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					Plant Evolution

   Chapter 21
What are the characteristics of plants?
• All plants are:
  – Eukaryotic
  – Multicellular with specialized tissues
  – Autotrophic
  – Cell wall made of cellulose
    Where did land plants originate?
• The land plants share a common ancestor with
  Chlorophyta, green algae
• Chlorophyta uses the same chlorophyll as land
• They store starch the same way as land plants
  • Other commonalities are similar genes in ribosomal RNA
  • Cell division that includes forming a cell plate
  • The same enzymes stored in cellular vesicles
            Why move to land?
• Land is an optimal place for plants, if they can
• Sunlight for photosynthesis can be accessed
  directly on land, it is not filtered through
• In order for plants to make the transition to
  land they had to solve a few problems that
  living on land introduced
• They solved these problems one at a time
    What problems did land plants face?
• Plants had to be able to conserve water
  – When plants live in water there is no need for water
• Plants had to be able to transport water and nutrients
  – Nutrients are diffused directly into the cells when the plants
    live in water
• Plants had to be able to anchor themselves
  – There was no need to anchor themselves in water
• Plants had to be able to reproduce sexually without
  • Sperm can swim to the egg when they live in water
• Plants needed to be able to protect their embryos
   The first problem, how do land plants
               conserve water?
• Water diffuses out of plant cells unless there is a
  water-proof covering protecting it
• Plants developed a water-proof cuticle, a waxy or
  lipid layer, on the outside of the plants, especially
• The cuticle introduced an additional problem
• If water cannot diffuse into the plants, gases cannot
• Plants developed stomata on the underside of
  leaves to allow gas exchange for photosynthesis
           How do stomata work?
• Stomata are openings in the underside of leaves to
  allow CO2 in and O2 out during photosynthesis
  – If gases can move through the stoma, so can water vapor
• The stomata are surrounded by guard cells that
  swell and shrink depending on the water content of
  the air surrounding the plants
  – If the water content is high, the guard cells swell opening
    the stomata
  – High water vapor means there is little diffusion of water
    out of the leaf
  – If the air is dry, the guard cells shrink conserving the
    water content inside the leaf
 What characteristics did the first land
 plants have? (seedless nonvascular)
• The first land plants had no transportation
  (vascular) system for nutrient distribution
• They had some ability to conserve water
• They still depended on water for the sperm to
  swim to an egg
• What did they look like and where did they
         Seedless, nonvascular plants
         Phylum Bryophyta (mosses)
• These were the first plants capable of surviving on
• They have no system for nutrient and water
  transport, they depend on diffusion (nonvacsular)
  – They are only a few centimeters in height
• The sperm has to swim to the egg (seedless)
  – They live close to one another so that the sperm can
    swim to the egg with a little water
• They have limited ability to conserve water
  – They live in moist areas
Is sexual reproduction the only method
       used by plants? (page 662)
• Vegetative reproduction is asexual reproduction and
  particularly important in simple plants
• Vegetative reproduction is the ability of a new plant
  to grow from parts of an existing plant
  – Roots, stems, and leaves can be used to grow new plants
  – Potato plants can be grown from pieces of potato
  – A pineapple plant can be grown from the top of a
• The resulting plant is a clone of the parent plant
       Seedless vascular plants (ferns)
• The next innovation was the development of a vascular
  system (which includes roots, stems and leaves)
   – Xylem is the vascular tissue that carries water and dissolved
     minerals from the root to the rest of the plant
   – Phloem is the vascular tissue that carries the products of
     photosynthesis from the leaf to the rest of the plant
• The first vascular systems were simple and most did not
  include a complete root system
• Vascular tissue allowed the plants to get bigger because they
  no longer had to depend solely on diffusion for water and
  nutrient delivery – water can travel farther
   – The size was still somewhat limited because of lack of, or a simple
     root system
• They still had to depend on the sperm swimming to the egg
   – They live in areas where there is sufficient moisture for the
     swimming sperm
   Vascular plants with seeds, cones
• Plants can now get taller because of a more
  complex root system to deliver water and nutrients
  – Roots also serve as an anchor for a taller plant
• These plants also did not require water for
• The pollen cone produces pollen which is the male
• The seed cones produce eggs (female gametophyte)
• The pollen gets distributed by the wind or by animals
• The seed cone is sticky and the pollen sticks to it
• The pollen fertilizes the egg inside the seed cone
  – The fertilized egg develops into a seed
  – Inside the seed is the embryonic plant
  – Seeds protect the embryonic plant until conditions are good
    enough for the young plant to survive
     • Allowed young plants to survive through the cold and/or dry seasons
• The seed is carried by the wind (helicopters) and
  animals (squirrels) to areas away from the parent plant
        Vascular Plants with seeds,
• Flowering plants are the most numerous and
  successful of all plants
• They have flowers to attract insects for cross
  pollination (produces variation)
• The ripened ovaries of the flowers produce fruit
• Fruit is an animals reward for seed distribution
          What are the plant divisions?
• Seedless nonvascular plants
   – Bryophyta (mosses)
   – Anthocerophyta (hornworts )
   – Hepaticophyta (liverworts)
• Seedless vascular plants
   – Lycophyta (club mosses)
   – Pterophyta (ferns)
• Seed producing, vascular plants
   –   Cycadophyta (cycads)
   –   Gnetophyta (Ephedra)
   –   Gingkophyta (Gingko)
   –   Coniferophyta (conifers, pines, firs, cypress, redwood)
   –   Anthophyta (all flowering plants)
            Other plant adaptations
• Leaves – The surface area of leaves allow plants to
  collect energy from the sun
• The large surface area also can be responsible for
  the loss of water
• Plants that live in humid areas have more surface
  area (tropical plants)
  – Grow quickly
• Plants that live in dry environments have less
  surface area to their leaves (cactus)
  – Grow slowly
  – Cactus have spines for leaves and stems that swell and
    store water
           Mineral absorption
• Plants absorb minerals dissolved in water
  through their roots
• Plants that live in soil that have few minerals
  and little nitrogen get minerals from another
  – “carnivorous” plants get minerals from insects
    that they trap
        Plants responses - tropisms
• A tropism is a plant’s response to an external
• Phototropism causes plants to grow toward light
• Gravitropism is a plant’s response to gravity
  – Positive grows toward gravity (roots)
  – Negative grows away from gravity (shoots)
• Thigmotropism is a plant’s response to mechanical
  – Causes vining
  – Causes growth away from wind
  – Causes leaves to close (venus fly trap)
                 Plant hormones
• Auxin – found in roots and shoots (apical meristem)
  – Apical meristem is growth area of plants
  – Causes cells to elongate
  – Causes apical dominance
  – Cutting off shoots causes lateral growth
• Gibberelins – stimulates cells division
  – Lack of gibberelin results in dwarf plants
• Ethylene – causes fruit to ripen
  – The only known plant hormone that is a gas
  – Produced n ripening fruit
  – Causes other fruit to ripen

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