Evolution and Diversity in Plants II Ecol 182 4122005

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Evolution and Diversity in Plants II - Ecol 182
4-12-2005
Downloaded at XXX pm on 4-11
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Figure 29.4 From Green Algae to Plants
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The Seed Plants

• Seed plants are the most derived tracheophytes.
• Gymnosperms (such as pines and cycads) four
  phyla
• Angiosperms (flowering plants) one phyla
• Big evolutionary innovations
• Evolution of a seed
• Reduction in gametophyte generation
• The haploid gametophyte is attached to and
  nutritionally dependent on the diploid sporophyte.

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Figure 30.2 The Relationship between Sporophyte
and Gametophyte Has Evolved (Part 1)
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The Seed Plants

• The seed plants are heterosporous
• Separate megasporangia and microsporangia
• Megaspores produce a single, haploid,
  multicellular female gametophyte in megasporangia
• Microspores meiotically divide to produce pollen
  grains in microsporangia
• Fertilization occurs through pollen tube
  elongation to the female gametophyte (which
  release two sperm)
• Resulting zygote divides until an embryonic stage
  is reached, when growth is halted (producing a
  seed).

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The Seed Plants

• A seed may contain tissues from three
  generations.
• Seed coat and megasporangium develop from the
  diploid sporophyte parent.
• In the megasporangium, the haploid female
  gametophyte tissue is of the next generation.
• The center of the seed contains a third
  generation, 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, and 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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The Gymnosperms Naked Seeds

• Fir, cedar, spruce, and pine all belong to
  Pinophyta
• Megaspores are produced in cones (modified stem
  bearing a tight cluster of scales specialized for
  reproduction)
• Microspores are produced in pollen strobili (a
  conelike cluster of scales that are modified
  leaves)

• About ½ of conifers have fruit-like tissues
  surrounding seeds that are eaten by animals
  resulting in dispersal in their feces (but they
  are not true fruits)

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Figure 30.6 The Life Cycle of a Pine Tree
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The Gymnosperms Naked Seeds

• Gymnosperms exhibit secondary growth
• Recall types of types of growth (animals versus
  plants)
• Determinate - body ceases to grow once adulthood
  is reached.
• Indeterminate body growth is potentially
  continuous
• Meristematic regions are localized regions of
  cell division.
• They produce new cells indefinitely
• When meristem cells divide, one daughter cell
  develops into another meristem cell the other
  develops specialization.
• Two meristem types
• Apical meristems give rise to the primary plant
  body.
• Lateral meristems give rise to the secondary
  plant body.
• Lateral meristems give rise to tissues
  responsible for stems and roots thickening to
  form wood.

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Figure 35.13 Apical and Lateral Meristems
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Figure 35.15 Tissues and Regions of the Root Tip
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Forming the Plant Body

• Secondary tissues derive from two lateral
  meristems vascular and cork cambium.
• Vascular cambium - a cylindrical tissue that form
  the secondary xylem, and the secondary phloem
• Cork cambium - produces the outermost layers of
  stems protecting tissues from H2O loss
  microorganisms.
• Growth in the diameter of the stems and roots,
  produced by these meristematic regions is called
  secondary growth.
• Wood is secondary xylem.
• Bark is everything produced external to the
  vascular cambium (including secondary phloem).

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Figure 35.14 A Woody Twig
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Gymnosperms Naked Seeds

• Gymnosperms (except Gnetophyta) have only
  tracheids, and simple phloem.
• Tracheids are simple xylem that conduct water
  throughout the plant body
• Tracheids undergo apoptosis and operate as empty
  cells (cell walls remain).
• Phloem are alive, and transport carbohydrates and
  other materials throughout the plant

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The Angiosperms Flowering Plants

• Phylum Angiospermae 257,000 species.
• Angiosperm means enclosed seed.
• The angiosperms are the most derived form of the
  tracheophytes
• the sporophyte generation is larger and has
  greater independence from the gametophyte
• the gametophyte is 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 (upcoming figure).
• They produce triploid endosperm.
• Their ovules and seeds are enclosed in a carpel
  (modified leaf).
• They have flowers (modified leaves).
• They produce fruit (at minimum mature ovary and
  seed).
• Their xylem contains vessel elements (specialized
  H2O transport) and fibers (structural integrity).
• Their phloem contains companion cells (assists
  with metabolic issues associated with transport).

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The Angiosperms Flowering Plants

• 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
• Results in endosperm - tissue that nourishes the
  embryonic sporophyte

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

• All the parts of a flower are modified leaves.
• Stamens - filament bearing anthers containing
  pollen-producing microsporangia.
• Pistil one or more carpels with a swollen base
  (ovary) containing megasporangia.
• Style is the apical stalk of the pistil (terminal
  surface receiving pollen is called the stigma)

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The Angiosperms Flowering Plants

• Specialized leaves (petals and sepals) are
  important for attracting pollinators
• Many angiosperms are animal-pollinated increasing
  the likelihood of outcrossing (in exchange for
  nectar or pollen)
• Coevolution has resulted in some highly specific
  interactions, but most plant-pollinator systems
  are not highly specific
• Evolutionarily ancient angiosperms have a large
  and variable number of floral structures (petals,
  sepals, carpels, and stamens)
• Evolutionary trend within the group reduction in
  number of floral organs, differentiation of
  petals and sepals, changes in symmetry, and
  fusion of parts.

