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The developmental origin of leaves
1. Earliest vascular plants had no leaves 2. Leaves have evolved at least twice -- microphylls
and megaphylls
3. Microphyll origins a. small projections formed called enations b. later, single vascular strand grew toward and into the
enation c. result is a microphyll, with single unbranched vein d. found only in one group of plants (Lycophyta)
4. Megaphyll origins a. ancestors had dichotomous branching b. ferns & all seed plants
http://www.ualr.edu/~botany/leaf_types.gif
Leaf Shapes and Functions
• Photosynthesis• Evapotranspiration• Minimizes
desiccation via cutin, epidermal hairs, and stomata
• Export nutrients• Storage of water• Defense • Anchorage (tendrils) • Insect capture
http://www.vancouver.wsu.edu/fac/robson/cl/natrs301/anatomy/petiole.htm
Basic Leaf Morphology
Pattern of Growth in Leaves
1. Determinate growth (after maturity growth ceases)
2. New leaves - produced from leaf primordia in the shoot apical meristem.
3. Leaves comprised of dermal, cortex, and vascular tissues
4. Why is it adaptive for a photosynthetic organ to be thin and flat?
http://www.esb.utexas.edu/mauseth/weblab/webchap6apmer/6.1-1.htm
1. Origin - leaf primordium at the shoot apical meristem. exogenous from the outer edge (vs endogenous in lateral root).
2. primordia attached to stem nodes
3. primordia arch over the zone of cell division (protection from herbivory and desiccation).
The Origin of the Leaf
Basic Anatomical Features
1. Vascular tissue restricted to the veins. – every cell in close proximity to a minor vein– move water to and also move photosynthate out of
each and every cell. 2. Blade has prominent midvein
• center of the leaf • major "artery" of the leaf• Parallel or reticulate
3. Dermal tissue - upper and lower epidermis.
4. Ground tissue = mesophyll, – palisade mesophyll = upper layer of elongated,
vertically arranged cells– spongy mesophyll = lower layer of loosely organized
cells with significant intercellular air spaces
Dichotomous venation in Ginkgo
Common in ferns - ancestral
Reticulate (net-like) venation
Crang & Vassilev Plant Anatomy CD
Dicot vs. Monocot Veination
Basic Anatomical Features
1. Vascular tissue restricted to the veins. – every cell in close proximity to a minor vein– move water to and also move photosynthate out of
each and every cell. 2. Blade has prominent midvein
• center of the leaf • major "artery" of the leaf• Parallel or reticulate
3. Dermal tissue - upper and lower epidermis.
4. Ground tissue = mesophyll, – palisade mesophyll = upper layer of elongated,
vertically arranged cells– spongy mesophyll = lower layer of loosely organized
cells with significant intercellular air spaces
http://www.ualr.edu/~botany/leafstru.gif
Bifacial and Unifacial Leaves
Esau 1977
Plant Anatomy CD
Bifacial Leaf - two sides are different
http://www.botany.hawaii.edu/faculty/webb/BishopWeb/KoaLeafComboXS500.jpg
Unifacial Leaf - Two sides are mirror images (more or less…)
Epidermis
1. abaxial & adaxial 2. stomata, flanked by
guard cells 3. Epi-, hypo-, or
amphistomatous4. cuticle 5. specialized
epidermal cells a. buliform cells b. trichomes,
glands
Buliform Cells http://www.esb.utexas.edu/mauseth/weblab/webchap10epi/10.5-
3.htm
Mesophyll tissue
1. mesophyll - "middle of the leaf"
2. palisade mesophyll a. located on adaxial side b. may contain more than 80% of the leaf's plastids c. controls light intensity and damage by reducing light
passing through
3. spongy mesophyll a. spongy appearance because of air spaces, allowing free
gas flow b. primary site of photosynthesis in vascular plants
http://www.park.edu/bhoffman/courses/bi225/recaps/leavesii.htm
http://www.ualr.edu/~botany/leaf_cs.jpg
Cross Section Through a Dicot Leaf (bifacial)
Differentiation of Mesophyll
Esau 1977
Vascular bundles (veins)
1. often enclosed by bundle sheaths of sclerenchyma fibers - why?
2. xylem on adaxial, phloem on abaxial side
Leaf Functions and Specializations
1. Sun vs shade leaves – sun leaves - smaller, thicker, more
mesophyll layers – shade leaves - larger, thinner, fewer
mesophyll layers
Shade and Sun leaveshttp://www.lima.ohio-state.edu/biology/images/shadleaf.jpg
Leaf Functions and Specializations, continued
2. Extreme environments • abscission • hydrophytes (aquatic plants) • xerophytes (desert plants)
Water Lily Leaf
http://images.google.com/imgres?imgurl=www.botany.hawaii.edu
1. Pine leaves ("needles") - low moisture (e.g. frozen ground in winter)
2. epidermis 3. hypodermis -beneath the
epidermis– one or more layers of thick-walled
cells – support and rigidity– protection
4. mesophyll - not divided into palisade and spongy layers.
5. transfusion tissue - surround xylem and phloem
6. endodermis - outer boundary of the transfusion tissue
7. resin canals - circular to elliptical cells in mesophyll (cells lining canal secrete resin)
8. sunken stomatal pores (common in desert plants)
Internal Anatomy of a Pine Leaf
http://www.ualr.edu/~botany/leaf_lab.html
Krantz Anatomy in C4 Plants Two stages of carbon fixation
1. Stage 1 - in MESOPHYLL CELL temporary fixation of CO2 cytoplasm into C4 molecule (no direct involvement of chloroplasts)• Transferred through plasmodesmata to the
bundle sheath cells
2. Stage 2 - in BUNDLE SHEATH CELL• C4 molecules broken down to CO2 again. • chloroplasts fix the CO2 into C3 intermediates to
build sugars
Diagram of a typical leaf. Typical C3 leaf, that is.
C4 typical leaf with photosynthetic cells in concentric rings around the vascular bundles.
Esau 1977
Krantz Anatomy
vascular bundle
bundle sheath cell
palisade mesophyll (Kranz-mesophyll) cell
water-storage parenchyma
cuticle
stomata
Esau 1977
Examples of Xeromorphic Leaves (Esau 1977)
Leaf Functions and Specializations, continued
3. Other leaf specializations – tendrils - elongated leaves for climbing
and attaching – spines - sharp stiff leaves for defense – bracts - floral leaves; often colorful to
attract pollinators– carnivory - leaf is modified to trap
insects for trace nutrients
http://www.soasoas.com/april/gallery/viewImg2.cgi?dir=sanDiego&id=Baby_Cactus_Spines
http://www.sarracenia.com/photos3/dmusc55.jpg