Leaf Development, Structure & Function

                                                                                                                                                                

leaf by sobur

I.             Leaf development and structure

                A.            Leaf development

                                1.             Leaf primordia initiated in region of apical meristem

                                2.             Primary meristematic tissue:  protoderm (epidermis), ground meristem (leaf mesophyll), and procambium (vascular tissue)

                B.            Leaf structure

                                1.             Epidermis

                                                a)            Usually no chloroplasts in epidermal cells - except in guard cells

                                                b)            Extra-thick wall on outside (atmospheric) surface of epidermal cells

                                                c)             Cutin & waxes form cuticle - usually thicker on upper epidermis

                                                d)            Stomata (stomates)

                                                                1)            "Holes" in epidermis to allow exchange of gases

                                                                2)            Bordered by guard cells which control stomatal opening

                                                                3)            Commonly found only on lower epidermis in dicots - both surfaces in monocots

                                2.             Mesophyll

                                                a)            All internal leaf tissue (except some vascular tissue)

                                                b)            In dicots, usually organized into 2 layers

                                                                1)            Palisade parenchyma - upper layer

                                                                                a)            Elongated cells, perpendicular to leaf surface

                                                                                b)            Numerous, large chloroplasts

                                                                                c)             Principle photosynthetic tissue of leaf

                                                                2)            Spongy parenchyma

                                                                                a)            Irregularly shaped cells with conspicuous air spaces

                                                                                b)            Fewer chloroplasts than palisade layer

                                                                                c)             Air spaces important in diffusion of gases

                                                c)             Most monocots & gymnosperms - not 2 layers - mesophyll similar to spongy parenchyma

                                3.             Vascular bundles

                                                a)            Transport materials to & from leaf

                                                b)            Larger bundles visible as leaf veins

                                                c)             Xylem found on top, phloem found on bottom of vascular bundles

                                                d)            Runs throughout mesophyll

                                                                1)            Most mesophyll within 1-2 cells of vascular bundle

                                                                2)            Found mostly within the spongy layer in dicots

                                                e)             Bundle sheath

                                                                1)            Tight sheath of cells surrounding the vascular bundles

                                                                2)            Controls movement of materials to & from vascular tissue

II.            Leaf Function

                A.            Photosynthesis

                                1.             Harvest light energy and convert it to ATP and NADPH

                                2.             Fix CO₂

                B.            Transpiration

                                1.             Driving force for xylem transport of water and dissolved nutrients (adhesion & cohesion)

                                2.             Evaporative cooling of leaves when temperature is high

                C.            Stomates - control gas exchange and transpiration (mechanism is osmosis at the guard cell level)