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Figure 30.8 Inflorescences
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The Angiosperms Flowering Plants

• Perfect flowers have both microsporangia and
  megasporangia.
• Imperfect flowers (have either, 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.
• Developing embryos consists of an embryonic axis
  and one or two cotyledons (seed leaves), which
  metabolize endosperm and may become
  photosynthetic.

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Figure 35.1 Monocots versus Eudicots
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Figure 35.2 Vegetative Organs and Systems
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Organs of the Angiosperms

• Two main types of root system taproot and
  fibrous root.
• Many eudicots have a taproot system a single,
  large, deep-growing primary root with smaller
  lateral roots.
• Monocots and some eudicots have a fibrous root
  system composed of numerous thin roots roughly
  equal in diameter.
• A fibrous root system holds soil in place very
  effectively.
• Some plants have adventitious roots, which arise
  from points along the stem where roots would not
  usually occur.

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Figure 35.3 Root Systems
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Angiosperm vascular systems

• Xylem in angiosperms consists of vessel elements
  in addition to tracheids
• Vessel elements also conduct water and are formed
  from dead cells.
• Vessel elements are generally larger in diameter
  than tracheids and are laid down end-to-end to
  form hollow tubes.
• Sieve tube elements (Phloem) in Angiosperms are
  stacked, similar to xylem
• Have adjacent companion cells that retain all
  organelles
• Companion cells may regulate the performance of
  the sieve tube members through their effects on
  active transport of solutes

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Figure 35.9 Plant Cell Types (Part 3)
Why is a greater diameter a big deal for the
evolution of plants?
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Figure 35.10 Evolution of the Conducting Cells
of Vascular Systems
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Figure 35.11 Sieve Tubes
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Angiosperms Flowering Plants

• Monocots - a single embryonic cotyledon (grasses,
  cattails, lilies, orchids, and palms)
• Eudicots - two cotyledons, and include the
  majority of familiar seed plants
• Additional clades - water lilies, star anise, and
  the magnoliid complex
• Big question in plant evolution what is the
  basal angiosperm?

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Plant Structure and Function I - Ecol 182
4-12-2005
Downloaded at XXX pm on 4-11
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• Uptake and Movement of Water and Solutes
• Transport of Water and Minerals in the Xylem
• Transpiration and the Stomata
• Translocation of Substances in the Phloem

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General problem in plant function

• Need for H2O for
• photosynthesis,
• Solute transport,
• temperature control,
• internal pressure for growth
• Plants obtain water and minerals from the soil
  via the roots
• in turn roots extract carbohydrates and other
  important materials from the leaves.
• Water enters the plant through osmosis
• but the uptake of minerals requires transport
  proteins.

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Uptake Movement of Water Solutes in Plants

• Osmosis is the diffusion of water through a
  membrane primary means of water transport in
  plants
• Osmotic potential, or solute potential,
  determines the direction of water movement across
  a membrane.
• Potential refers to the potential energy
  contained in the system measured
• Dissolved solutes have the effect of lowering the
  concentration of water (changing the potential
  energy).
• Greater solute concentration results in a more
  negative solute potential and a greater the
  tendency of water to diffuse to the solution.

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Uptake Movement of Water Solutes in Plants

• Water potential is the tendency of a solution to
  take up water from pure water (Y).
• Water potential of a system is the sum of the
  negative solute potential (ys) and the (usually
  positive) pressure potential (yp).
• y ys yp
• Solute potential, pressure potential, and water
  potential are measured in megapascals (Mpa).

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Figure 5.8 Osmosis Modifies the Shapes of Cells
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Figure 36.2 Water Potential, Solute Potential,
and Pressure Potential
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Figure 36.4 Apoplast and Symplast
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Figure 36.5 Casparian Strips
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Transport of Water and Minerals in the Xylem

• The adhesion-cohesiontension theory of water
  movement
• The concentration of water vapor is higher inside
  the leaf than outside, so water diffuses out of
  the leaf through the stomata (this is
  transpiration).
• This creates a tension in the mesophyll that
  draws water from the xylem of the nearest vein
  into the apoplast surrounding the mesophyll cells
• The removal of water from the veins, in turn,
  establishes tension on the entire volume of water
  in the xylem, so the column is drawn up from the
  roots.

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Figure 36.8 The TranspirationCohesionTension
Mechanism
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Transport of Water and Minerals in the Xylem

• Hydrogen bonding results in cohesion (sticking
  of molecules to one another).
• The narrower the tube, the greater the tension
  the water column can stand.
• Maintenance of the water column also occurs
  through adhesion of water molecules to the walls
  of the tube.

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Transport of Water and Minerals in the Xylem

• The key elements in water transport in xylem
• Transpiration
• Tension
• Cohesion
• The transpirationcohesiontension mechanism does
  not require energy.
• At each step, water moves passively toward a
  region with a more negative water potential.

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Transport of Water and Minerals in the Xylem

• Mineral ions in the xylem sap rise passively with
  the solution.
• Transpiration also contributes to the plants
  temperature regulation, cooling plants in hot
  environments.