1. The Shoot System II: The Form and Structure of Leaves
Chapter 8

2. Functions of Leaves Photosynthesis Transpiration Specialized functions
Release oxygen, synthesize sugars
Transpiration
Evaporation of water from leaf surface
Specialized functions
Water storage
Protection

3. Comparison of Monocot and Dicot Leaves
Type
Shape of blade
Venation
Description
Monocot
Strap-shaped *blade
Parallel vascular bundles
Leaf bases usually wrap around stem
Dicot
Thin, flat blade
Netted pattern of vascular bundles
Petiole holds blade away from stem
*blade – portion of leaf that absorbs light energy

4. Leaf Blade Broad, flat surface for capturing light and CO2
Two types of leaves
Simple leaves
Compound leaves

5. Leaf Blade Simple leaves Leaves with a single blade Examples Poplar
Oak
Maple

6. Leaf Blade Compound leaves Blade divided into leaflets Two types
Palmately compound
Leaflets diverge from a single point
Example: red buckeye
Pinnately compound
Leaflets arranged along an axis
Examples: black locust, honey locust

7. Leaf Blade Advantages of compound leaves
Spaces between leaflets allow better air flow over surface
May help cool leaf
May improve carbon dioxide uptake

8. Petiole Narrow base of most dicot leaves
Leaf without petiole – sessile
Vary in shape
Improves photosynthesis
Reduces extent to which leaf is shaded by other leaves
Allows blade to move in response to air currents

9. Sheath Formed by monocot leaf base wrapping around stem Ligule
Keeps water and dirt from getting between stem and leaf sheath
Auricles
In some grass species
Two flaps of leaf tissue
Extend around stem at juncture of sheath and blade

10. Sheath Why does grass need mowing so often?
Grass grows from base of sheath
Intercalary meristem
Allows for continued growth of mature leaf
Stops dividing when leaf reaches certain age or length

11. Leaf Veins Vascular bundles composed of xylem and phloem
Type of venation
Example
Description
Parallel
Monocots
Several major veins running parallel from base to tip of leaf
Minor veins perpendicular to major veins
Netted
Dicots
Major vein (midvein or midrib) runs up middle of leaf
Lateral veins branch from midvein
Open dichotomous
Ferns and some gymnosperms
Y-branches with no small interconnecting veins

12. Epidermis Covers entire surface of blade, petiole, and leaf sheath
Continuous with stem epidermis
Usually a single layer of cells
Cell types
Epidermal cells
Guard cells
Subsidiary cells
Trichomes

13. Epidermal Cells Appear flattened in cross-sectional view
Outer cell wall somewhat thickened
Covered by waxy cuticle
Inhibits evaporation through outer epidermal cell wall

14. Stomatal Apparatus Cuticle blocks most evaporation
Opening needed in epidermis for controlled gas exchange
Two guard cells + pore stoma
Subsidiary cells
Surround guard cells
May play role in opening and closing pore

15. Stomatal Apparatus Guard cells + subsidiary cells stomatal apparatus
Functions of stoma
Allows entry of CO2 for photosynthesis
Allows loss of water vapor by transpiration
Cools leaf by evaporation
Pulls water up from roots

16. Stomatal Apparatus Stomata usually more numerous on bottom of leaf
Stomata also found in
Epidermis of young stem
Some flower parts

17. Trichomes Secretory Short hairs
Stalk with multicellular or secretory head
Secretion often designed to attract pollinators to flowers
Short hairs
Example: saltbush (Atriplex)
Hairs store water, reflect sunlight, insulate leaf against extreme desert heat

18. Trichomes Mat of branched hairs Specialized trichomes
Example: olive tree (Olea europea)
Act as heat insulators
Specialized trichomes
Leaves modified to eat insects as food

19. Mesophyll Two distinct regions in dicot leaf Substomatal chamber
Palisade mesophyll
Spongy mesophyll
Substomatal chamber
Air space just under stomata

