1. Slide 1 of 32 Copyright Pearson Prentice Hall 23–4 Leaves

2. 23-4 Leaves Slide 2 of 32 Copyright Pearson Prentice Hall Leaf Structure How does the structure of a leaf enable it to carry out photosynthesis?

3. 23-4 Leaves Slide 3 of 32 Copyright Pearson Prentice Hall Leaf Structure The structure of a leaf is optimized for absorbing light and carrying out photosynthesis.

4. 23-4 Leaves Slide 4 of 32 Copyright Pearson Prentice Hall Leaf Structure To collect sunlight, most leaves have thin, flattened sections called blades. Blade Stem Bud Petiole Simple leaf Compound leaf Leaflet

5. 23-4 Leaves Slide 5 of 32 Copyright Pearson Prentice Hall Leaf Structure The blade is attached to the stem by a thin stalk called a petiole. Blade Stem Bud Petiole Simple leaf Compound leaf Leaflet

6. 23-4 Leaves Slide 6 of 32 Copyright Pearson Prentice Hall Leaf Structure Simple leaves have only one blade and one petiole. Blade Stem Bud Petiole Simple leaf Compound leaf Leaflet

7. 23-4 Leaves Slide 7 of 32 Copyright Pearson Prentice Hall Leaf Structure Compound leaves have several blades, or leaflets, that are joined together and to the stem by several petioles. Blade Stem Bud Petiole Simple leaf Compound leaf Leaflet

8. 23-4 Leaves Slide 8 of 32 Copyright Pearson Prentice Hall Leaf Structure Leaves are covered on the top and bottom by epidermis. Epidermis

9. 23-4 Leaves Slide 9 of 32 Copyright Pearson Prentice Hall Leaf Structure The epidermis of many leaves is covered by the cuticle. Epidermis Cuticle

10. 23-4 Leaves Slide 10 of 32 Copyright Pearson Prentice Hall Leaf Structure The cuticle and epidermal cells form a waterproof barrier that protects tissues inside the leaf and limits the loss of water through evaporation. The vascular tissues of leaves are connected directly to the vascular tissues of stems.

11. 23-4 Leaves Slide 11 of 32 Copyright Pearson Prentice Hall Leaf Structure In leaves, xylem and phloem tissues are gathered together into bundles that run from the stem into the petiole. In the leaf blade, the vascular bundles are surrounded by parenchyma and sclerenchyma cells.

12. 23-4 Leaves Slide 12 of 32 Copyright Pearson Prentice Hall Leaf Structure All these tissues form the veins of a leaf. Xylem Phloem Vein

13. 23-4 Leaves Slide 13 of 32 Copyright Pearson Prentice Hall Leaf Functions Most leaves consist of a specialized ground tissue known as mesophyll. Palisade mesophyll Spongy mesophyll

14. 23-4 Leaves Slide 14 of 32 Copyright Pearson Prentice Hall Leaf Functions The layer of mesophyll cells found directly under the epidermis is called the palisade mesophyll. These closely-packed cells absorb light that enters the leaf. Palisade mesophyll

15. 23-4 Leaves Slide 15 of 32 Copyright Pearson Prentice Hall Leaf Functions Beneath the palisade mesophyll is the spongy mesophyll, a loose tissue with many air spaces between its cells. Spongy mesophyll

16. 23-4 Leaves Slide 16 of 32 Copyright Pearson Prentice Hall Leaf Functions The air spaces connect with the exterior through stomata. Stomata are porelike openings in the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf. Stoma

17. 23-4 Leaves Slide 17 of 32 Copyright Pearson Prentice Hall Leaf Functions Each stoma consists of two guard cells. Guard cells are specialized cells that control the opening and closing of stomata by responding to changes in water pressure. Guard cells

18. 23-4 Leaves Slide 18 of 32 Copyright Pearson Prentice Hall Leaf Functions Transpiration The surfaces of spongy mesophyll cells are kept moist so gases can enter and leave the cells easily. Water evaporates from these surfaces and is lost to the atmosphere.

19. 23-4 Leaves Slide 19 of 32 Copyright Pearson Prentice Hall Leaf Functions Transpiration is the loss of water through its leaves. This lost water is replaced by water drawn into the leaf through xylem vessels in the vascular tissue.

20. 23-4 Leaves Slide 20 of 32 Copyright Pearson Prentice Hall Leaf Functions Gas Exchange How does gas exchange take place in a leaf?

21. 23-4 Leaves Slide 21 of 32 Copyright Pearson Prentice Hall Leaf Functions Plant leaves allow gas exchange between air spaces in the spongy mesophyll and the exterior by opening their stomata. Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water.

22. 23-4 Leaves Slide 22 of 32 Copyright Pearson Prentice Hall Leaf Functions Guard cells are specialized cells that control the stomata. Stomata open and close in response to changes in water pressure within guard cells.

23. 23-4 Leaves Slide 23 of 32 Copyright Pearson Prentice Hall Leaf Functions When water pressure within guard cells is high, the stoma open.

24. 23-4 Leaves Slide 24 of 32 Copyright Pearson Prentice Hall Leaf Functions When water pressure within guard cells decreases, the stoma closes.

25. 23-4 Leaves Slide 25 of 32 Copyright Pearson Prentice Hall Leaf Functions Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis. Stomata are open in daytime, when photosynthesis is active, and closed at night, to prevent water loss. In hot, dry conditions stomata may close even in bright sunlight, to conserve water.