Transcript Understanding Our Environment

Leaves
Outline
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Overview
Leaf Arrangements and Types
Internal Structures of Leaves
 Stomata
 Mesophyll and Veins
Specialized Leaves
Autumnal Changes in Color
Abscission
Relevance of Leaves
Overview
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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).
Overview
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Pinnately compound leaves have leaflets in
pairs along the rachis (extension of the
petiole), 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.
Overview
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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 of
hydathodes (openings at tip of leaf veins).
Leaves are sealed off at the base of their petiole
and drop during fall (abscission)
Leaf Arrangements and Types
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Leaves vary in:
- in size: duckweeds (1 mm wide) to palms (6 meters
long)
- shapes: flattened, tubular, feathery, cup-shaped,
spine-like, needle-like
- texture: smooth, hairy, slippery, sticky, waxy, glossy
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Leaves are attached to stems at nodes, with stem regions
between nodes known as internodes.
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Leaf arrangement on stem (Phylotaxy) generally occurs in
one of three ways:
- Alternate (spiral): one leaf per node
- Opposite: two leaves per node
- Whorled: three or more leaves per node
Leaf Arrangements and Types
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Arrangement of veins in a leaf or leaflet blade may
also be pinnate or palmate.
 Pinnately veined leaves have one primary vein
called 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 (netted or reticulate
venation)
 Dichotomous venation: veins fork evenly and
progressively from base of blade (Gingko leaf)
Fig. 7.4
Internal Structure of Leaves
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Epidermis is a single layer of cells covering
the entire surface of the leaf.
 Upper epidermal cells are devoid of
chloroplasts.
 Waxy cutin called the cuticle is often
present
 Different glands (occur in the form of
depressions, protuberances, or
appendages on the leaf surface) may also
be present in the epidermis. Glands
secrete stick substances.
Stomata
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Lower epidermis of most plans is perforated by numerous
stomata (opening formed between two guard cells).
 Guard cells originate from the same parent cell as the
rest of the epidermis but differ in that they contain
chloroplasts.
- Primary function includes regulating gas exchange
between leaf interior and the atmosphere, and
regulating the evaporation of water entering the plant
at the roots.
 Guard cells are surrounded by thickened but
flexible cell wall. Water pressure inside each pair
of guard cells regulates their expansion or deflate
resulting in the formation of an opening (stoma)
between guard cells when they expand or the
collapse of the opening when they deflate.
Mesophyll and Veins
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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
 Monocot leaves usually do not have mesophyll
differentiated into palisade and spongy.
Veins (Vascular bundles) are scattered throughout the
mesophyll.
 Consist of xylem and phloem tissues surrounded by the
bundle sheath.
 Some monocot leaves (grasses) have large, thin-walled
bulliform cells on either side of the midrib toward the
upper surface. Under dry conditions bulliform cells partly
collapse causing the leaf blade to fold or roll which in turn
reduces transpiration.
Fig. 7.10
Specialized Leaves
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Shade Leaves
 Since they receive less total light, they tend to be
thinner, have fewer mesophyll layers and
chloroplasts, and fewer hairs than leaves on the
same tree exposed to direct light (sun leaves).
Leaves of Arid Regions
 Many have thick cuticle, leathery leaves and few
and sunken stomata.
 Some have succulent, water-retaining leaves, or
dense, hairy coverings.
 Pine needles have a thick cuticle and a layer of
thick-walled cells (hypodermis) beneath the
epidermis.
Fig. 7.11b
Fig. 7.12
Specialized Leaves
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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.
Fig. 7.14
Specialized Leaves
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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 (e.g. honey
locust).
 Prickles - Outgrowths from the epidermis
or cortex (e.g. roses, raspberries).
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Required for Reproduction or Display
Specialized Leaves
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Storage Leaves – Succulent leaves, retain water in
large, thin-walled parenchyma with no chloroplasts.
Fleshy leaves (e.g. onions, lily bulbs) store
carbohydrates.
Flower-Pot Leaves - Urn-Like Pouches which
become home to ant colonies. Ants carry in soil and
waste products and the flower pot then produces
adventitious roots.
Window Leaves – In some plants of Kalahari desert
(South Africa) leaves are shaped like ice cream
cones and are buried in ground.
Reproductive Leaves - New plants at tips.
Floral Leaves – Bracts (e.g. poinsettia).
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Specialized Leaves
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Insect-Trapping Leaves
 Pitcher Plants
 Sundews
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Specialized Leaves
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Insect-Trapping Leaves
 Venus’s Flytraps
 Bladderworts
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Autumnal Changes in Leaf Color
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Cholorplasts of mature leaves contain
several groups of pigments.
 Chlorophylls - Green
 Carotenoids – carotenes (yellow) and
xanthophylls (pale yellow).
Water soluble anthocyanins (red or blue) and
betacyanins (red) may also be present in the
vacuole.
Mechanism of fall color development
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Water soluble anthocyanins accumulate in the
vacuoles. If the cell sap is acidic it turns red, if
its alkaline it turns blue.
A reduction in daylight causes the green
chlorophyll to break and the yellow color of
carotenoids and pale yellow color of xanthophylls
to predominate.
Leaf Abscission
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Deciduous plants drop their leaves seasonally.
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At the abscission zone near the base of the petiole two
layers of cells are differentiated due to hormonal changes
associated with aging.
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One layer (several cells thick) at the stem side forms the
protective layer whose cells are coated with suberin.
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On the leaf side, a separation layer is differentiated. The
cells swell and become gelatinous. Due to seasonal
changes (low temperature, shorter days, less light intensity)
the pectins of the middle lamella break down by enzymes
making the attachement of leaf to stem very loose. Wind or
rain can then break the leaf off the stem.
Leaf Abscission Zone
Human and Ecological Relevance of Leaves
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Landscaping
Food
Dyes
Ropes and Twine
Drugs
 Tobacco
 Marijuana
Insecticides
Waxes
Review
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Overview
Leaf Arrangements and Types
Internal Structures of Leaves
 Stomata
 Mesophyll and Veins
Specialized Leaves
Autumnal Changes in Color
Abscission
Relevance of Leaves
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