Roots, stems, and leaves

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Roots, stems, and leaves
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Roots

• Absorb water and dissolved nutrients
• Anchor plants in the ground
• Holds soil in place to prevent erosion
• Protect from harmful soil bacteria and fungi
• Transport water and nutrients to the rest of the
  plant
• Hold plant upright against forces such as wind
  and rain

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Types of roots

• Taproot
• Primary root grows long and thick while secondary
  roots remain small
• Some store sugars and starches
• Hickory, oak, carrots, dandelion, radish
• Fibrous root
• Branch so that no single root grows larger than
  the rest
• Help prevent topsoil from being washed away
• Grasses, rye,

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Root structure and growth

• A mature root has an outside layer, epidermis,
  and a central cylinder of vascular tissue
• Ground tissue fills the area between
• Surface covered with root hairs
• Produce a larger surface area for water
  absorption
• root grows in length at tip
• Root cap protects fragile new cells as it forces
  its way through the soil

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Stems

• Support system for the plant body
• Must be strong enough to hold up leaves and
  branches
• Holds leaves up to the sunlight
• Transport system that carries nutrients
• Must be able to lift water from roots to the
  leaves
• Defense system that protects plant against
  predators and disease
• Produce leaves, branches, and flowers

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Stem structure

• Surrounded by thick cell walls and waxy
  protective covering
• Nodes- where leaves are attached
• Internodes- regions between nodes
• buds- undeveloped tissue that can produce new
  stems and leaves
• Found where leaves attach to nodes

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Stem growth

• Primary growth- new cells are produced at the
  tips of roots and shoots
• Secondary growth- must increase in thickness as
  well as length
• Takes place in lateral meristems
• Vascular cambrium, cork cambrium, and cork
• Vascular cambrium- produces vascular tissue and
  increases the thickness of stems over time
• Cork cambrium- produces outer coverings of stems

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Formation of wood

• Actually layers of xylem
• Heartwood- old nonfunctioning xylem
• Helps support the tree
• Usually darkens with age
• Sapwood- surrounds heartwood
• active in fluid transport
• Usually lighter in color
• Tree rings formed from alternation of dark and
  light bands formed from early and late wood
  during the growing season
• Each ring corresponds to a year of growth

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Formation of bark

• Bark includes all of the tissue outside of the
  vascular cambrium
• Phloem, cork, and cork cambrium
• As new xylem is produced phloem must grow to
  accommodate the larger size of the tree
• Phloem is pushed outward and the expansion causes
  it to split and fragment
• The cork cambrium surrounds the cortex and
  produces a thick protective layer of cork

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leaves

• Main photosynthetic systems
• Optimized for absorbing light and carrying out
  photosynthesis
• Broad, flat surfaces help increase the amount of
  light absorbed

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Leaf structure

• Blades- thin flattened section that collects
  sunlight
• Petiole- thin stalk that attaches leaf to stem
• Cuticle and epidermal cells form waterproof
  barrier
• Vascular tissue of leaves connect directly to the
  vascular tissue of the stems

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Leaf functions

• Photosynthesis
• Bulk of leaf is mesophyll
• Site where photosynthesis occurs
• Palisade mesophyll- absorbs light
• Stomata- surrounded by guard cells, allow gas
  exchange
• Transpiration-the loss of water through leaves
• Replaced by water drawn into the leaf through
  xylem
• Surface of leaf kept moist for gas exchange

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Leaf functions cont.

• Gas exchange
• Take in carbon dioxide and give off oxygen during
  photosynthesis
• Take in oxygen and give off carbon dioxide during
  cellular respiration
• Stomata- site for gas exchange
• Open just enough to allow photosynthesis to take
  place, but not so much that they lose an
  excessive amount of water
• Usually open during the daytime and close at
  night
• Guard cells- control opening of stomata
• Stomata open and close in response to changes in
  water pressure within guard cells