angiosperms Flowering Plants

• A number of synapomorphies, or shared derived
  traits, characterize the angiosperms
• They have double fertilization.
• They produce triploid endosperm.
• Their ovules and seeds are enclosed in a carpel.
• They have flowers.
• They produce fruit.
• Their xylem contains vessel elements and fibers.
• Their phloem contains companion cells.

22
The Angiosperms Flowering Plants

• In double fertilization, two male gametes
  participate in fertilization events within the
  megagametophyte.
• One sperm combines with the egg to produce a
  diploid zygote.
• The other sperm combines with two other haploid
  nuclei of the female gametophyte to form a
  triploid nucleus.
• This gives rise to the endosperm, triploid tissue
  that nourishes the embryonic sporophyte during
  its early development.

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The Angiosperms Flowering Plants

• The carpel is a modified leaf that encloses the
  ovules and seeds of the angiosperms.
• Vessel elements are specialized
  water-transporting cells present within the xylem
  of angiosperms.
• Fiber is another distinct cell type found in the
  xylem of angiosperms. It plays an important role
  in supporting the plant body.
• Companion cells are a unique type of cell found
  in angiosperm phloem.

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The Angiosperms Flowering Plants

• All the parts of a flower are modified leaves.
• Stamens are composed of a filament that bears an
  anther containing the pollen-producing
  microsporangia.
• The pistil is composed of one carpel or two or
  more fused carpels. It has a swollen base called
  the ovary that contains the megasporangia.
• The style is the apical stalk of the pistil, and
  the terminal surface that receives pollen grains
  is called the stigma.

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Figure 30.7 A Generalized Flower
26
The Angiosperms Flowering Plants

• Flowers often have several other specialized
  leaves
• The inner ones are called petals (collectively,
  the corolla).
• The outer ones are called sepals (collectively,
  the calyx).
• These leaves often play roles in attracting
  animal pollinators to the flower.
• The calyx commonly protects the immature flower
  in bud.
• The sepals, petals, stamens, and carpels are
  usually in circular arrangements called whorls
  and attached to a stalk called the receptacle.

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The Angiosperms Flowering Plants

• Perfect flowers have both microsporangia and
  megasporangia.
• Imperfect flowers have either functional
  megasporangia or microsporangia, but not both.
• Monoecious species produce both types of
  imperfect flowers on the same plant.
• In dioecious species, a plant produces either
  megasporangiate or microsporangiate flowers but
  not both.

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Figure 30.8 Inflorescences
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The Angiosperms Flowering Plants

• The flowers of the most evolutionarily ancient
  angiosperms have a large and variable number of
  tepals, carpels, and stamens.
• Evolution within the angiosperms has resulted in
  many modifications of this early condition
  reduction in number of floral organs,
  differentiation of petals and sepals, changes in
  symmetry, and fusion of parts.

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Figure 30.9 Flower Form and Evolution
31
The Angiosperms Flowering Plants

• Carpels and stamens are thought to have evolved
  from leaflike structures.
• The carpels later fused and became more buried in
  the receptacle tissue.
• Natural selection has favored pistils with long
  styles and stamens with long filaments, probably
  to increase the likelihood of successful
  fertilization.

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The Angiosperms Flowering Plants

• Most angiosperms are animal-pollinated. Animals
  visit flowers to obtain nectar or pollen, and in
  the process often carry pollen from one plant to
  another.
• The animals have affected the evolution of
  plants, and plants have affected the evolution of
  animals.
• This coevolution has resulted in some highly
  specific interactions, with certain plant species
  being pollinated by only one or very few animal
  species.
• Most plantpollinator interactions are not highly
  specific.

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The Angiosperms Flowering Plants

• Angiosperms are heterosporous.
• The ovules are contained within carpels, rather
  than being exposed on the scales as in
  gymnosperms.
• The ovule develops into a seed containing the
  products of double fertilization, a diploid
  zygote and a triploid endosperm.
• The embryo consists of an embryonic axis and one
  or two cotyledons, or seed leaves. The cotyledons
  may digest the endosperm, and later expand and
  become photosynthetic.

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Figure 30.11 The Life Cycle of an Angiosperm
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The Angiosperms Flowering Plants

• The ovary of a flowering plant develops into a
  fruit after fertilization.
• A fruit may consist only of the mature ovary and
  its seeds, or it may include other parts of the
  flower or structures associated with it.
• Simple fruits develop from a single carpel or
  several united carpels.
• Aggregate fruits develop from several separate
  carpels of a single flower.
• Multiple fruits are formed from a cluster of
  flowers.
• Accessory fruits are derived from parts in
  addition to the carpel and seeds.

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Figure 30.12 Fruits Come in Many Forms and
Flavors (Part 1)
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Figure 30.12 Fruits Come in Many Forms and
Flavors (Part 2)
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The Angiosperms Flowering Plants

• Angiosperm clades
• The monocots have a single embryonic cotyledon
  and include the grasses, cattails, lilies,
  orchids, and palms.
• The eudicots have two cotyledons, and include the
  majority of familiar seed plants, including most
  herbs, vine trees, and shrubs.
• Clades other than the eudicots and monocots
  include the the water lilies, star anise, and the
  magnoliid complex.

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Figure 30.13 Evolutionary Relationships among
the Angiosperms
40
Figure 30.14 Monocots and Eudicots Are Not the
Only Surviving Angiosperms (Part 1)
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Figure 30.14 Monocots and Eudicots Are Not the
Only Surviving Angiosperms (Part 2)
42
Figure 30.15 Monocots (Part 1)
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Figure 30.15 Monocots (Part 2)
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Figure 30.16 Eudicots (Part 1)
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Figure 30.16 Eudicots (Part 2)
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Figure 30.16 Eudicots (Part 3)
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The Angiosperms Flowering Plants

• The question of how the angiosperms first arose
  is still unanswered. Several questions complicate
  efforts to answer this question.
• What morphological characters should be selected
  as important?
• What algorithms should be applied to computerized
  analysis of data?
• Are all molecular differences and similarities
  significant?
• Which fossils should be chosen for comparisons?
• What is the likelihood that evidence of double
  fertilization can be found in ancient fossils?