Leaves are the major site for photosynthesis in most plants. These leaves have a flat blade specialized for catching sunlight. Leaves also collect carbon dioxide from the air. Leaves vary greatly from plant to plant and are useful in classification and identification.

• Leaf parts:
  Any of these parts may be lacking or reduced.
  • Blade - The expanded portion of a leaf.
  • Petiole - The leaf stalk connecting the blade to the stem.
  • Stipules - A pair of appendages at the base of the petiole. They may protect the young leaf and may be modified into spines or tendrils.
  
  
• Leaf characteristics:

  • Simple and compound leaf structure:
    • Simple leaves have a single blade.
    • Compound leaves have more than one blade on a single petiole. The multiple blades of a compound leaf are called leaflets.
      • Palmately compound leaves have leaflets arranged like the fingers of a hand.
      • Pinnately compound leaves have leaflets arranged on either side of an axis, resembling a feather.
      • Trifoliolate leaves have leaflets arranged in threes, like clover.
      • Compound leaves are sometimes twice divided. These leaves are called twice-compound.

  • Leaf attachment:
    • Petiolate - The blade is attached to the stem by a petiole.
    • Sessile - The blade is attached directly to the stem without a petiole.

  • Leaf arrangement:
    • Opposite - Two leaves grow opposite each other at each node.
    • Alternate - One leaf grows at each node. The leaves alternate sides along the stem.
    • Whorled - Several leaves grow around a single node.

  • Leaf shapes:
    • Linear - Narrow from base to tip.
    • Elliptic - Oval-shaped.
    • Ovate - Wide at the base and narrow at the tip.
    • Cordate - Heart-shaped.

  • Leaf margins:
    • Entire - The edge of the leaf is smooth.
    • Serrate - The edge of the leaf is finely toothed.
    • Lobed - The edge of the leaf is deeply indented.

  • Leaf venation: The system of principal veins in the leaf blade.
    • Parallel - Major veins arise at the base, remain more or less parallel, and converge at the tip of the leaf.
    • Net-veined or Reticulate:
      • Pinnate - Major veins diverge from one large mid-vein, with smaller network connections between.
      • Palmate - Several large veins arise from the base of the leaf like the fingers of a hand.

  • Leaf surfaces: The presence or absence of hairs, the kinds of hairs, and the presence of other surface features, such as glands, combine to give many leaf characteristics. There are over 25 terms used to describe leaf surfaces. This amount of detail is beyond the scope of our class.

What is the most serious problem leaves must overcome?

The three principal tissues of a leaf:

Review leaf tissue information from Biology Class before continuing.

• Epidermis - the external layer (one cell thick) of cells covering the leaf surface. The cells form an extra-thick cell wall on their outer surface, onto which a water-repellent layer of cutin and waxes is secreted, restricting evaporation of water. To allow for gas exchange through this layer, the epidermis has openings called stomates. The stomate is a slit-like intercellular space between two specialized epidermal cells called guard cells which open and close the pore. While the stomates may occur on either surface of the leaf, they are usually more abundant on the lower epidermis, where hundreds of stomates commonly occur per square millimeter of leaf surface. The leaves of most woody plants have stomates only on the lower epidermis.

  

• Mesophyll - the cells making up the internal leaf tissue, except the vascular bundles. In most leaves, the mesophyll is differentiated into an upper layer of one or more tiers of elongated palisade parenchyma cells and a lower layer of more irregularly shaped spongy parenchyma cells. Chloroplasts are usually larger and more numerous in the palisade cells. For this reason, the palisade layer is the principal photosynthetic tissue of the leaf. When light falling on a leaf passes through the palisade layers and enters the spongy tisssue, the irregular air spaces of this tissue scatter and reflect it back and forth through the tissue several times. This gives pigments in the leaf not just one, but several chances to absorb incident light, helping them capture weakly absorbed wavelengths. An elaborate system of air-filled intercellular spaces runs between the cells of both the palisade and spongy layers, but is especially obvious in the spongy tissue. This system permits rapid diffusion of CO₂ within the leaf from the stomates to the photosynthesizing mesophyll cells.

• Vascular bundles - the visible veins of the leaf which connect through the petiole to the vascular tissue of the stem. The vascular bundles are made up of the microscopic cells conducting water into the leaf and photosynthetic products out of the leaf in a process known as translocation. During the day, translocation normally does not keep pace with photosynthesis, causing carbohydrates to accumulate in the leaf. Photosynthetic products disappear during the night because of continued translocation.

Energy capture by leaves:

• Canopy structure: As a plant grows it increases its total leaf area available for energy capture and its total photosynthesis, as long as all its leaves are fully exposed to the sun. But as leaves grow in size and number, they start to shade one another, reducing the energy they receive. Many plants counter this reduction with an ability to position their leaves for minimal mutual shading, a pattern called a leaf mosaic. However, once the total leaf area exceeds the area of the land on which the plant is growing, mutual shading is usually unavoidable. Photosynthesis by a leaf more than two or three leaves down in the canopy may not even equal its respiration. The leaf becomes a liability rather than an asset to the plant.

  Because the photosynthesis of leaves exposed to full sun is usually light-saturated, these leaves waste a good part of the energy they absorb. To increase the total photosynthesis, a plant tips its upper leaves at a fight angle to the sun, cutting down the amount of light falling on them and allowing more light to fall on the lower, light-starved leaves.

• Shade plants: Ordinary plants, adapted for living in full sunlight, are called sun plants. Shade plants are species adapted to low-light habitats. Shade leaves are usually larger and much thinner than sun leaves. Since under low light the plant has only limited photosynthetic production, it is advantageous to make the products go as far as possible for new leaf construction. Because shade leaves have less cell material to maintain and contain fewer mitochondria per unit area, they respire substantially more slowly than sun leaves, and it takes less photosynthesis to equal the leaf's respiration rate. By their adaptation, shade plants are able to live in places so shady that sun plants simply die, even though the photosynthesis of shade leaves is inherently no more efficient than that of sun leaves.

• Stomatal diffusion: Stomatal openings, the route for entry of CO₂ into the leaf, usually comprise less than 1% of the epidermal surface. The need for very efficient diffusional uptake of CO₂ to support a reasonable photosynthetic rate by leaves creates their most serious problem, the evaporative loss of water, transpiration.

  Research Links:

  • Botany 210 - Texas A&M

   

   

   

   

   

   

   

     

   

  Transporation in leaves helps water move up a plant.
  Loss of water from plant leaves through transporation produces low pressure at the top of xylem tubes. This transporation pull greatly increases the ability of plants to take water to the top of a plant.

   

   

   

   

   

   

   

     

   

  The greatest problem that leaves must overcome is water loss.
  Leaves must have openings to allow the intake of CO₂, but these same openings increase water loss in leaves. While some water must be released during transporation, leaves cannot release too much water.