20. Mesophyll Type Cell type Location Description Palisade mesophyll
Palisade parenchyma, tightly packed, column shaped, oriented at right angles to leaf surface
Usually on upper surface
Cells tightly packed, absorb sunlight more efficiently
Spongy mesophyll
Spongy parenchyma cells, irregularly shaped, abundant air spaces
Usually located on bottom surface
Irregular cell shape, abundant air spaces allow more efficient air exchange

21. Mesophyll Dicot midrib (midvein) Bundle sheath
Xylem in upper part of bundle
Phloem in lower part of bundle
Bundle sheath
Single layer of cells surrounding vascular bundle
Loads sugars into phloem
Unloads water and minerals out of xylem

22. Formation of New Leaves
Originate from meristems
Leaf primordia – early stages of development

23. Formation of New Leaves
Steps in leaf formation
Initiated by chemical signal
Location in leaf depends on plant’s phyllotaxis
Cells at location begin dividing
Becomes leaf primordium
Shape of new leaf determined by how cells in primordium divide and enlarge

24. Cotyledons Seed leaves Primarily storage organs
Slightly flattened, often oval shaped
Usually wither and die during seedling growth
Example of exception – bean plant
Cotyledons enlarge and conduct photosynthesis

25. Heterophylly Different leaf shapes on a single plant
Types of heterophylly
Related to age of plant
Example: ivy (Hedera helix)
Juvenile ivy leaves – three lobes to leaves
Adult ivy leaves – leaves are not lobed

26. Heterophylly
Environment to which shoot apex is exposed during leaf development
Example: marsh plants
Water leaves
Leaves developing underwater are thin with deep lobes
Air leaves
Shoot tip above water in summertime develops thicker leaves with reduced lobing

27. Heterophylly Position of leaf on tree Shade leaves Sun leaves
Develop on bottom branches of tree
Mainly exposed to shade
Leaves are thin with large surface area
Sun leaves
Develop near top of same tree
Exposed to more direct sunlight
Leaves are thicker and smaller

28. Adaptations for Environmental Extremes
Xerophytes
Grow in dry climates
Leaves designed to conserve water, store water, insulate against heat
Sunken stomata
Thick cuticle
Sometimes multiple layers to epidermis

29. Adaptations for Environmental Extremes
Xerophytes
Abundance of fibers in leaves
Help support leaves
Help leaf hold shape when it dries
Examples
Oleander (Nerium oleander)
Fig (Ficus)
Jade plant (Crassula argentea)

30. Adaptations for Environmental Extremes
Hydrophytes
Grow in moist environments
Lack characteristics to conserve water
Leaves
Thin
Thin cuticle
Often deeply lobed
Mesophytes
Grow in moderate climates

31. Leaf Modifications Spines Tendrils Cells with hard cell wall
Pointed and dangerous to potential predators
Tendrils
Modified leaflets
Wrap around things and support shoot

32. Leaf Modifications Bulbs
Thick leaves sometimes referred to as bulb scales
Store food and water
Modified branches with short, thick stem and short, thick storage leaves

33. Leaf Modifications Plantlets Leaves have notches along margins
Meristem develops in bottom of each notch that produce a new plantlet
Plantlet falls off leaf and roots in soil
Form of vegetative (asexual) reproduction
Example
Air-plant (Kalanchoe pinnata)

34. Leaf Abscission Abscission – separation
Result of differentiation and specialization at region at base of petiole called abscission zone
Weak area due to
Parenchyma cells in abscission zone are smaller and may lack lignin in cell walls
Xylem and phloem cells are shorter in vascular bundles at base of petiole
Fibers often absent in abscission zone

35. Leaf Abscission Abscission zone weakens
Cells in vascular bundles become plugged
Leaf falls off
Leaf scar
Scar that remains when leaf falls off
Sealed over with waxy materials which block entrance of pathogens

36. Environmental Abscission Controls
Cold temperatures
Short days
Induce hormonal changes that affect formation of abscission zone
Leaves move nutrients back into stem
Leaves lose color
Leaves fall off tree
Leaves decompose and recycle nutrients