Exercise 10: The Leaf

• Leaf

  • function: photosynthesis
  • major site of transpiration
• Phyllotaxy

  • arrangement of leaves along stem
• Phyllotaxy

  • based on the number of leaves attached to a single node
• Alternate

  • phyllotaxy: one leaf attached to single node
• Opposite

  • phyllotaxy: two leaves in one single node
• Whorled

  • phyllotaxy: more than two leaves in a single node
• Petiolate

  • petiole attaches the blade to the node
• Petiole

  • stalk-like structure
• Sessile

  • blade is attached to the node directly
• Simple

  • complexity: single, undivided lamina attached to single node
• Compound

  • complexity: lamina is separated into discrete leaflets where each is borne by a stalk
• Petiolule

  • a stalk where each leaflet is attached
• Trifoliate

  • leaves with three leaflets
• 2 classifications of compound leaves:

  1. pinnate
  2. palmate
• First-order pinnately compound

  • compound leaves where leaflets/petiolules are attached to a central axis
• Rachis

  • central axis where leaflets/petiolules are attached
• Second-order pinnately compound

  • leaflets are attached first to another axis called rachilla, then rachillae are attached to rachis
• Palmately compound

  • leaflets are attached to a central point at the end of petiole
• Stipule

  • nodal outgrowths which may resemble buds, leaves or thorns
• Stipule

  • usually occur perpendicular to petiole's axis or across petiole itself
• Stipule is present in every node while axillary buds are only observable in a select few
• Arrangement of vascular tissue in leaf is marked by its venation
• 2 classifications of venation

  1. reticulate
  2. parallel
• Reticulate venation

  • secondary veins branch off the midrib, split into lower-order veins and last order veins, end into the ground tissue
• Parallel venation

  • lower-order veins are indistinguishable from one another and run parallel to midrib
• Outline

  • general shape of lamina
  • determined by its length-to-width ratio relative to is apical and basal halves
• Margin

  • edges of lamina where undulations and incisions may occur
• Base

  • point of the petiole's or sheath's attachment
• Apex

  • lamina's tip
• Modification type: bracts

  Specimen: bougainvillea or poinsettia

  Modified part: lamina

  Function: attracts pollinators
• Modification type: carnivorous leaf

  Specimen: pitcher plants

  Modified part: entire leaf

  Function: heterotrophic nutrition
• Modification type: meristematic leaves

  specimen: kataka-taka

  Modified part: margin

  function: asexual reproduction
• Modification type: phyllodes

  specimens: acacia, citrus, bitter-wood

  modification part: petiole/rachilles

  function: additional photosynthetic activity
• modification type: pseudostem

  specimen: banana

  modification part: leaf sheath

  function: structural support
• modification type: spines

  specimen: cactus

  modified part: entire leaf

  function: defense and prevent water loss
• modification type: storage leaves

  specimen: Aloe vera

  modified part: entire leaf

  function: food storage