LEAVES

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LEAVES
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The leaf is generally an expanded and flattened
green structure growing out at the node of the
stem. Its green color is due to chlorophyll
pigments.
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Blade
A thin, flattened green structure
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Leaf stalk or Petiole

• A cylindrical or flattened narrow structure which
  supports and holds the blade upright.
• Provides maximum exposure of the blade to
  sunlight.
• Serves for conduction
• With petiole petiolate without a petiole
  sessile.

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In some monocots

• Petiole is lacking
• The blade is supported by a flattened structure
  called the leaf sheath which clasps the stem.

Leaf Sheath
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• Outgrowths at the junction of the sheath and
  blade may be present and are collectively
  referred to as the ligule.
• This performs a protective function of preventing
  water and dirt from accumulating between the
  sheath and the stem.

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Stipule

• A pair of appendages or outgrowth at the base of
  some dicotyledonous leaves.
• With stipule stipulate without stipule
  exstipulate.

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Apex
Leaf Blade
Leaf Margin
Base
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Leaf variations
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Nature of Leaf Blade
Simple leaf

• The blade consists of only one piece.
• Petiole of a leaf is directly attached to the
  stem.

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Compound Leaf

• The blade is divided into separate segments
  called leaflets.
• Stalk of leaflets is the petiolule.
• Outgrowth, usually in pairs, may be found at the
  base of the petiolule stipel.

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Pinnately Compound Leaf
Leaflets are attached to a rachis which is an
extension of the petiolule.
Simple pinnately compound leaf
There is a single rachis to which the leaflets
are attached.
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Bipinnately compound leaf
First rachis or primary rachis branches into
secondary rachises where the leaflets are
attached.
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Tripinnately Compound Leaf
There are rachises of the first, second and third
order.
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Palmately Compound Leaf
Leaflets radiate from the upper end or tip of the
leaf stalk.
Central rachis
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Venation
Arrangement of veins or vascular strands of a
leaf blade.
Netted Leaf Venation

• The veins branch profusely and form a network
  over the blade.
• Commonly found in dicots.

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Pinnately Netted
Midrib is present from which smaller veins and
their branches ramify or spread out in all
directions throughout the blade.
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Palmately Netted

• Several principal veins arise from the tip of the
  petiole and spread fanlike through the blade.
• The principal veins produce smaller veins that
  from a network.

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Radiately netted
Several principal veins radiate from around the
tip of the petiole and break up into smaller
veins to form a network throughout the blade.
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Parallel Leaf Venation

• The veins do not form a network.
• Common among monocots.

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Parallel venation with the veins parallel to the
midrib
Parallel venation with the veins at acute or
right angles to the midrib
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Phyllotaxy
The system of leaf arrangement on the stem.
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Alternate or Spiral
Only one leaf develops at each node
Texas sugarberry tree (Celtis laevigata)
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Opposite
Two leaves develop opposite each other at a node.
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Whorled or Verticillate
Three or more leaves develop equidistantly around
the node.
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Fasciculate
Two or more leaves develop at only one side of
the node.
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Midrib Cross Section
Upper Epidermis
Xylem
Phloem
Lower Epidermis
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Dicot Leaf Cross Section
Upper Epidermis
Palisade Parenchyma
Xylem
Phloem
Lower Epidermis
Trichome
Stomata
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Monocot Leaf Cross Section
Upper Epidermis with cuticle
Mesophyll layer
Lower Epidermis with cuticle
Vascular bundle
Stomata
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Monocot Leaf Cross Section
Phloem
Xylem
Bundle Sheath
There is no differentiation of mesophyll into
spongy and palisade parenchyma (Isobilateral or
Equifacial Leaves)
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Isobilateral or Equifacial Leaves
There is no differentiation of mesophyll into
spongy and palisade parenchyma
These vertically oriented leaves have their two
surfaces receiving direct sunlight.
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Bulliform cells
Large, bubble-shaped epidermal cells that occur
in groups on the upper surface of the leaves of
many grasses. Loss of turgor pressure in these
cells causes leaves to "roll up" during water
stress
Monocot Leaf Cross Section
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Dorsiventral or Bifacial Leaves

• Have their upper or adaxial surfaces darker green
  in color
• Presence of palisade mesophyll (upper) and spongy
  mesophyll (lower) layers

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For Support
Hooks
Supporting leaf bases that grow close together to
form a false trunk
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Floaters
Tendrils
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For Absorption
Insectivorous leaves
Thin, uncutinized epidermis
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For Attraction
Petalloid bracts
Bright coloration of entire blade or portion of
the blade
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For Reproduction
Leaf and plantlet develop at certain parts of the
leaf
Tip, base or margin
Tip of petiole
Leaf Cuttings
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For Protection
Spiny Leaves
Bud Scales
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For Protection
Stipular spines
Leaves reduced to spines
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For Protection
Apical or Marginal Spines
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For Storage
Fleshy or thickened blades
Bulbs
Pocket leaves