Leaves

Leaf Structure
 The leaves of plants exhibit far more variation in shape (morphology) than stems and roots. Leaf shape, size, venation pattern, margins, tips and bases are all used in identification of plant species. Were you to take a class in plant identification, you would learn dozens of new terms just to describe leaf features.

In general, however, most leaves do have a two common features:
blade (or sometimes lamina), the flattened portion of the leaf and petiole , or leaf stalk, which attaches the leaf to the stem. Leaves which do not have a petiole are sessile . Stipules, small leaf-like growths near the base of the petiole, may or may not be present.

Blade, Petiole, Stem, Bud, Stipules

Leaf venation patterns are an important distinction between monocots and dicots. Monocots usually have parallel veins; dicots have netted veins, generally with a significant mid vein. Dicot leaves may have pinnate venation or palmate venation. The Ginkgo tree, which is a gymnosperm, has leaves with dichotomous venation. It is unique.

Leaf blades may be simple , dissected or compound with leaflets. A compound leaf can be distinguished from a simple leaf by the location of buds. Compound leaves may be pinnately compound or palmately compound. Compound leaves with three leaflets, such as clover, are said to be ternate . The phyllotaxy, or arrangement of leaves on the stem may be alternate, opposite or whorled.

The many variations in leaves provide for better survival in specific habitats. For example, thickened epidermis and epidermal hairs protect and minimize water loss for leaves of dry areas. Large air spaces make floating leaves buoyant. Dissected leaves in water offer less resistance to water force.

There are also differences in leaves found in sun and in shade, and in some juvenile and adult leaf forms.
Leaf Functions

  • Photosynthesis
    The major function of any leaf is photosynthesis. As you observe the structure of leaves in lab this week, note the design of the leaf relative to what it needs for photosynthesis: Light, water, CO2 and chlorophyll.
  • Transpiration
    Evaporation of water through leaf surfaces, which creates a tension (or negative pressure) which helps draw water upward through the xylem tissue from roots and stems.
  • Gas exchange
    CO2 in for the process of photosynthesis
    O2 in or out, depending on diffusion gradients. Oxygen is used in respiration in every living cell, and is a by-product of photosynthesis


     Tissues of the Leaf (Anatomy) (surface to surface)
    Leaf Epidermis
     Special cells in leaf epidermis
     Leaf Mesophyll
     Dicot Leaf Mesophyll
     Monocot Leaf Mesophyll
     Vascular Tissue in Leaves (Veins)
     Provides support Veins branch extensively throughout the mesophyll of leaves.
    Vein Patterns
     In dicots , the veins form a network throughout the mesophyll, branching from the mid vein. In a leaf cross section, the large mid vein is conspicuous, with bundle sheath extensions which often go from epidermis to epidermis layer, providing additional support. There may be some secondary growth in the mid vein. Branching veins may be seen either in cross section or in longitudinal section. Larger branching veins may also have bundle sheath extensions.

    In monocots , the parallel veins are seen in cross section, along with the cross section of the leaf, and are more or less uniform in size and distribution across the leaf's interior.

    Some grasses may have large thin-walled cells along both sides of the midvein in the upper epidermis. These cells, called bulliform cells , help the leaf to fold or roll inward during water deficit periods. This folding of the leaf probably minimizes evaporation.

    In addition, many monocots have enlarged bundle sheath cells, surrounding the veins, which have a function in photosynthesis for some plants.

    Some monocots also have variations in guard cell structure . Rather than having thickened inner walls, the walls of the guard cells of some monocots are thickened at the ends, so the guard cells have more of a "dumbbell shape than a bean shape. The guard cells are also associated with specialized adjacent cells, called subsidiary cells.


     Some Leaf Variations

    Modifications for Habitat
    Hydromorphic Leaves
     Submerged Leaves
     Plants which grow completely underwater will typically have very dissected leaves, or narrow linear leaves. These shapes minimize resistance to water currents, and probably prevent damage to tissues which would be more likely with a broad surface.

    Some aquatic plants produce dissected leaves where submerged, and less dissected shapes when the leaves are produced above the surface of the water.

