Adapting to Terrestrial Living

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Adapting to Terrestrial Living

• 288,700 species of plants are now in existence
• These are terrestrial
• However, green algae, the likely ancestors of
  plants, are aquatic and not well adapted to
  living on land
• Three challenges had to be overcome
• 1. Mineral absorption
• 2. Water conservation
• 3. Reproduction on land

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Mineral Absorption

• Plants require relatively large amounts of six
  inorganic minerals
• Nitrogen, potassium, calcium, phosphorus,
  magnesium, sulfur
• Plants absorb these materials through their roots
• The first plants developed symbiotic associations
  with fungi
• These mycorrhizae enabled plants to extract
  minerals from rocky soil

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Water Conservation

• To avoid drying out, plants have a watertight
  outer covering, termed the cuticle

• Stomata (singular, stoma) are pores in the
  cuticle that allow gas and vapor exchange

Regulate opening and closing of the stoma
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Reproduction on Land

• Spores developed as a means to protect gametes
  from drying out on land
• In a plant life cycle, there is alternation of
  generations
• Diploid with haploid

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• The diploid generation is called the sporophyte
• The haploid generation is called the gametophyte
• As plants evolved, the sporophyte tissue dominated

Two types of gametophytes
Vascular plant
Primitive plant
Gametophyte barely visible
Mostly gametophyte
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Plant Evolution

• Four key evolutionary innovations serve to trace
  the evolution of the plant kingdom

• 1. Alternation of generations
• Plants developed a more dominant diploid phase of
  the life cycle
• 2. Vascular tissue
• Transports water and nutrients throughout the
  plant body
• Thus plants were able to grow larger and in drier
  conditions

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Plant Evolution

• 3. Seeds
• Protected the embryo, thus allowing plants to
  dominate their terrestrial environments
• 4. Flowers and fruits
• Flowers protected the egg and improved the odds
  of its fertilization
• Fruits surrounded the seeds and aided in their
  dispersal

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Nonvascular Plants

• Only two phyla of living plants lack a vascular
  system

The simplest of all living plants

• Liverworts (Phylum Hepaticophyta)
• Hornworts (Phylum Anthocerophyta)

• Mosses (Phylum Bryophyta) were the first plants
  to evolve strands of specialized conduction cells
• The conducting cells do not have specialized wall
  thickenings
• Thus, a primitive vascular system, at the most

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The Evolution of Vascular Tissue

• The first vascular plant appeared approximately
  430 million years ago (mya)

• Early plants became successful colonizers of land
  through the development of vascular tissue

• Efficient water- and food-conducting systems

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• Early vascular plants exhibited primary growth
• Growth by cell division at the tips of the stem
  and roots
• About 380 mya, vascular plants developed a new
  pattern of growth, secondary growth
• New cells are produced in regions around the
  plants periphery
• Thus, plants could become thick-trunked and
  taller
• Note
• The product of plant secondary growth is wood

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Seedless Vascular Plants

• Two phyla of modern-day vascular plants lack
  seeds
• Ferns (Phylum Pterophyta)
• Club mosses (Phylum Lycophyta)
• Both have free-swimming sperm that require free
  water for fertilization
• By far, the largest group are ferns
• 12,000 living species

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The Life of a Fern

• Ferns have both gametophyte and sporophyte
  individuals, each independent and self-sufficient
• Gametophyte
• Produces eggs and sperm
• These unite to form the zygote, which develops
  into the sporophyte
• Sporophyte
• Bears and releases haploid spores
• These germinate to form gametophytes

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Frond Vertical leaves
Rhizome Horizontal stem
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Evolution of Seed Plants

• Seeds are embryo covers that protect the
  embryonic plant at its most vulnerable stage
• Seed plants produce two kinds of gametophytes

• Male gametophytes
• Called pollen grains
• Arise from microspores

• Female gametophytes
• Contains the egg
• Develops from a megaspore produced within an ovule

• Pollination is the transfer of pollen by insects,
  winds
• Thus, there is no need for free water for
  fertilization

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• All seed plants are derived from a single common
  ancestor
• There are five living phyla

• Four are gymnosperms
• Ovules not completely enclosed by sporophyte at
  time of pollination

• Fifth is angiosperms
• Ovules completely enclosed by a vessel of
  sporophyte tissue, the carpel, at time of
  pollination

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• A seed has three parts
• 1. A sporophyte plant embryo
• 2. A source of food for the embryo called
  endosperm
• 3. A drought-resistant protective cover

