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. Leaf Blade Leaf Margin Apex Base Leaf variations Simple leaf The blade consists of only one piece. Petiole of a leaf is directly attached to the stem. Nature of Leaf Blade 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 Simple pinnately compound leaf Leaflets are attached to a rachis which is an extension of the petiolule. 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 Netted Leaf Venation Arrangement of veins or vascular strands of a leaf blade. The veins branch profusely and form a network over the blade. Commonly found in dicots. Midrib is present from which smaller veins and their branches ramify or spread out in all directions throughout the blade. Pinnately Netted 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 at acute or right angles to the midrib Parallel venation with the veins parallel 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 Upper Epidermis Palisade Parenchyma Xylem Phloem Lower Epidermis TrichomeStomata Dicot Leaf Cross Section Monocot Leaf Cross Section Upper Epidermis with cuticle Mesophyll layer Lower Epidermis with cuticle Vascular bundle Stomata Monocot Leaf Cross Section Phloem Xylem Bundle Sheath There is no differentiation of mesophyll into spongy and palisade parenchyma (Isobilateral or Equifacial Leaves) There is no differentiation of mesophyll into spongy and palisade parenchyma Isobilateral or Equifacial Leaves 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 Supporting leaf bases that grow close together to form a false trunk Hooks FloatersTendrils For Absorption Thin, uncutinized epidermis Insectivorous leaves For Attraction Petalloid bracts Bright coloration of entire blade or portion of the blade For Reproduction Leaf and plantlet develop at certain parts of the leaf Tip, base or margin Tip of petioleLeaf Cuttings For Protection Bud Scales Spiny Leaves For Protection Stipular spinesLeaves reduced to spines For Protection Apical or Marginal Spines For Storage Fleshy or thickened blades BulbsPocket leaves