Plant Biology - The Shoot System Leaves - Quick Guides

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Study GuidePlant Biology–The Shoot System: Leaves1. External Features, Origin, and Internal Structure1.1 External Features of Leaves (Basic Leaf Terms)Botanists use many specific terms to describe leaves. These terms help scientists correctly identifyand classify plants. While the vocabulary may seem heavy, each term describes something simpleand visible.Leaf Arrangement on the StemLeaves are attached to the stem at points callednodes. Based on how many leaves grow at eachnode, we see three main patterns:•Alternate: One leaf per node•Opposite: Two leaves per node•Whorled: Three or more leaves per nodeSimple and Compound Leaves•Asimple leafhas one blade attached to a petiole (leaf stalk).•Acompound leafhas its blade divided into smaller units calledleaflets.

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Study GuideCompound leaves can be:•Pinnate: Leaflets arranged along a central axis (rachis)•Palmate: Leaflets spread out from a single pointScientists also describe leaves using terms for:•Vein pattern (venation)•Overall shape•Tip shape•Leaf margin (smooth, toothed, or lobed)•Surface texture (hairy or smooth, upper or lower side)1.2 Origin and Development of a LeafLeaves begin forming at theshoot apex(the growing tip of the stem). They develop from cells justbelow the outermost layer, called theprotoderm.Leaf Primordium Formation•Rapid cell division creates a small bump called aleaf primordium.•Special growing regions form the futureupper and lower surfacesof the leaf blade.Connection to Stem Vascular TissueA strand of conducting tissue called theleaf tracegrows from the stem into the developing leaf. Thisensures continuity ofxylem and phloemthroughout the plant.At the point where the leaf traceleaves the stem, aleaf gapappears.•Despite the name, this isnot a hole.•It is simply an area without xylem and phloem, filled with parenchyma cells.Growth Pattern of Leaves•The lower (abaxial) surface grows faster than the upper (adaxial) surface.•This causes the young leaf to bend inward, protecting the shoot tip.Leaves stop dividing cells early and then grow only by cell expansion.This is calleddeterminategrowth.In contrast, the shoot apex grows continuously (indeterminate growth).

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Study Guide1.3 Internal Structure of a Typical LeafA typical leaf hasthree main tissue regions:1.Epidermis2.Mesophyll3.Vascular tissue (veins)1.4 Epidermis–The Protective Outer LayerTheepidermisis a single layer of tightly packed cells covering both surfaces of the leaf.Key Features:•Cells fit together like paving stones•Usuallyno chloroplasts, except inguard cells•Covered by a waxycuticle(made of cutinand wax) that reduces water lossTrichomes (Leaf Hairs)•Extensions of epidermal cells•Can be hairs or scales•Help protect the leaf by:oDeterring herbivoresoProducing toxic or unpleasant chemicals1.5 Stomata–Openings for Gas ExchangeAstomais a tiny pore that allows gases and water vapor to move in and out of the leaf.Structure of a Stoma:•Twokidney-shaped guard cells•One pore between them•Surrounded bysubsidiary cells

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Study GuideHow Stomata Open and Close:•When guard cells absorb water, they becometurgid→ pore opens•When they lose water, they becomeflaccid→ pore closesThis movement controls:•Water loss (transpiration)•Oxygen and carbon dioxide exchangeDistribution of Stomata:•May be on:oBoth surfacesoOnly upper or lower surface•Submerged aquatic plantsusually lack stomata entirely1.6 Mesophyll–The Photosynthesis ZoneThemesophyllmakes up most of the leaf and contains many chloroplasts. It lies between the upperand lower epidermis.Palisade Mesophyll (Upper Layer)•Long, column-shaped cells•Tightly packed•Containsmore chloroplasts•Main site of photosynthesisSpongy Mesophyll (Lower Layer)•Irregular-shaped cells•Loosely arranged•Large air spaces for gas exchangeTogether, these tissues formchlorenchyma(chloroplast-rich parenchyma).The spongy mesophyllalso functions asaerenchymabecause of its air spaces.Water loss and gas exchange occur at thewet surfaces of mesophyll cells.Stomata simply act asdoorwaysto the outside.

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Study Guide1.7 Supporting Tissues in the MesophyllLeaves also need strength and support.•Collenchyma: Strengthens veins•Sclereids: Hard cells scattered in the mesophyll•Fibers: Common in monocot leavesThese tissues prevent the leaf from collapsing or tearing.1.8 Veins (Vascular Tissue)–Transport System of the LeafVeins form a branching network that connects the leaf to the stem and roots.Structure of a Vein:•Xylem: Upper side → carries water and minerals•Phloem: Lower side → carries sugars•Surrounded by abundle sheath(parenchyma or fibers)Additional Support:•Bundle sheath extensionsconnect veins to the epidermis•Provide extra strength to the leaf bladeAs veins branch, they become thinner and end near mesophyll cells.This ensures:•Quick delivery of raw materials•Easy transport of photosynthetic productsThe bundle sheath also helpsretain materialswithin the vascular system.

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