    Xeromorphic Leaves
     Plants which live in arid environments are subject to drought, and often, intense sunlight. Such plants are called xerophytes. These plants are subjected to intense evaporation of water, a resource which is often in short supply. Many such plants have a number of leaf modifications to prevent excessive water loss.
    Some xeromorphic plants may have succulent leaves, and may have stomata which open at night rather than in the daylight, again to minimize water loss.

    Conifer Leaves (Needles)
     The needles of conifers have many of the same adaptations of xeromorphic leaves. Conifers predominate in areas that have long, cold winters and dryer summers. To survive conifers are adapted to conditions of moisture stress. A typical conifer needle will have sunken stomata , often in channels on the lower surface, thick cuticle , hypodermis layers, and, in addition, an endodermis surrounding the vein (or rarely, veins). As with other conifer organs. the leaves will have resin canals, lined with parenchyma.


    Other Leaf Variations
    Shade/Sun
     Leaves in the shade exhibit etiolation (small leaves, long stems, thinner blades, etc.)

     Age
     In some plants, the shape of leaves changes from the juvenile regions of the plant to those which are more mature. English ivy leaves show these changes
    Leaves and Water Relations in Plants
     Stomata are open in daytime which permits diffusion of CO2 into the leaf for photosynthesis. At the same time water is lost through the stomata by a process called transpiration. The intercellular spaces of plant tissues are near 100% humidity, and the stomata are openings into the environment, which is usually not at 100% humidity. The diffusion gradient for water is from the leaf to the environment. This creates serious problems for water maintenance. The resolution of this problem is the closure of stomata at night so that water loss is restricted to the daytime hours when the plant is actively using CO2. It is important to remember that the primary function of stomata is gas exchange, a subject that will be discussed later.

    Transpiration also plays a role in the movement of water throughout the plant as we shall also discuss later. Transpiration loss is significant. In corn fields, as much as 90% of the water absorbed by the roots is lost by transpiration.


    Other Water Movement - Positive Root Pressure
     Simple diffusion pressure in roots moves H2O upward - often forcing the H2O to be exuded from vein tips in leaves, a phenomenon called guttation. The special leaf tip cells are called hydathodes.

    Guttation occurs when there is high soil moisture and low evaporation stress. You can typically see guttation in Seattle during the spring and early summer, in the early mornings before the sunlight evaporates the water. Grasses and many herbaceous plants, such as strawberries. guttate nicely. Guttation can not be observed during rain, since the rain drops coat the plant surfaces, and should not be confused with dew, which can condense from the atmosphere onto plant surfaces.

    Leaf Abscission (Loss of leaves)
    Loss and replacement of leaves in plants is a normal process. A perennial which loses all of its leaves at one time in response to seasonal or climate differences, is termed deciduous . All leaves, however, have a finite life span, and are lost from the plant, to be replaced by newer leaves produced in the shoot meristems. The process of leaf loss is called abscission , and is controlled by hormones.

    The abscission zone is located at the base of the petiole in a region of undifferentiated, small parenchyma cells. Their walls contain no lignin, and the vascular cells in the abscission zone are also reduced in size.

    The process of abscission is initiated and proceeds as follows:

    The hormones (which will be discussed later) involved in leaf abscission are:
    Auxin
    ABA (Abscissic acid)
    Ethylene

    Associated with abscission, and occurring prior to the loss of the leaf, is a process which involves degrading and moving a number of solutes, minerals and other substances from the leaf tissue into the stem and roots. The degradation of chlorophyll and change in leaf color which results, is often referred to as "fall color". As chlorophyll degrades the natural carotenoids become prominent. In addition, some leaves may accumulate anthocyanins in their vacuoles, adding reds or bluish reds to the fall color. Photoperiod is important in triggering the fall change in pigmentation.

    Some Leaf Modifications (Discussed with modified shoots)
  • Bud scales
  • Spines
  • Tendrils
  • Bracts
  • Bulbs

     Humans and Leaves
  • Leaves are used extensively in medicine and nutrition, as well as other human economical endeavors.

  • Food and Seasonings (Too many to list)
  • Beverages such as teas

  • Medical uses

  • Drug uses

    Other Leaf Uses

  • Insecticides
  • Waxes
  • Many, many Aromatic Oils

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