Figure 23–18 The Internal Structure of a Leaf

Figure 23–18 The Internal Structure of a Leaf Cuticle Veins Epidermis Palisademesophyll Xylem Vein Phloem Spongymesophyll Epidermis Stoma Guardcells

Leaf Structures & Functions • Epidermis – layer of tough, irregularly shaped cells; covered by cuticle; form waterproof barrier that protects tissues and limits water loss through evaporation • Mesophyll – photosynthesis in most plants occurs here • Palisade mesophyll – tall columnar, cells; closely packed • Spongy mesophyll – loose; many air spaces that connect to exterior through stomata

Stomata – porelike openings in the underside of the leaf; allows carbon dioxide and oxygen to diffuse into and out of leaf • Guard cells – control the opening and closing of stomata by responding to changes in water pressure • 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 • In general, stomata are open during the daytime when photosynthesis is active, and closed at night. • May be closed during hot, dry conditions to reduce water loss.

Vein – vascular tissue connected to vascular tissue of stems; transport system • xylem – carries water from roots to rest of plant • phloem – carries nutrients and food made by photosynthesis

Function of Guard Cells Section 23-4 Guard cells Guard cells Inner cell wall Inner cell wall Stoma Stoma Open Stoma Closed

Figure 23–18 The Internal Structure of a Leaf