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