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Botany Basics 3 Crop Science 1 Fall 2004 September 28, 2004 Leaf Function Principal function Photosynthesis to absorb sunlight to manufacture plant sugars Surfaces flattened to present a large area for efficient light absorption Leaf Structure Blade expanded thin structure on either side of the midrib and usually the largest, most conspicuous part of a leaf Leaf Structure Petiole Stem-like appendage which holds the leaf away from the stem The base is attached to the stem at a node Vary in length or may be lacking entirely, in which case the leaf blade is described as sessile or stalkless Leaf Structure Leaf axil The node where a petiole meets a stem Contain axillary buds single buds or bud clusters Either active or dormant Will develop into stems or leaves under the right conditions Leaf Blade Structure A leaf blade is composed of several layers Epidermis Layer of thick, tough cells on the top and bottom Primary function is to protect the other layers of leaf tissue The arrangement of epidermal cells determines the leaf's surface texture Some leaves have hairs (pubescence), which are extensions of epidermal cells Cuticle Part of the epidermis Produces a waxy layer called cutin Protects the leaf from dehydration and disease The amount of cutin on a leaf increases with increasing light intensity Move plants from shade into full sunlight gradually to allow the cutin layer to build up and protect the leaves from rapid water loss or sunscald Repels water Many pesticides contain a spray additive to help the product adhere to, or penetrate, the cutin layer (surfactant) Guard Cells Special epidermal cells Open and close in response to environmental stimuli (changes in weather and light) Conditions that cause a loss of water (high temperature, low humidity), or the absence of light stimulate guard cells to close Regulate the passage of water, oxygen, and carbon dioxide into and out of the leaf through tiny openings called stomata In most species, the majority of the stomata are located on the underside of leaves Mesophyll Located between the upper and lower epidermis Divided into: dense upper layer (palisade mesophyll) lower layer that contains lots of air space (spongy mesophyll) Located within the mesophyll cells are chloroplasts Where photosynthesis takes place Types of Leaves There are many kinds of plant leaves: Foliage Scale leaves Seed leaves Spines and tendrils Modified Leaves Storage leaves Bracts Foliage Most common and conspicuous leaves Primary location of photosynthesis Scale leaves Cataphylls Found on rhizomes and buds Enclose and protect rhizomes and buds Seed leaves Cotyledons Found on embryonic plants Store food for the developing seedling Spines and Tendrils Found on plants such as barberry and pea plants Protect a plant Help support stems Storage Leaves Found on bulbous plants and succulents Store food Bracts Often are brightly colored The showy structures on dogwoods and poinsettias are bracts, not petals Venation The vascular bundles of xylem and phloem extend from the stem, through the petiole, and into the leaf blade as veins. The term venation refers to how veins are distributed in the blade. There are two principal types of venation: parallel-veined and net-veined Venation Parallel-Veined Leaves Numerous veins run essentially parallel to each other and are connected laterally by minute, straight veinlets Occur most often on monocotyledonous plants The most common type of parallel veining is found in plants of the grass family, whose veins run from the leaf's base to its apex Another type of parallel venation is found in plants such as banana whose veins run laterally from the midrib Net-Veined Leaves Also called reticulate-veined Veins branch from the main rib or ribs and subdivide into finer veinlets then unite in a complicated network More resistant to tearing Occur on dicotyledonous plants Pinnate vs. Palmate Either Pinnate The veins extend laterally from the midrib to the edge Apples, cherries, and peaches Palmate The principal veins extend outward, like the ribs of a fan, from the base of the leaf blade Grapes and maples Common Blade Shapes Lanceolate Longer than wide and tapering toward the apex and base Linear Narrow, several times longer than wide, and of approximately the same width throughout Common Blade Shapes Cordate Heart-shaped Broadly ovate, tapering to an acute apex, with the base turning in and forming a notch where the petiole is attached Common Blade Shapes Elliptical About two or three times as long as wide, tapering to an acute or rounded apex and base Ovate Egg-shaped, basal portion wide, tapering toward the apex Common Margin Forms Entire Smooth edge with no teeth or notches Crenate Rounded teeth Dentate Teeth ending in an acute angle pointing outward Common Margin Forms Serrate Small, sharp teeth pointing toward the apex Incised Sharp, deep, irregular teeth or incisions Lobed Incisions that extend less than halfway to the midrib Common Apex Shapes Acute Ending in an acute angle, with a sharp, but not acuminate, point Acuminate Tapering to a long, narrow point Obtuse Tapering to a rounded edge Common Base Shapes Cuneate Wedge-shaped; triangular with the narrow end at the point of attachment Obtuse Tapering to a rounded edge Cordate Turning in and forming a notch Plant Identification Leaves are useful for plant identification: Leaf's venation Blade and margin shapes Apex and base shape Leaf type Leaf arrangement Leaf Type Simple leaves Leaf blade is a single, continuous unit Compound leaves Composed of several separate leaflets arising from the same petiole Some leaves are doubly compound Leaf type can be confusing, because a deeply lobed simple leaf may look like a compound leaf Leaf Arrangement Opposite Positioned across the stem from each other, with two leaves at each node Alternate (spiral) Leaves are arranged in alternate steps along the stem, with only one leaf at each node Leaf Arrangement Whorled Arranged in circles along the stem Rosulate Arranged in a rosette around a stem with extremely short nodes Leaves as Food Leaf blade Chives, collards, dandelions, endives, kale, leaf lettuce, mustard, parsley, spinach and Swiss chard Cluster of fleshy leaf bases Leeks, onions, and Florence fennel Petiole Celery and rhubarb