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
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.ualr.edu/~botany/leaf_types.gif
Basic Leaf Morphology
http://www.vancouver.wsu.edu/fac/robson/cl/natrs301/anatomy/petiole.htm
Pattern of Growth in Leaves
1.
2.
3.
4.
Determinate growth (after maturity growth ceases)
New leaves - produced from leaf primordia in the
shoot apical meristem.
Leaves comprised of dermal, cortex, and vascular
tissues
Why is it adaptive for a photosynthetic organ to be thin
and flat?
The Origin of
the Leaf
http://www.esb.utexas.edu/mauseth/w
eblab/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).
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.
4.
Dermal tissue - upper and lower epidermis.
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 (netlike) 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.
4.
Dermal tissue - upper and lower epidermis.
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
Bifacial Leaf - two sides
are different
Plant Anatomy CD
Unifacial Leaf - Two sides are
mirror images (more or
less…)
http://www.botany.hawaii.edu/faculty/webb/BishopWeb/KoaLeafComboXS500.jpg
Epidermi
s
1. abaxial & adaxial
2. stomata, flanked by
guard cells
3. Epi-, hypo-, or
amphistomatous
4. 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
Cross Section Through a
Dicot Leaf (bifacial)
http://www.park.edu/bhoffman/courses/bi225/recaps/leavesii.htm
http://www.ualr.edu/~botany/leaf_cs.jpg
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
Internal Anatomy of a Pine Leaf
1.
Pine leaves ("needles") - low moisture
(e.g. frozen ground in winter)
epidermis
hypodermis -beneath the epidermis
2.
3.
–
–
–
4.
5.
6.
7.
8.
one or more layers of thick-walled cells
support and rigidity
protection
mesophyll - not divided into palisade
and spongy layers.
transfusion tissue - surround xylem
and phloem
endodermis - outer boundary of the
transfusion tissue
resin canals - circular to elliptical cells
in mesophyll (cells lining canal secrete
resin)
sunken stomatal pores (common in
desert plants)
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
cuticle
water-storage parenchyma
palisade mesophyll
(Kranz-mesophyll) cell
bundle sheath cell
vascular bundle
stomata
Examples of Xeromorphic Leaves (Esau 1977)
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