Leaf Introduction PDF

 Origin and Definition a lateral outgrowth from a plant stem that is typically a flattened expanded variably shaped greenish organ, functions primarily in food manufacture by photosynthesis  Types of leaves 1-Foliage leaves: The ordinary green leaves which carry t...

 Origin and Definition a lateral outgrowth from a plant stem that is typically a flattened expanded variably shaped greenish organ, functions primarily in food manufacture by photosynthesis  Types of leaves 1-Foliage leaves: The ordinary green leaves which carry the normal leaf function 2- Prophyll (fore-leaves): The first leaves born on the main stem, they reduced and simple in structure 3- Bracts: Leaves having a flower or a group of flowers in their axil they are small, simple in structure and may be coloured 4- Scale leaves: Occur in subterranean and some aerial stems, they are membranous and devoid of chlorophyll. They may be leathery covering the winter buds or fleshy 5- Floral leaves: The floral parts including sepals, petals, stamens and carpels 6- Modified leaves: When whole leaf or part of it modified into special structure for specific function The foliage leaves - It is the commonest type of leaves and it the type that has a medicinal effect - They are green in colour due to presence of chlorophyll except certain cases; the yellow and orange colours are due to the pigments left after the disintegration of the chlorophyll  Phyllotaxis - It is the arrangement of the leaves on the stem. - It is classified to the following type Phyllotaxis Alternate Opposite Whorled or spiral  Alternate or spiral - When the leaves are inserted singly at nodes.  Opposite - when a pair of leaves is developed at each node -Opposite decussate if, the two leaves are opposite and each pair is inserted at right angles to the pairs above and below it. Opposite decussate  Whorled - Three or more leaves attach at each point or node on the stem.  Structure of the Leaf - The complete leaf is composed of three parts 1- Leaf base 2- Petiole or leaf-stalk 3- Lamina or leaf-blade  1- Leaf base - It is the part of the leaf at which it is attached to the stem. It forms a close tube around the stem or a large sheath enclosing the stem or part of the stem. - Sometimes the leaf base is small and develops lateral outgrowth called stipules. - Both the leaf base and stipules protect the young axillary buds.  2- Petiole or leaf-stalk - It is the leaf stalk that connects the blade (lamina) to the twig. - It serves to carry the lamina away from the stem to place it suitable to the light and to conducts materials to and from the lamina. - The leaves are described according to the petiole to A- Petiolated - Have a petiole attaches to the blade inside from the blade margin. B- Sessile - Do not have a petiole; the blade (lamina) attaches directly to the stem.  3- Lamina or leaf-blade - The thin, usually flat part of leaf excluding petiole, having a green colour. Divisions or classification of the lamina (blade) 1- Simple leaf 2- Lobed or divided leaf 3- Compound leaf  1- Simple leaf - leaves have continuous undivided blade. - The lamina has many shapes as lanceolate, ovate, so on not compound, not divided into secondary units  2- lobed or divided leaf - It may cut up into a number of lobes connected with one another by undivided portion -The leaf shape may have deeply divided lobes, but the gaps between lobes do not reach to the vein. palmately lobed pinnately lobed three or more lobes segments or lobes radiating fan-like from a along each side of a common base common axis  3- Compound leaf -When leaves are completely segmented into a number of separate leaflets arranged on a central axis called rachis. Types of compound leaves: 1- Binate: With two leaflets 2- Ternate: Binate With three leaflets, two lateral and one terminal 3- Palmate: Ternate With five or more leaflets radiating fan-like from a common basal point of attachment Palmate 4-Pinnate: With several leaflets arranged on both sides of the rachis A- Paripinnate: when terminates with two leaflets B- Imparipinnate: when terminates with single leaflets C- Bi-pinnate: two pinnate Paripinnate Imparipinnate Bi-pinnate - The important points required for the general description of the lamina of a simple leaf as well as leaflets shape base apex margin venation surface  Shapes of lamina lanceolate ovate elliptical cordate needle linear oblong obovate auriculate spatulate oblanceolate  Apex - The common terms used for description are 1- Acute - If the two sides of the margin form acute angle at the top. 2- Acuminate - If the apex is more prolonged into a point and is longer than in the acute.  3- Obtuse - Blunt or rounded 4- Micronate - Acute apex terminating in a sharp horny point 5- Emarginate - with an acute apical deep notch or incision. 6- Recurved curved outwards or backwards.  Margin - It can be described as following 1- Entire when quite smooth 2- Dentate when having teeth pointed straight outwards.  