leaf structure - external/internal 

• external form - initiated by primordial in apical meristems
  • main sites of photosynthesis on land
  • cell enlargement/division >> leaves expand
  • microphyll - leaf w/ 1 vein
  • megaphyll - has several veins, leaves gap in cylinder once it branches off
  • veins - collection of xylem/phloem, parallel in monocots, networked in dicots
  • simple leaves - undivided blades
  • compound leaves - blades divided into leaflets
  • pinnately compound - leaflets arranged in pairs
  • palmately compound - leaflets radiate from a center
  • leaves alternately/oppositely arranged, or in whorls (leaf circle)
• internal form - transparent epidermis covers each leaf
  • no chloroplasts in epidermal cells
  • mesophyll - tissue between upper/lower epidermis, contains vascular bundles and majority of photosynthesis
  • palisade mesophyll - closer to upper epidermis, contains chlorenchyma
  • spongy mesophyll - closer to lower epidermis, contains many air spaces for gas exchange
• stomata - surrounded by guard cells, regulates water mov’t/gas exchange
  • K+ controls guard cells
  • water enters osmotically >> guard cells swell >> stomata closes
  • opens in the day, closes at night