Plant Structure and Growth

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Plant Structure and Growth

• Chapter 35

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The Plant Body

• Plant structures are directly related to the
  environment in which they are found.
• Plants evolved special structures when they began
  to colonize the land around 430 million years
  ago.
• Plants have 3 basic organs roots, stems, and
  leaves.

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The Root System

• Roots
• Anchor plants in soil
• Absorb minerals
• Absorb water
• Store food

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Fibrous Root

• Characterized by having a mass of similarly sized
  roots.
• Most monocots have fibrous root systems. 
• Some fibrous roots are used as storage for
  example sweet potatoes form on fibrous roots. 
• Plants with fibrous roots systems are excellent
  for erosion control, because the mass of roots
  cling to soil particles.

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Taproots

• Characterized by having one main root (the
  taproot) from which smaller branch roots emerge. 
  
• When a seed germinates, the first root to emerge
  is the radicle, or primary root.
• Taproots can be modified for use in storage
  (usually carbohydrates) such as those found in
  sugar beet or carrot. 
• Taproots are also important adaptations for
  searching for water, as those long taproots found
  in mesquite and poison ivy.

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Adventitious Roots

• Some plants have roots that grow aboveground from
  stems or sometimes leaves.
• These function as props to keep the plant upright.

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The Shoot System Stems and Leaves

• Shoots consist of stems and leaves
• Shoots systems may be vegetative or leaf bearing
  OR
• Reproductive, or flower bearing.

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Stems

• The stem is the major aerial support system in
  most plants.
• Sometimes called a stalk or trunk, it holds up
  the plant into the air and provides a pathway for
  fluid transport between the shoot and the root.
• The stem also may represent a huge volume for
  storage of materials. Sometimes the stored
  materials are for retrieval and later use
  sometimes the stored materials are toxins put
  into the trunk as permanent waste or as material
  to prevent trunk rot by microorganisms.

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• A stem is an alternating system of nodes and
  internodes.
• Nodes are the point at which a leaf is attached.
• Internodes are the stem segments between leaves.
• The axil is the angle formed by the angle between
  the leaf and stem.
• An axillary bud is located in the axil. An
  axillary bud has the potential to form a
  vegetative branch.

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• The tip or apex of a shoot is where the terminal
  bud is located.
• Leaves develop at this bud and there is a series
  of nodes and internodes.
• The terminal bud inhibits the growth of axillary
  buds due to a phenomenon called apical dominance.

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Apical Dominance

• This thought to be an evolutionary adaptation to
  increase exposure to light.

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Stem Functions

• Support of Leaf Display
• Support of Flower Display
• Support of Fruit Display
• Conduct water and minerals up from soil
• Conduct water and nutrients
• Photosynthesis
• Storage of water, etc
• Defense (i.e. spines, thorns, etc.)
• Anchorage (especially in vines)

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Leaves

• The main photosynthetic organ of plants.
• Made up of a blade and a petiole.
• The blade is the leaf itself and the petiole is
  the stalk.
• Grasses and some other plants do not have
  petioles.
• Monocots and Dicots have different leaf
  arrangements.

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Plant Cell Types

• Plant tissues are made up of 3 types of cells
• Parenchyma
• Collenchyma
• Sclerenchyma

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Parenchyma

• A generalized plant cell type, parenchyma cells
  are alive at maturity.
• They function in
• storage,
• photosynthesis,
• and as the bulk of ground and vascular tissues.
• Palisade parenchyma cells are elogated cells
  located in many leaves just below the epidermal
  tissue.
• Spongy mesophyll cells occur below the one or two
  layers of palisade cells.
• Ray parenchyma cells occur in wood rays, the
  structures that transport materials laterally
  within a woody stem.

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• Parenchyma cells also occur within the xylem and
  phloem of vascular bundles.
• The largest parenchyma cells occur in the pith
  region, often, as in corn (Zea ) stems, being
  larger than the vascular bundles.
• Pith ground tissue that is internal to the
  vascular tissue.
• Cortex ground tissue that is external to the
  vascular tissue.

