1. Plant Structure and Function - Leaves
Objectives of today’s class:
Learn the typical structure of plant leaves
Develop an understanding of how these structures support photosynthesis in the leaf

2. Photosynthesis Photosynthesis can be described in this series
of coupled reactions:
ADP &
NADP
H2O
(CH2O)
Light
Chlorophyll
ATP &
NADPH O2
CO2 & H2O
LIGHT
REACTION
CALVIN
CYCLE

3. Design features for a leaf
Exposed to sunlight
Large surface area
Capable of exchanging gases
CO2 in, O2 out
Import minerals and water
Not obtained from atmosphere
Export fixed carbon to “sinks”
Control water loss
Resist biotic and abiotic stresses

4. A “typical” leaf stem axil with axillary bud leaf blade internode node
petiole

5. The leaf blade
Broad expanded part of the leaf frequently has the following characteristics:
Large surface area
Thin with a small distance between upper and lower surfaces

7. The leaf blade These anatomical features:
Maximize the surface area while minimizing volume
Reduce the distance that gases must diffuse through the leaf
Leaf structure varies to allow plants to survive and grow under diverse conditions.

9. Leaf tissues - epidermis
epi - upon
dermis - skin
The outer layer of cells
Comprised of a number of different cell types

10. Leaf tissues - epidermis
Cell types of the epidermis
Epidermal cells
Most abundant, arranged in a number of ways

11. Leaf epidermis Epidermal cells usually lack chloroplasts
Epidermis is also covered by a waxy cuticle
Secreted from epidermal cells
Impermeable to water

12. Leaf epidermis There are other specialized cells in the epidermis
Guard cells, forming stomata
Trichomes, leaf hairs

13. Stomata and Guard Cells
Stomata are pores in the epidermis that lead to intercellular spaces in the leaf (from the Greek “stoma”, meaning mouth)
Found on both upper and lower surfaces of the leaf, more prevalent on bottom
Formed by specialized guard cells

14. Stomata and Guard Cells
Crescent shaped cells
Inner wall is thickened
When guard cells are turgid, stomata are open; pores close when cells are not turgid

15. Stomata and Guard Cells
Guard cells regulate gas exchange and water loss from the leaf
Guard cells open and close depending on environmental and developmental signals

16. Why are stomata important for photosynthesis?
Photosynthesis requires efficient gas exchange through stomata
Interior of the leaf is moist, so a large amount of water is lost through stomata
>90% of water loss occurs via stomata
Cuticle is impermeable to water
When plants cannot get enough water, stomata close to preserve water at the expense of photosynthesis

17. Why are stomata important for photosynthesis?
Plants using C3 photosynthesis open stomata during the day, close them at night
Some plants have a mechanism that allows them to fix CO2 at night (stomata open) in order to minimize water loss during the day - CAM plants

18. Other specialized epidermal cells
Trichomes and glands
Cellular protrusions from epidermis
These have a variety of forms and serve a number of protective functions
Stinging hairs can prevent predation, e.g. on nettles

19. Other specialized epidermal cells
Trichomes and glands
Globular trichomes release compounds that are toxic to insects
Secretory hairs allow plants to secrete compounds

20. Mesophyll Tissue Occupies most of the internal tissue of the leaf
Comprised of two cell types
Palisade parenchyma cells
Spongy parenchyma cells

21. Mesophyll Tissue Palisade parenchyma cells Elongated cells
One to three cell layers thick
Contain many chloroplasts
Primary site for photosynthesis

22. Mesophyll Tissue Spongy parenchyma cells More randomly arranged
Air spaces between cells
Fewer chloroplasts

23. Mesophyll Tissue Mesophyll tissue is designed for:
Interception of light energy
Fixation of CO2
Exchange of gases

24. Vascular tissue Visible as veins distributed throughout the leaf
Required for transport of material to and from the leaf
Water and nutrients in, photosynthetic
products out

25. Monocot vs. Dicot Leaf Veins

26. Vascular tissue
Organized as bundles containing xylem and phloem

27. Vascular tissue Xylem Phloem
Distribution of water and minerals transported from root
Phloem
Transport of fixed carbon compounds from the leaf to the rest of the plant

28. Leaf morphology and arrangement
Leaf morphology varies between species
A stable characteristic that can be used for plant identification
Simple leaves
Single leaf blade at each node

29. Compound leaves
Multiple leaflets

30. Leaf morphology and arrangement
Other characteristics used to describe leaves include:
How they are attached to the stem
Shape of leaves
Leaf margin

32. Leaf Modifications Leaves can be modified to serve other functions:
The scales of many bulbs (e.g. daffodil) are leaves that serve as storage tissue
Tendrils (on pea, morning glory, etc.) are modified leaves that allow plants to vine
Spines or thorns on some plants are modified forms of leaves that protect the plant from predation

33. Leaf Modifications
Leaves can be modified to serve other functions

34. Organ Tissue Cell Type Leaf Epidermal Guard cells Hair cells Epidermis
Palisade
Spongy
Leaf
Mesophyll
Xylem
Phloem
Sclerenchyma
Vascular
System