The Leaf Tissues & Gas Exchange

 Air (gases) can enter cells via passive diffusion ◦ This would take a long time  Leaf has developed specialized cells to maximize its ability to exchange gases

 Epidermis on top and bottom of leaf is very thin and clear  Tiny openings called stomata are formed by guard cells ◦ This allows gas to exchange easily

 Stomata regulate movement of gases ◦ Carbon dioxide and oxygen can enter and leave the leaf  Direction is determined by a concentration gradient  Majority found on lower surface of leaf

 A plant hormone  Mediator in adaption of plant to stress ◦ Stomata opening and closing are sensitive to abscisic acid ◦ Rapid onset when water uptake from roots is insufficient to keep up with transpiration

 This is the process of water loss through the stomata Film of water is always on cell surface Means water is continually being lost by evaporation When water is not available, guard cells become limp, closing the stomata

Water enters root hair cells and moves through cells by osmosis Water rises in xylem due to adhesion and cohesion Evaporation has a cooling effect

 Xero means dry  Phyte means plant ◦ Therefore, a xerophyte is a plant able to survive in an environment with very little water

 Adaptations to the habitats can include: ◦ Reduced leaves  Spines ◦ Thickened waxy cuticle ◦ Reduced number of stomata ◦ Stomata surrounded by hairs ◦ Water storage tissue ◦ Low growth forms

 Between upper and lower epidermis is the mesophyll ◦ Two very different types of cells here  Palisade tissue cells (parenchyma) just below upper epidermis  Spongy mesophyll tissue (parenchyma) below palisade and above lower epidermis

 Palisade Tissue ◦ Long, rigid, rectangular ◦ Tightly packed ◦ Responsible for photosynthesis  Many chloroplasts found here ◦ Require carbon dioxide, produce oxygen

 Spongy mesophyll tissue ◦ Loosely packed, irregularly shaped, less rigid cells ◦ Increased space allows more gas diffusion  Moves oxygen towards stomata for release  Moves carbon dioxide from the air to the palisade tissue

 Provides leaf with water for transpiration & photosynthesis ◦ Removes sugars produced in photosynthesis  Xylem & phloem linked together in a vascular bundle

 No organs specific for gas exchange - occurs via diffusion ◦ The leaf is not the only place that gas exchange occurs ◦ Pores in bark and herbaceous plants are called lenticels  Provide pathway for gas exchange  Also provide pathway for an opening for transpiration