3- Serrate - when having sharp teeth directed towards the apex 4- Crenat when having numerous minute rounded lobes.  Base - The base of the lamina is described as Symmetric Asymmetric Decurrent when the when the base when the base base is equal continuous is unequal on on both downwards as both sides of sides of the wings along the the midrib midrib petiole.  Venation - The lamina is traversed by vascular strands these form the veins; these are generally project on the lower side of the leaf. - The vein in the middle of the lamina is more strongly developed and termed the midrib. - The disposition and arrangement of the veins is known as venation - Venation of the lamina can be described as following 1- Parallel - when several veins of equal size run side by side parallel to each other.  2- Reticulate - When the lateral veins arise in a pinnate form one or more main veins and these branches off from one another, which run in all directions forming network appearance. 3- Pinnately Reticulate With one main vein from which lateral veins extended to the margin in a regular manner. 4- Palmately Reticulate With several main veins arising from the top of the midrib.  Surface - The upper surface of the lamina is usually dark green, while the lower is paler. - The surface can be described as Smooth puncate dotted with projection formed by oil-glands glabrous when free from hairs spinose when be set with spines hairy when contains hairs  Structure of the lamina - A leaf typically consists of the following tissues 1- epidermis - Covers the upper and lower surfaces 2- Mesophyll 3- veins  1- Epidermis - The epidermis is the outer layer of cells covering the leaf blade. It is a protective layer of primary origin, formed of a single layer or some times of two or more layers - The layer is usually transparent (cells lack chloroplasts) - coated on the outer surface with a waxy cuticle that prevents water loss.  The epidermal cells are tabular with no intercellular spaces except for the with pores called stomata that enable oxygen and carbon dioxide to move in and out of the leaf -The anticlinal walls may be: 1- straight in Senna 2- Wavy in Belladonna 3- Beaded due to unequal thickening or thickening in the angles as in Digitalis  - Hairs grow out from the epidermis in many species. The epidermal cells may extend to the outside to form projections varying in shape and size. - If these projections are short and conical, they are called papillae, but if long and well protruding, they form the trichomes or haris. head 4- cruciferous stomata Anisocytic stomata have three unequally sized subsidiarycells associated with the guard cell pair, where one of these three cells is smaller than the other two.  2- Mesophyll - It is divided into two layers Mesophyll palisade spongy layer layer A- palisade layer - Formed of one or more layers of cylindrical, columnar cells having the long axis perpendicular to the epidermis. - They show narrow intercellualr spaces. It is usually interupted by the cortical tissue in the midrib region, but in some cases the upper palisade may form a continous layer T. S. in lamina region  B- Spongy layer - Composed of loosely arranged cells of irregular shape, this layer contains fewer chloroplasts and has many intercellular spaces for the diffusion of gases (required for respiration and photosynthesis), linked to the outside by means of stomata. - According to the arrangement of the mesophyll, the leaf is described Mesophyll Isobilateral Dorsiventaral leaf leaf  Isobilateral leaf - The mesophyll is symmetric on both sides showing one or more layers of palisade cells on both the upper and lower epidermis. Dorsiventaral leaf - The mesophyll is not symmetric showing only one palisade layer below the upper epidermis while the rest of mesophyll is formed of irregular parenchyma.  Cortical tissue - It is present mainly in the midrib -region. It may be formed - of parenchyma or collenchyma -or both. Endodermis -It is the inner most layer of the cortical tissue, is usually indistinguishable. - When recognisable it often takes the form of starch sheath as in digitalis leaf.  The pericycle - It lies between the phloem and the cortical tissue; it is mostly parenchymatous, sometimes collenchymatous or may consist of lignified fibers below the vascular bundles. - Another patch of pericyclic fibers may also be present above the vascular bundles. Veins (vascular system) - The veins are made up of xylem, which brings water from the stem into the leaf, and phloem, which containing the glucose produce by photosynthesis in the leaf. - The vascular bundles appear in T. S. as an arc open above with - the phloem towards the lower surface - and the xylem towards the upper surface - The cambium is not always distinguishable, Xylem (X. vessels, xylem parenchyma, and medulallary rays) Phloem: Sieve tubes, companion cells, phloem parenchyma