The developmental origin of leaves Earliest vascular plants had no leaves Leaves have evolved at least twice -- microphylls and megaphylls 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 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 Origin - leaf primordium at the shoot apical meristem. exogenous from the outer edge (vs endogenous in lateral root). primordia attached to stem nodes primordia arch over the zone of cell division (protection from herbivory and desiccation). http://www.esb.utexas.edu/mauseth/weblab/webchap6apmer/6.1-1.htm

Basic Anatomical Features 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. Blade has prominent midvein center of the leaf major "artery" of the leaf Parallel or reticulate 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 (net-like) venation Crang & Vassilev Plant Anatomy CD

Dicot vs. Monocot Veination

Basic Anatomical Features 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. Blade has prominent midvein center of the leaf major "artery" of the leaf Parallel or reticulate 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

Epidermis 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

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) 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

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 Pine leaves ("needles") - low moisture (e.g. frozen ground in winter) epidermis hypodermis -beneath the epidermis 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 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 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)

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 http://www.soasoas.com/april/gallery/viewImg2.cgi?dir=sanDiego&id=Baby_Cactus_Spines

http://www.sarracenia.com/photos3/dmusc55.jpg