LEAF
It is composed of: Leaf a leaf stalk called petiole; if it lacks leaf is sessile; the expanded part called lamina or blade; a strand of vascular tissue (veins) in the blade; a pair of leafy outgrowth (stipules) arising at the base of the petiole; the sheath, the base of the lamina enclosing stem or stalk; Leaf may be: simple, if blade is not divided in segments or composite if blade is divided in various segments (leaflets)
Simple leaves
Sessile leaves Leaf sessile and with sheath enclosing stem of corn
Composite leaves
Some leaflets change into tendrils which enable the plant to hold its position securely Pinnate leaf, having separate leaflets along each side of a common stalk (rachis)
Limited by upper and lower epdidermis, there is the leaf mesophyll, a chlorenchyma tissue composed of the palisade mesophyll (usually on adaxial side), with tightly packed, columnar cells, at right angle to the epidermis, each containing many chloroplasts, and the spongy mesophyll lying beneath the palisade layer (on abaxial side), comprising cells of irregular shape, poor in chloroplasts, separated by large spaces which is the site of gaseous exchange for photosynthesis and respiration. Anatomy
Anatomy Dorsiventral leaf, having the dorsal and ventral surfaces different each other. mesophyll The main site of photosynthesis is the palisade layer because of the higher number of chloroplasts and greater surface entrapping light. In a dorsiventral leaf stomata are mostly on the abaxial surface.
Lower epidermis with stomata and trichomes
Dicotyledone Monocotyledone Scattered and parallel arrangment of stomata on the leaf of Dicotyledone and Monocotyledone
Anatomy Distribution of stomata on the leaf (dorsiventral) of a hydrophytic plant.
Anatomy Leaf of a xerophyte (oleander) with thick cuticle, multicellular epidermis and stomata in depressions surrounded by protective trichomes.
Anatomy Isobilateral leaf of a Monocotyledone having both surfaces similar to each other; spongy layer is limited upper and lower by 2 palisade layers.
Anatomy Leaf-area index (LAI) Palisade cells number out of leaf surface area may represent a pharmacognostic parameter useful to recognize plant drugs.
Veins are vascular bundles composed of primary xylem and phloem. Minor veins collect organic compounds produced during photosynthesis; major veins only conduct them towards the other parts of the plant. Together with epidermal cuticle they provide support to leaf owing to the presence of collenchyma and sclerenchyma.
Anatomy Mesophyll structure is representative of the plant photosynthetic system t.s. of sugar cane leaf having photosynthetic C 4 pathway. Mesophyll cells are surrounded by bundle sheath containing chloroplasts.
Anatomy t.s. of wheat leaf having photosynthetic C 3 pathway. Bundle sheath is formed by 2 layers (parenchyma outside, sclerenchyma inside), both lacking of chloroplasts.
Leaf abscission Leaf abscission occurs at an abscission zone or layer developing across the base of a petiole, where hydrolytic enzymes reduce cell adhesion through the disruption of cell wall polysaccharides (e.g. cellulose and pectine) causing the leaf to fall off. Below the abscission zone, a protective layer, composed of corky cells, protect the stem and isolates the leaf before abscission. The process (normally happening in autumn) is controlled by ethylene.
Leaf modification Spines of Cactus are leaf modifications to reduce excessive water loss due to high temperatures
Pharmaceutical importance of leaves Leaves are sources of numerous secondary metabolites such as alkaloids (e.g. tobacco, belladonna, tea), glycosides (e.g. digitalis, aloe), tannins (e.g. tea), essential oils (e.g. mints, eucalyptus), juices (e.g. aloe), etc.