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LEAVES. 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. Blade. A thin, flattened green structure. Leaf stalk or Petiole.

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LEAVES

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LEAVES


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.


Blade

A thin, flattened green structure


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.


In some monocots:

  • Petiole is lacking

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

Leaf Sheath


  • 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.


Stipule

  • A pair of appendages or outgrowth at the base of some dicotyledonous leaves.

  • With stipule – stipulate; without stipule – exstipulate.


Apex

Leaf Blade

Leaf Margin

Base


Leaf variations


Nature of Leaf Blade

Simple leaf

  • The blade consists of only one piece.

  • Petiole of a leaf is directly attached to the stem.


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.


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.


Bipinnately compound leaf

First rachis or primary rachis branches into secondary rachises where the leaflets are attached.


Tripinnately Compound Leaf

There are rachises of the first, second and third order.


Palmately Compound Leaf

Leaflets radiate from the upper end or tip of the leaf stalk.

Central rachis


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.


Pinnately Netted

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


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.


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.


Parallel Leaf Venation

  • The veins do not form a network.

  • Common among monocots.


Parallel venation with the veins parallel to the midrib

Parallel venation with the veins at acute or right angles to the midrib


Phyllotaxy

The system of leaf arrangement on the stem.


Alternate or Spiral

Only one leaf develops at each node

Texas sugarberry tree (Celtis laevigata)


Opposite

Two leaves develop opposite each other at a node.


Whorled or Verticillate

Three or more leaves develop equidistantly around the node.


Fasciculate

Two or more leaves develop at only one side of the node.


Midrib Cross Section

Upper Epidermis

Xylem

Phloem

Lower Epidermis


Dicot Leaf Cross Section

Upper Epidermis

Palisade Parenchyma

Xylem

Phloem

Lower Epidermis

Trichome

Stomata


Monocot Leaf Cross Section

Upper Epidermis with cuticle

Mesophyll layer

Vascular bundle

Lower Epidermis with cuticle

Stomata


Monocot Leaf Cross Section

Phloem

Xylem

Bundle Sheath

There is no differentiation of mesophyll into spongy and palisade parenchyma (Isobilateral or Equifacial Leaves)


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.


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


Dorsiventral or Bifacial Leaves

  • Have their upper or adaxial surfaces darker green in color

  • Presence of palisade mesophyll (upper) and spongy mesophyll (lower) layers


For Support

Hooks

Supporting leaf bases that grow close together to form a false trunk


Floaters

Tendrils


For Absorption

Insectivorous leaves

Thin, uncutinized epidermis


For Attraction

Bright coloration of entire blade or portion of the blade

Petalloid bracts


For Reproduction

Leaf and plantlet develop at certain parts of the leaf

Tip, base or margin

Leaf Cuttings

Tip of petiole


For Protection

Spiny Leaves

Bud Scales


For Protection

Stipular spines

Leaves reduced to spines


For Protection

Apical or Marginal Spines


For Storage

Fleshy or thickened blades

Bulbs

Pocket leaves


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