Used for food storage
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• Seeds have greatly improved the adaptations of
  plants to living on land
• 1. Dispersal
• Facilitate migration dispersal
• 2. Dormancy
• Postpone development until conditions are
  favorable
• 3. Germination
• Permit embryonic development to be synchronized
  with habitat
• 4. Nourishment
• Offer an energy source of young plants

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Gymnosperms

• Gymnosperms are nonflowering seed plants
• They include four phyla
• Conifers (Coniferophyta)
• Cycads (Cycadophyta)
• Gnetophytes (Gnetophyta)
• Ginkgo (Ginkgophyta)

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• Conifers
• The most common (and familiar) of the gymnosperms

• Include
• Pine, spruce, cedar, redwood and fir trees

• Conifers are trees that produce their seeds in
  cones
• Seeds (ovules) develop on scales within cones and
  are exposed at the time of pollination

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• Cycads
• Have short stems and palmlike leaves

• The predominant land plant in the Jurassic Period

Acts like a plant standing on its head!

• Gnetophytes
• The most closely related to angiosperms
• Only three types of plants all unusual

• Have flagellated sperm

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• Ginkgo
• Only one living species exists
• The maidenhair tree, Ginkgo biloba

• Resistant to air pollution
• Reproductive structures found on different trees
• Have flagellated sperm

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The Life of a Gymnosperm

• Conifer trees form two kinds of cones

• Large seed cones contain the female gametophyte
• Small pollen cones contain pollen grains

• Pollen grains are carried by wind to the seed
  cones
• Fertilization yields a zygote
• The zygote matures into a seed
• Seeds are dispersed into new habitats

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Dominant form of the life cycle
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Rise of the Angiosperms

• Angiosperms comprise 90 of all living plants
• gt 300,000 species
• Virtually all our food is derived, directly or
  indirectly from them
• In gymnosperm reproduction, pollen grains are
  carried passively by the wind
• Angiosperms have evolved a more direct way of
  transferring pollen
• Induce animals to carry it for them
• How?
• Flowers!

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Rise of the Angiosperms

• Flowers are the reproductive organs of
  angiosperms
• A flower employs bright colors to attract insects
  and nectar, to induce the insects to enter the
  flower
• There they are coated with pollen grains, which
  they carry with them to other flowers

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• A flower consists of four concentric circles, or
  whorls, connected to a base called the recepatcle
• Outermost whorl (Sepals)
• Protects flower from physical damage
• Second whorl (Petals)
• Attracts pollinators
• Third whorl (Stamens)
• Produces pollen grains in the anther
• Innermost whorl (Carpel)
• Produces eggs in the ovary
• Rising from the ovary, is a slender stalk, the
  style, with a sticky tip, the stigma

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Petals
Fused carpel
Stamens
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Why Are ThereDifferent Kinds of Flowers?

• Different insect pollinators are attracted to
  specific types of flowers
• The most numerous insect pollinators are bees
• Bees are first attracted by the odor of nectar
• They then focus on the flowers color and shape
• Bee-pollinated flowers are usually yellow or blue

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How a bee sees a flower
Bee covered in pollen
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• Other pollinators include
• Moths
• Attracted to scented, white or pale-colored
  flowers

• Flies
• Attracted to foul-smelling brown flowers

Pollen on beak

• Hummingbirds
• Attracted to red flowers
• These are not typically visited by insect
  pollinators

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Improving SeedsDouble Fertilization

• Within their seeds, angiosperms produce a
  special, highly nutritious tissue called the
  endosperm
• The male gametophyte contains two sperm

• One fertilizes the egg to form the diploid (2n)
  zygote
• The other fuses with two polar nuclei to form the
  triploid (3n) endosperm

• This process is called double fertilization
• It is only found in angiosperms and gnetophytes

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• Angiosperms are divided into two groups

• Dicotyledons or dicots

• Embryos have two cotyledons
• Evolved earlier

• Monocotyledons or monocots

• Embryos have a single cotyledon
• Evolved later

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• Angiosperms are divided into two groups

• Dicotyledons or dicots

• Have leaves with netlike veins
• Have flower parts in fours and fives

• Monocotyledons or monocots

• Have leaves with parallel veins
• Have flower parts in threes

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Improving Seed DispersalFruits

• A fruit is a mature ripened ovary containing
  fertilized seeds
• Fruits aid in the dispersal of seeds to new
  habitats
• By animals
• By water
• By wind

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By animals
By water
By wind