1. 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

2. Leaf Shapes and Functions
Photosynthesis
Evapotranspiration
Minimizes desiccation via cutin, epidermal hairs, and stomata
Export nutrients
Storage of water
Defense
Anchorage (tendrils)
Insect capture

3. Basic Leaf Morphology

4. 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?

5. 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).

6. 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

7. Dichotomous venation in Ginkgo
Common in ferns - ancestral

8. Reticulate (net-like) venation
Crang & Vassilev Plant Anatomy CD

9. Dicot vs. Monocot Veination

10. 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

12. Bifacial and Unifacial Leaves
Esau 1977

13. Bifacial Leaf - two sides are different
Plant Anatomy CD

14. Unifacial Leaf - Two sides are mirror images (more or less…)

15. 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

16. Buliform Cells http://www. esb. utexas

18. 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

19. Cross Section Through a Dicot Leaf (bifacial)

20. Differentiation of Mesophyll
Esau 1977

21. Vascular bundles (veins)
often enclosed by bundle sheaths of sclerenchyma fibers - why?
2. xylem on adaxial, phloem on abaxial side

22. Leaf Functions and Specializations
1. Sun vs shade leaves
sun leaves - smaller, thicker, more mesophyll layers
shade leaves - larger, thinner, fewer mesophyll layers

23. Shade and Sun leaveshttp://www. lima. ohio-state

24. Leaf Functions and Specializations, continued
2. Extreme environments
abscission
hydrophytes (aquatic plants)
xerophytes (desert plants)

25. Water Lily Leaf

26. 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)

27. 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

28. Diagram of a typical leaf. Typical C3 leaf, that is.
C4 typical leaf with photosynthetic cells in concentric rings around the vascular bundles.

29. Esau 1977

30. Krantz Anatomy cuticle water-storage parenchyma
palisade mesophyll (Kranz-mesophyll) cell
bundle sheath cell
vascular bundle
stomata

31. Examples of Xeromorphic Leaves (Esau 1977)

32. 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

33. http://www. soasoas. com/april/gallery/viewImg2. cgi