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					The Evolution of Seed Plants
30                            The Evolution of Seed Plants

     • The Seed Plants
     • The Gymnosperms: Naked Seeds
     • The Angiosperms: Flowering Plants
30                                           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.
30                                           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.
30                                               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.
30                                              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.
Figure 30.3 Pollen Grains
30                                          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.
30                          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
Figure 30.4 Diversity among the Gymnosperms (Part 1)
Figure 30.4 Diversity among the Gymnosperms (Part 2)
Figure 30.4 Diversity among the Gymnosperms (Part 3)
Figure 30.4 Diversity among the Gymnosperms (Part 4)
30                         The Gymnosperms: Naked Seeds

     • All gymnosperms exhibit secondary growth—their
       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
30                          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.
Figure 30.5 Cones and Strobili (Part 1)
Figure 30.5 Cones and Strobili (Part 2)
30                            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
     • 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.
Figure 30.6 The Life Cycle of a Pine Tree
30                     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 tracheophytes—the
       sporophyte generation becomes larger and more
       independent of the gametophyte, while the
       gametophyte becomes smaller and more
       dependent on the sporophyte.
30                        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.
30                        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
     • This gives rise to the endosperm, triploid tissue that
       nourishes the embryonic sporophyte during its early
30                       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
     • 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.
30                         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
     • 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.
Figure 30.7 A Generalized Flower
30                        The Angiosperms: Flowering Plants

     • Flowers often have several other specialized leaves:
         The inner ones are called petals (collectively, the
         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
     • The sepals, petals, stamens, and carpels are usually
       in circular arrangements called whorls and attached
       to a stalk called the receptacle.
30                   The Angiosperms: Flowering Plants

 • Perfect flowers have both microsporangia and
 • 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.
Figure 30.8 Inflorescences
30                      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.
Figure 30.9 Flower Form and Evolution
30                      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
30                       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
     • 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
     • Most plant–pollinator interactions are not highly
30                       The Angiosperms: Flowering Plants

     • Angiosperms are heterosporous.
     • The ovules are contained within carpels, rather
       than being exposed on the scales as in
     • 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.
Figure 30.11 The Life Cycle of an Angiosperm
30                         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.
Figure 30.12 Fruits Come in Many Forms and Flavors (Part 1)
Figure 30.12 Fruits Come in Many Forms and Flavors (Part 2)
30                         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.
Figure 30.13 Evolutionary Relationships among the Angiosperms
Figure 30.14 Monocots and Eudicots Are Not the Only Surviving Angiosperms (Part 1)
Figure 30.14 Monocots and Eudicots Are Not the Only Surviving Angiosperms (Part 2)
Figure 30.15 Monocots (Part 1)
Figure 30.15 Monocots (Part 2)
Figure 30.16 Eudicots (Part 1)
Figure 30.16 Eudicots (Part 2)
Figure 30.16 Eudicots (Part 3)
30                      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
          Which fossils should be chosen for
          What is the likelihood that evidence of double
           fertilization can be found in ancient fossils?

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