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Collenchyma

• Collenchyma cells support the plant.
• These cells are characterized by thickenings of
  the wall
• They are alive at maturity.
• They tend to occur as part of vascular bundles or
  on the corners of angular stems.

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Sclerenchyma

• Sclerenchyma cells support the plant.
• They often occur as bundle cap fibers.
• Sclerenchyma cells are characterized by
  thickenings in their secondary walls.
• They are dead at maturity.

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Plant Tissues

• Plant tissues fall into three fundamental
  categories (among a few others)
• dermal tissue
• ground tissue
• vascular tissue

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Dermal Tissue

• Dermal tissues generally occupy the "skin" layer
  of all plant organs.
• They are responsible for environmental
  interaction
• light passage,
• gas exchange,
• pathogen recognition,
• color display, etc

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Ground Tissue

• This tissue occupies the space between the dermal
  tissues and the vascular tissues.
• In roots the ground tissue may store sugars or
  starches to fuel the spring sap flow.
• In leaves, the ground tissue is that layer doing
  photosythesis, the mesophyll.
• In many species ground tissues produce
  intracellular crystals that paralyze potential
  herbivores.

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Vascular

• The vascular tissues of higher plants (Kingdom
  Plantae) are divided into two sections xylem and
  phloem.
• Xylem mostly water transport
• Phloem- mostly organic matter transport (food)

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Xylem

• Xylem functions in the transport of minerals from
  the soil up the plant.
• In the spring sap flow of woody trees, however,
  the xylem briefly carries lots of sugars and
  other nutrients from root storage to "jump start"
  the production of leaves.

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• Xylem is basically a sclerenchyma tissue (it
  contains some parenchyma.)
• Some of that sclerenchyma is fiber which does not
  conduct. The remainder, however, is all
  conductive.
• There are two fundamental cell types
• tracheids and
• vessels.

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Tracheids

• Tracheids are thick-walled, have no cytoplasm,
  and conduct from cell-to-cell through perforation
  plates in the end walls or through pits in the
  side walls.
• The earliest matured tracheids have annular,
  spiral, or reticulate wall thickenings.
  Later-maturing tracheids have pitted walls.

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Vessels

• Vessels are also thick-walled, dead, hollow
  cells, but lack end walls.
• They are typically much larger in diameter and
  are therefore major pipes in the plumbing for
  water movement.
• The wall thickenings in vessels are pitted.

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Phloem

• Phloem functions in the
• transport of sugars,
• amino acids,
• and other small molecules from the leaf to the
  rest of the plant.
• Obviously, it is therefore a bidirectional flow
  down to the roots, but also up to the apical
  buds, flowers, fruits, and seeds.

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• Phloem is basically a parenchyma tissue (it
  contains some fibers (some very important such as
  those in flax that we use to make linen), but the
  rest is simple parenchyma (non-conductive) or the
  two specialized phloem cell types Sieve elements
  and companion cells.

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Sieve Elements

• Sieve elements are living cells with typical thin
  walls.
• The end walls are perforated and called sieve
  plates.
• The pores are essentially well-developed
  plasmodesmata.
• Since the rate of flow through sieve elements is
  high, the cytoplasm is very simple, with no
  developed organelles (no nucleus, no
  mitochondrion, no chloroplast).

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Companion Cells

• To maintain the living state of the sieve element
  cells, they are directly associated with
  well-developed, adjacent companion cells by means
  of plasmodesmata.
• The companion cell has a typical nucleus and
  cytoplasm and does enough biochemistry to keep
  the sieve elements and the companion cells
  themselves alive.
• Plasmodesmata are narrow channels that act as
  intercellular cytoplasmic bridges to facilitate
  communication and transport of materials between
  plant cells.

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