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Plant Biology - The Shoot System Leaves

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    Study Guide Plant Biology – The Shoot System: Leaves 1. External Features, Origin, and Internal Structure 1. 1 External Features of Leaves (Basic Leaf Terms) Botanists use many specific terms to describe leaves. These terms help scientists correctly identify and classify plants. While the vocabulary may seem heavy, each term describes something simple and visible. Leaf Arrangement on the Stem Leaves are attached to the stem at points called nodes . Based on how many leaves grow at each node, we see three main patterns: • Alternate : One leaf per node • Opposite : Two leaves per node • Whorled : Three or more leaves per node Simple and Compound Leaves • A simple leaf has one blade attached to a petiole (leaf stalk). • A compound leaf has its blade divided into smaller units called leaflets .

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    Study Guide Compound leaves can be: • Pinnate : Leaflets arranged along a central axis (rachis) • Palmate : Leaflets spread out from a single point Scientists 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 Leaf Leaves begin forming at the shoot apex (the growing tip of the stem). They develop from cells just below the outermost layer, called the protoderm . Leaf Primordium Formation • Rapid cell division creates a small bump called a leaf primordium . • Special growing regions form the future upper and lower surfaces of the leaf blade. Connection to Stem Vascular Tissue A strand of conducting tissue called the leaf trace grows from the stem into the developing leaf. This ensures continuity of xylem and phloem throughout the plant. At the point where the leaf trace leaves the stem, a leaf gap appears. • Despite the name, this is not 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 called determinate growth . In contrast, the shoot apex grows continuously ( indeterminate growth ).

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    Study Guide 1. 3 Internal Structure of a Typical Leaf A typical leaf has three main tissue regions : 1. Epidermis 2. Mesophyll 3. Vascular tissue (veins) 1. 4 Epidermis – The Protective Outer Layer The epidermis is a single layer of tightly packed cells covering both surfaces of the leaf. Key Features: • Cells fit together like paving stones • Usually no chloroplasts , except in guard cells • Covered by a waxy cuticle (made of cutin and wax) that reduces water loss Trichomes (Leaf Hairs) • Extensions of epidermal cells • Can be hairs or scales • Help protect the leaf by: o Deterring herbivores o Producing toxic or unpleasant chemicals 1. 5 Stomata – Openings for Gas Exchange A stoma is a tiny pore that allows gases and water vapor to move in and out of the leaf. Structure of a Stoma: • Two kidney - shaped guard cells • One pore between them • Surrounded by subsidiary cells

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    Study Guide How Stomata Open and Close: • When guard cells absorb water, they become turgid → pore opens • When they lose water, they become flaccid → pore closes This movement controls: • Water loss (transpiration) • Oxygen and carbon dioxide exchange Distribution of Stomata: • May be on: o Both surfaces o Only upper or lower surface • Submerged aquatic plants usually lack stomata entirely 1. 6 Mesophyll – The Photosynthesis Zone The mesophyll makes up most of the leaf and contains many chloroplasts. It lies between the upper and lower epidermis. Palisade Mesophyll (Upper Layer) • Long, column - shaped cells • Tightly packed • Contains more chloroplasts • Main site of photosynthesis Spongy Mesophyll (Lower Layer) • Irregular - shaped cells • Loosely arranged • Large air spaces for gas exchange Together, these tissues form chlorenchyma (chloroplast - rich parenchyma). The spongy mesophyll also functions as aerenchyma because of its air spaces. Water loss and gas exchange occur at the wet surfaces of mesophyll cells . Stomata simply act as doorways to the outside.

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    Study Guide 1. 7 Supporting Tissues in the Mesophyll Leaves also need strength and support. • Collenchyma : Strengthens veins • Sclereids : Hard cells scattered in the mesophyll • Fibers : Common in monocot leaves These tissues prevent the leaf from collapsing or tearing. 1. 8 Veins (Vascular Tissue) – Transport System of the Leaf Veins 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 a bundle sheath (parenchyma or fibers) Additional Support: • Bundle sheath extensions connect veins to the epidermis • Provide extra strength to the leaf blade As veins branch, they become thinner and end near mesophyll cells. This ensures: • Quick delivery of raw materials • Easy transport of photosynthetic products The bundle sheath also helps retain materials within the vascular system.

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