Plant Leaves: Types and Functions

• The leaf is a lateral outgrowth/appendage with limited growth, differing in structure and organization from the stem.
• Leaves are flattened and have a bud or branch in their axil, consisting mainly of primary tissues without nodes or internodes.
• Leaves assimilate glucose and starch from water and carbon dioxide through photosynthesis, using sunlight.

Different Kinds of Leaves

• Cotyledons (seed leaves) store food for the embryo.
• Foliage leaves are ordinary green leaves and are responsible for photosynthesis.
• Floral leaves compose the reproductive parts of the flower, including sepals, petals, stamens, and carpels.
• Bract leaves have a flower in their axils; the axillary bud modifies into a flower.
• Scale leaves occur on subterranean stems, are thin, membranous, lack chlorophyll, and protect inner parts.
• Modified leaves are adapted into special structures to perform specific functions, such as spines for protection in orange plants, or tendrils for climbing in grapes.

Morphological Structure of a Dicot Leaf

• Leaves exhibit variations in shape, which aids in identification.
• A leaf consists of a broad, flat lamina/blade, and a stalk called the petiole.
• Some leaves possess stipules, which are small leaf-like outgrowths at the base.
• The petiole attaches the blade to the stem.
• The lamina/blade is broad and flat and collects light for photosynthesis.
• Veins contain vascular tissue.
• The axil is the point where the leaf attaches to the stem's node.
• It is the intersection of the petiole and the stem and sometimes has an axillary bud.
• Stipules are outgrowths at the base of the leaf.

Forms of Leaves

• Simple leaves have blades that are not divided into distinct, separate leaflets.
• Entire (single) blades are leaves that are not divided.
• Lobed leaves have lobes.
• Pinnately lobed leaves have lobes arranged on the sides of a central axis, like a feather.
• Palmately lobed leaves have lobes spreading radially from a point, similar to fingers on a hand.
• Compound leaves consist of blades divided into smaller leaflets.
• Pinnate leaves possess leaflets that arise along the sides of the rachis, which extends from the petiole; leaflets are in even numbers, called paripinnate, or odd numbers, called imparipinnate.
• Palmate leaves are divided into 5 or more leaflets radiating from the top of the petiole.

Venation

• Parallel venation is characteristic of monocotyledons, with veins running approximately parallel to each other.
• Multicostate venation involves multiple main veins or ribs radiating from the leaf's base.
• Unicostate venation involves a single main vein running through the center of the leaf blade.
• Netted venation is characteristic of dicotyledons.
• Pinnately netted veins branch off or radiate from a central midvein.
• Palmately netted leaves have several major veins that radiate out from a point.

Leaf Arrangement (Phyllotaxis)

• Alternate leaf arrangement: leaves occur one leaf per node.
• Opposite leaf arrangement: nodes are opposite each other with two leaves per node.
• Opposite decussate: successive pairs of leaves are at right angles to each other (typical for the mint family).
• Whorled: three or more leaves arise from one node.

Histology of Leaves

• Ordinary leaf consists of the upper and lower epidermis, mesophyll, cortical tissues, and vascular system.
• The lower surface of the leaf has more stomata that facilitate gaseous exchange.
• Mesophyll lies between the two epidermal layers.
• Cortical tissues surround the vascular bundle in the mesophyll.

Microscopic Leaf Characteristics

• These include form and number of stomata, trichomes, and crystals such as calcium oxalate.

Senna (Alexandrian & Indian Senna)

• Origin: Dried leaflets of Cassia acutifolia (Alexandrian Senna) and Cassia angustifolia (Indian Senna); Family Leguminosae.
• Active constituents: Anthraquinone glycosides (Sennoside A, B, C, D, which upon hydrolysis yields Sennidin A, B, C, D) and mucilage (carbohydrates).

Confirmatory Chemical Tests:

• Borntrager's test for free anthraquinones: chloroformic extract with NH4OH gives a rose (pink) to intense red color in the aqueous layer.
• Modified borntrager's test for combined anthraquinones: chloroformic extract with NH4OH gives a rose (pink) to intense red color in the aqueous layer.
• Medicinal Uses: It is used as a laxative and powerful cathartic due to anthraquinones glycosides.
• Precautions: This shouldn't be taken for more than 10 days due to bowel muscle weakening; side effects include a bitter taste and nausea; mixed with aromatic herbs like Mentha to decrease nausea.
• Contraindications: Should not be taken during pregnancy.
• Morphology: Petiole is petiolate.
• Key features are: obovate/lanceolate shape, entire margin, compound/pinnate, paripinnate form, reticulate/pinnately reticulate venation, and alternate phyllotaxis.
• Key Elements: The KOH mount of the powdered plant shows palisade cells, upper/lower epidermal cells with paracytic stomata, warty hairs (non-glandular, unicellular), crystal sheath (with calcium oxalate prisms), and calcium oxalate clusters; fibers can sometimes be seen due to the transparent crystal sheath.

Digitalis (Foxglove)

• Origin: dried leaves of Digitalis purpurea and Digitalis lanata, Scrophulariaceae family.
• Active constituents: cardiac glycosides (Digoxin, Digitoxin, Lanatoside A, Lanatoside C) and saponin glycosides.

