1. Leaf Structure and Function
Chapter 32
Leaf Structure and Function

2. Leaves typically consist of
Broad flat blade
Stalk-like petiole
Some also have
Small stipules (small, leaf-like outgrowths from the base)

3. Parts of a leaf

4. Leaves may be Simple (having a single blade)
Compound (having a blade divided into two or more leaflets)

5. Simple, pinnately compound and palmately compound leaves

6. Leaf arrangement on a stem may be
Alternate (one leaf at each node)
Opposite (two leaves at each node)
Whorled (three or more leaves at each node)

7. Leaf arrangement may be alternate, opposite, or whorled

8. Leaves may have Parallel venation Netted venation
Pinnately netted (with several major veins radiating from one point
Palmately netted (with veins branching along the entire length of the midvein

9. Venation patterns include parallel, pinnately netted, and palmately netted

10. Major tissues of the leaf
Epidermis
Photosynthetic ground tissue
Xylem
Phloem

11. Epidermis Covers upper and lower surfaces of the leaf blade
Coated by a waxy cuticle enabling plant to survive a terrestrial existence

12. The thick, waxy cuticle and sunken stomata are two structural adaptations that enable Pinus to retain its needles throughout the winter

13. Epidermis, cont.
Has stomata permitting gas exchange for photosynthesis; each surrounded by
Two guard cells, often associated with subsidiary cells providing a reservoir of water and ions

14. Tissues in a typical leaf blade

15. Mesophyll consists of photosynthetic parenchyma cells
Palisade mesophyll (functions primarily for photosynthesis
Spongy mesophyll (functions primarily for gas exchange)

16. Leaf veins have
Xylem (to conduct water and essential minerals to the leaf)
Phloem (to conduct sugar produced by photosynthesis to the rest of the plant)

17. Monocot leaves All have parallel venation
Some do not have mesophyll differentiated into distinct palisade and spongy layers
Some have dumbbell-shaped guard cells, unlike more common bean-shaped guard cells

18. Cross section of a monocot leaf

19. Variation in guard cells
Guard cells of dicots and many monocots are bean-shaped
(b) Some monocot guard cells are dumbbell-shaped

20. Dicot leaves All have netted venation
All have mesophyll differentiated into distinct palisade and spongy layers
All have bean-shaped guard cells

21. Cross section of a dicot leaf

22. Photosynthesis and leaf structure
Broad, flattened leaf blade is efficient collector of radiant energy
Stomata open diurnally for gas exchange and close nocturnally to conserve water
Transparent epidermis allows light into leaf for photosynthesis

23. Photosynthesis and leaf structure, cont.
Air spaces in mesophyll tissue permit rapid diffusion of
CO2 and water into mesophyll cells
Oxygen out of mesophyll cells

24. With regard to the opening of stomata, blue light triggers
Activation of ATP synthase in the guard cell plasma membrane
Synthesis of malic acid
Hydrolysis of starch

25. Mechanism of stomatal opening

26. Physiological changes accompanying stomatal opening and closing
When malic acid ionizes, protons (H+) are produced
Protons are pumped out of the guard cells by ATP synthase

27. Physiological changes, cont.
As protons leave guard cells, an electrochemical gradient forms on the two sides of the guard cell plasma membrane
Electrochemical gradient drives uptake of potassium ions through voltage-activated potassium channels into guard cells

28. Physiological changes, cont.
Chloride ions are also taken into guard cells through ion channels
These osmotically active ions increase the solute concentration in the guard cell vacuoles

29. Physiological changes, cont.
Resulting osmotive movement of water into guard cells causes them to become turgid, forming a pore
As the day progresses, potassium ions slowly leave guard cells

30. Temporary wilting

31. Physiological changes, cont.
Starch is hydrolyzed to sucrose, which increases in concentration in the guard cells
Stomata close when water leaves guard cells due to decline in concentration of sucrose (osmotically active solute)

32. Physiological changes, cont.
Sucrose is converted to starch (osmotically inactive)
Some environmental factors affecting stomatal opening and closing
Light or darkness
CO2 concentration
Water stress
Plant’s circadian rhythm

33. Rate of transpiration affected by environmental factors, such as
Is loss of water vapor from aerial parts of plants
Occurs primarily through stomata
Rate of transpiration affected by environmental factors, such as
Temperature
Wind
Relative humidity

34. Transpiration represents a trade-off for plants
Beneficial because of CO2 requirement
Harmful because of need to conserve water

35. Guttation and transpiration
Guttation, the release of liquid water from leaves of some plants, occurs through special structures when
Transpiration is negligible and
Available soil moisture is high

36. Guttation

37. Guttation and transpiration, cont.
Is the loss of water vapor
Occurs primarily through the stomata

38. Leaf abscission Loss of leaves that often occurs
With approach of winter (temperate climates) or
At beginning of dry period (tropical climates with wet and dry seasons)

39. Leaf abscission, cont.
Complex process involving changes occurring prior to leaf fall
Physiological
Anatomic
Abscission zone develops where petiole detaches from stem

40. Leaf abscission, cont.
From leaves to other plant parts, the following are transported
Sugars
Amino acids
Many essential minerals
Chlorophyll breaks down
Carotenoids and anthocyanins become evident

41. Abscission zone

42. Examples of modified leaves
Spines deter herbivores
Tendrils grasp other structures (to support weak stems)
Bud scales protect
Delicate meristematic tissue
Dormant buds

43. Leaves of Mammilaria are modified to form spines

44. Leaves of Echinocystis lobata are modified to form tendrils

45. A terminal bud and two axillary buds of an Acer twig have overlapping bud scales to protect buds

46. Examples of modified leaves, cont.
Bulbs are short underground stems with fleshy leaves specialized for storage
Succulent leaves serve for water storage
Leaves of insectivorous plants trap insects

47. The leaves of bulbs such as Allium cepa are fleshy for storage of food materials and water

48. The succulent leaves of Senecio rowleyanus are spherical to minimize surface area, thereby conserving water