Leaves

• Leaf

  • In most vascular plants, the leaf is the main photosynthetic organ.
  • Also functions to exchange gases with the atmosphere, dissipate heat, and defend themselves from herbivores and pathogens.
• Functions

  1. Photosynthesis
  2. Gas exchange
  3. Transpiration
  4. Guttation
  5. Abscission
• In general, a leaf consists of a flattened blade and a stalk, the petiole, which joins the leaf to the stem at a node
• Grasses and many other monocots lack petioles; instead, the base of the leaf forms a sheath that envelops the stem.
• STRUCTURE: Lamina/blade, Petiole, Leaf sheath, Pseudopetiole, Veins, Stipules
• =In identifying angiosperms according to structure, taxonomists rely mainly on floral morphology, but they also use variations in leaf morphology, such as leaf shape, the branching pattern of veins, and the spatial arrangement of leaves
• Leaf type: pattern of division of a leaf into discrete components or segments
• Simple leaf: one bearing a single, continuous blade
• Compound leaf: is one divided into two or more, discrete leaflets.
• first (largest) division of a leaf is termed a pinna; the ultimate divisions are termed pinnules
• Compound leaves may withstand strong wind with less tearing. They may also confine some pathogens that invade the leaf to a single leaflet, rather than allowing them to spread to the entire leaf.
• Pinnately compound or pinnate leaf - one with leaflets arranged along a central axis, the rachis
• imparipinnate or odd-pinnate - if a pinnate leaf has a terminal leaflet (and typically an odd number of leaflets)
• paripinnate or even-pinnate - if it lacks a terminal leaflet (and has an even number of leaflets)
• Bipinnately compound or bipinnate leaf - with two orders of axes, each of which is pinnate (equivalent to a compound leaf of compound leaves).
• Tripinnately compound or tri-pinnate - a compound leaf with three orders of axes
• Palmately compound or palmate - A compound leaf in which four or more leaflets arise from a common point
• Leaf Attachment: nature of the joining of the leaf to the stem
• Petiolate - with a petiole
• Sessile - without a petiole
• Amplexicaul - clasps the stem most, but not all, of its circumference
• Perfoliate - the base of the blade completely surrounds the stem
• Connate-perfoliate - typically two opposite leaves fuse basally and completely surrounds the stem
• Sheathing leaf attachment - a flattened leaf base (sheath) partially or wholly clasps the stem
• Decurrent - If a leaf appears to extend down the stem from the point of attachment, as if fused to the stem
• Leaf Venation: pattern of veins and vein branching
• Primary vein - major vein (or veins) of a leaf, with respect to size
• Secondary veins: smaller, lateral veins that “branch off,” from the primary vein
• Midrib or costa - If a simple leaf has a single, primary vein

  Midvein - primary vein of the leaflet of a compound leaf
• Dichotomous - veins successively branch distally into a pair of veins of equal size and orientation
• Uninervous - there is a central midrib with no lateral veins
• Parallel - primary and secondary veins are essentially parallel to one another, the ultimate veinlets being transverse (at right angles)
• Netted or reticulate - the ultimate veinlets form an interconnecting netlike pattern
• Leaf margin: Outline of the leaf
• Teeth - small sharp-pointed or rounded projections or lobes along the sides, technically extend no more than 1/8 of the distance to the midrib, or midvein

  • If further than 1/8 of this distance, then these are lobed, cleft, parted, or divided
• Entire - margin without teeth
• Serrate - sawlike teeth, sharp-pointed and ascending (the lower side longer than the upper)
• Dentate - sharklike teeth that point outward at right angles to the margin outline (the upper and lower sides about the same length)
• Crenate - rounded to obtuse teeth, that point outward at right angles or shallowly ascend
• Spinose - teeth bearing sharp, stiff, spinelike processes