Confirmatory Chemical Tests

• Baljet test: alcoholic extract with Baljet reagent (picric acid + 10% NaOH/sodium picrate) yields orange-yellow color.
• Keller-Killiani test: acetic acid extract with FeCl3 + Conc H2SO4 yields reddish-brown ring at the node of the two layers, and the upper layer turns bluish-green.
• Froth test: powdered drug with water gives a persistent froth that remains for 5 minutes.
• Medicinal uses: cardiotonic, strengthens the heartbeat, increases force of contraction, enables slower/stronger more regular heartbeats, requires less oxygen; prolonged use increases heart load and fatigue.
• Precautions: It is excreted slowly with cumulative effects (potentially fatal); preferred as long-term medication.
• Morphology: petiolate petiole; acute apex, lanceolate and decurrent shape, entire but ciliated margin, simple form, reticulate anastomosing venation, exstipulate symmetric and decurrent apex, alternate phyllotaxis.
• Key microscopical Elements include: epidermal cell fragments (beaded anticlinal walls, anomocytic stomata, striated cuticle); 3 types of glandular hairs (bicellular biserriate head/unicellular stalk, unicellular head/stalk, unicellular head/multicellular uniserriate stalk); 2 types of non-glandular hairs (collapsed and multicellular uniserriate); lacks calcium oxalate.

Tea

• Origin: Prepared leaves and leaf bud of Thea sinensis L. (Camellia thea Link.), Family Theaceae.
• Part used: The two adjacent young leaves with stem.

Types of Tea

• Green tea undergoes rapid drying at high temperatures, which stops the oxidase enzyme resulting in no fermentation, and no oxidation of tannins, with no change in chlorophyll color.
• Black tea undergoes slow drying at no high temperature, which results in complete oxidation by oxidase enzyme, complete fermentation, oxidation of tannins, and changes in chlorophyll color.
• Active constituents: Contains caffeine-phlobatannins complexes.
• Green tea caffeine-phlobatannins complex is soluble due to destroyed enzyme by temperature
• Black tea contains insoluble phlobaphenes (tannins) and free caffeine through hydrolysis.
• Green tea has more Flavonoids, less caffeine and is used as an antioxidant and antitumor agent.

Confirmatory Chemical Tests

• Murexide test: for caffeine; aqueous decoction, evaporate, residue with Conc. HCl + H2O, then evaporate till dryness, yellow color forms and gives a crimson red residue with ammonia vapors; violet color is formed.
• Ferric chloride test: for tannins; aqueous extract with FeCl3 gives a green color.
• KOH test: for flavonoids glycosides; acidic extract with KOH yields a yellow color.
• Froth test: for saponin glycosides; powdered drug shaken with water gives a persistent froth.
• Medicinal uses (caffeine): CNS stimulant, diuretic, weak smooth muscle relaxant.
• Medicinal uses (tannins): antidiarrheal (constipating).
• Precautions: Green tea is antioxidant and anticancer.It can cause gastritis, nervous irritability, anemia, constipation, habituation and increased heart rate.

Herb in Botany

• A plant with a non-woody, non-permanent stem that dies down to the ground after fruiting

Herb in Pharmacognosy

• A drug composed of a tender part of the plant axis (stem with leaves, flowers, fruits).
• Includes aerial shoots (cannabis), whole aerial parts (lobelia), and thallus (carrageen/ergot).
• Definition: stem is the axis and bears leaves/flowers.
• Structure: it is formed segments/internodes linked to node where leaves arise.

Types Of Stem

• Aerial
• Subterranean

Aerial Stem Functions

• Supports/displays leaves, flowers, and fruits to favor activities.
• Conducts water / minerals to leaves.
• Transports elaborated foods from leaves to roots. Macroscopical Characteristics include shape, kinds of, branching, surface, and fracture.

Shape Of The Stem

• Cylindrical (Euphorbia)
• Angular (with 3 sides -- Triangular -- or 4 sides -- Quadrangular /Square like Mentha -- or 5 Sides --Pentagonal)
• Ribbed (alternate ridges as in Ricinus communis
• Winged (marked with wings e.g. Lobelia).
• Flattened (either normally or by compression of Belladonna
• Twisted (by winding as in Convolvulus)

Kind of Stem

• Herbaceous (soft, easily broken, annuals with little/no thickening in monocots/dicots like Mentha where stem consists of primary tissues, few secondary)
• Succulent (fleshy, stored water e.g. Euphorbia)
• Woody (shrubs/trees with secondary xylem like Eucalyptus)
• Creeping (creeping along ground/root with adventitious roots.)
• Climbing (weak, attaches to support using tendrils/prickles/hooks/adventitious roots. e.g. Pisum sativum)
• Twining (attaches via support by winding, thin, long, with distant internodes: Vine, Convolvulus)

Anatomy of Dicot stem

• Ground tissue: pith, cortex and medullary rays.
• Vascular: bundles may connect through interfascicular cambium within medullary rays.
• Bundles are ring-shaped w/primary xylem towards pith.

Mentha

• The source is dried leaves/flowering tops of Mentha piperita and Labiatae family.
• Primary constituents include: volatile oil (Menthol 35-55, Menthone 10-40%), flavonoids, phenolic acids, tannins, and triterpenes.
• Confirmatory tests: Sudanese 3 for any volatile oil provides red color.
• Vanillin = yellowish when crystals of menthol and H2SO4 are with each other. It also contains H2O

Further Mentha info

• It is sometimes known to be both Carminative and Antispasmodic H2SO4 yields with KHO and yellow
• Has something to do with volatile oils and flavonoids
• Medicinal uses: gastrointestinal and IBS disorders and is thought to increase digestive use for liver and bile
• Side-effects consist of esophageal sphincter issues and heart burn
• Morphology: stem has erector quadrangle

Structure

• Lamina shape with acute is serrate
• Opposite decussate
• With presence of flower spike and vessels on each axes