Leaves
In this chapter you will learn about leaves by comparing
them with solar panels and by discussing their general functions, morphology,
and dimensions. You will learn about basic leaf types and specific forms and
arrangements as well as the internal structure of leaves, including epidermis
and cuticle, stomata, glands, mesophyll, and veins. You will learn about
specialized leaves, including tendrils, spines, flower pot leaves, window
leaves, reproductive leaves, floral leaves, and insectivorous leaves. You will
learn about autumnal color changes and abscission and make some observations on
the human and ecological relevance of leaves.
At the end of this chapter the successful student will be able to
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- All leaves originate as primordia in the buds.
- At maturity, most leaves have a stalk (petiole) and a flattened blade
(lamina) with a network of veins (vascular bundles).
- Leaves of flowering plants are associated with leaf gaps and have an
axillary bud at the base.
- May be simple (single blade) or compound (divided into leaflets).
- Pinnately compound leaves have leaflets in pairs along the rachis, while
palmately compound leaves have all the leaflets attached at the same point at
the end of the petiole.
- Pinnately compound leaves may be further subdivided an thus be referred
to as bipinnately compound.
- Leaves are attached to stems at nodes, with stem regions
between nodes known as internodes.
- Phylotaxy (leaf arrangement) generally occurs in one of three ways:
- Alternate
- Opposite
- Whorled
- Arrangement of veins in a leaf or leaflet blade may also be pinnate or
palmate.
- Pinnately veined leaves have a main midvein within a midrib.
- Secondary veins branch from midvein.
- Palmately veined leaves have several primary veins that fan out from
the base of the blade.
- Parallel in monocots
- Divergent in dicots (reticulate venation)
- Recording information about leaves in our lab notebook (click the small
picture for a closer look)
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- Green leaves capture sunlight and thus go through photosynthesis.
- Lower surfaces of leaves are dotted with stomata which allow carbon
dioxide to enter and oxygen and water to diffuse out.
- Guard Cells control stomatal opening.
- Transpiration occurs when water evaporates from the leaf surface.
- Guttation - Root pressure forces water out hydathodes.
- Epidermis is a single layer of cells covering the entire surface of the
leaf.
- Upper epidermal cells are devoid of chloroplasts.
- Waxy cuticle often present.
- Different glands may also be present in the epidermis
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- Stomata
- Lower epidermis of most plans is perforated by numerous stomata.
- Guard cells originate from the same parent cell, and contain chloroplasts.
- Primary function includes regulating gas exchange between leaf interior
and the atmosphere, and the evaporation of water.
- Cell water pressure regulates guard cells which in turn regulate stomata.
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Mesophyll and Veins
- Most photosynthesis takes place in the mesophyll between the two epidermal
layers.
- Palisade Mesophyll - Uppermost layer Contain most of leaf’s chloroplasts.
- Spongy Mesophyll - Lower layer
- Veins (Vascular bundles) are scattered throughout the mesophyll.
- Consist of xylem and phloem tissues surrounded by the bundle sheath.
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- Shade Leaves
- Leaves in the shade receive less total light, thus tend to be thinner
and have fewer hairs than leaves on the same tree exposed to direct light.
- Leaves of Arid Regions
- Many have thick, leathery leaves and few stomata.
- Some have succulent, water-retaining leaves, or dense, hairy coverings.
- Tendrils
- Modified leaves that curl around more rigid objects helping the plant to
climb or support weak stems.
- Become coiled like a spring as they develop.
- When contact is made, the tip curls around the object, and the
direction of the coil reverses.
- Spines, Thorns, and Prickles
- Spines - Modified leaves designed to reduce water loss and protect from
herbivory.
- Thorns - Modified stems arising in the axils of leaves of woody plants.
- Prickles - Outgrowths from the epidermis or cortex.
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- Storage Leaves - Succulents
- Flower-Pot Leaves - Urn-Like Pouches
- Window Leaves - Leaves buried in ground.
- Reproductive Leaves - New plants at tips.
- Floral Leaves - Bracts
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- Insect-Trapping Leaves
- Insect-Trapping Leaves
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- Autumnal Changes in Leaf Color
- Cholorplasts of mature leaves contain several groups of pigments.
- Chlorophylls - Green
- Carotenoids - Yellows
- In fall, chlorophylls break down and other colors are revealed.
- Water soluble anthocyanins (red or blue) and betacyanins (red) may also be
present in the vacuole.
- Abscission
- Deciduous plants drop their leaves seasonally.
- Occurs as a result of changes in an abscission zone near the base of the
petiole of each leaf.
- Cells of the protective layer become coated and impregnated with suberin.
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- Landscaping
- Food
- Dyes
- Ropes and Twine
- Drugs
- Insecticides
- Waxes
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Last modified:
October 08, 2004 by
Cynthia Herbrandson 
© Copyright 1999, Kellogg Community College.
All rights reserved.
