Leaves

• leaves are formed by the apical meristem
• leaves grow at nodes from leaf primordia
• enations and microphylls are not leaves
• enations are outgrowths of the stem epidermis that do not have vascular tissue
• microphylls are small leaves with a single strand of vascular tissue associated with protostele and have no leaf gap
• Megaphylls are large leaves with several to many veins, associated with a leaf gap or leaf trace gap. They have branched or multiple vascular bundles.
• leaves extend from stem in many planes
• leaflets are in one plane
• leaves have axillary bud and leaflet don't have axillary bud
• The basic parts of a leaf are: lamina (blade) and petiole (part of stem)
• sessile is when a leaf blade is attached directly to stem (no petiole)
• Stipules are scale or leaf like appendages at leaf base. They often fall off after full maturity (protect embryonic leaf)
• There are two leaf shapes: simple and compound
• Simple leaf has an undivided blade
• Compound leaf is where the blade is divided into two or more smaller leaflets. May be determinate or indeterminate
• Pinnate leaf: leaflets arise from rachis
• Palmate leaf: has no rachis, just a petiole. it resembles a "palm"
• oddpinnate: leaflets in rows, 1 at tip

  
• even pinnate: leaflets in rows, two at tip
• heteroplastic development (megaphylls): leaf form changes during plant development
• heterophylly: leaf form changes, depending on environment
• Pinnate venation - a leaf with veins branching out from a mid vein. "like a feather"
• palmate venation - small veins branching from more than one main vein
• reticulate venation - net like pattern of veins (network)
• parallel venation - parallel lines of veins running lengthwise along the blade (e.g grass)
• dichotomous venation - a pattern of veins that have two branches, one going to each side o the leaf
• phyllotaxy - leaf arrangement on a stem
• types of leaf arrangements: spiral (helical), whorled, distichous (1 plane - alternate) , opposite (1 plane), decussate (2 planes)

  
• Mesophytes: require moist environments
• xerophytes: adapted to dry environments
• hydrophytes: adapted to very wet environments (may be submerged)
• Dermal system of megaphylls: epidermis and cuticle (reduce water loss), epidermal cells, guard cells, trichomes
• guard cells: two cells that regulate opening and closing of stomata pore
• trichomes: tiny spikelike projections on some leaves for protection
• stomata: openings through which CO2, water, oxygen, and other gases pass.
• Guard cells: the two cells that border a stoma
• • stomata and guard cells may occur on upper &/or lower leaf surface
• stomata and guard cells may be sunken in xerophytic leaves (stomatal crypt)
• stomata and guard cells may be absent in hydrophytes
• hydathodes: pores in leaf surface (often modified stomata that lack ability to open and close). they allow exudation of water droplets (guttation)