The Leaf

         Leaves are perhaps the most conspicuous parts of plants and also the most important.  Being rich in chloroplasts, they function in photosynthesis.  In general, they are characterized by their thin, expanded form and are able to maintain their shape by an internal skeleton of vascular tissue which is continuous with the vascular tissue of the stem.  Thus, the vascular tissue carries food to all parts of the plant, gives support to the leaf, and brings water to the sites of photosynthesis.

         The internal arrangement of the tissues of the leaf can be seen in cross sections cut at right angles to the broad surface of the blade, (Figure 5).  The outermost layer of cells, which extends over the surface of the leaf, is the epidermis.  The interior of the leaf, between the upper and lower epidermis, is called the mesophyll which corresponds to the cortex of the root and stem.  The mesophyll is further differentiated into an upper region (the palisade mesophyll) and a lower region (the spongy mesophyll).  The vascular tissue extends through the spongy mesophyll.  Examine the prepared slide of a cross section through the leaf of Lilium sp.  (Slide #14) and identify the following, highlighted structures.

           

Figure 5A: Diagram of Leaf Cross Section.

Figure 5B: Microscopic View of Leaf Cross Section.

         1.                        Epidermis -- a continuous layer of cells extending over the upper and lower surfaces of the leaf.  Locate both the upper epidermis and the lower epidermis, each with a cuticle.  The cuticle is a waxy substance, which prevents water loss through epidermal cells and protects the leaf against mechanical injury and foreign invasion.  The lower epidermis contains a number of stomata, each of which is located between two highly specialized guard cells.  The guard cells regulate the opening and closing of the stomata by changes in turgor pressure.  The function of the guard cell is to regulate the exchange of gases for the photosynthetic process.

         2.                        Mesophyll -- an area corresponding to the cortex of the root and stem.  It is composed of a palisade mesophyll and a spongy mesophyll.  The palisade mesophyll is a region of packed cells located beneath the epidermis.  This region is composed of chlorenchyma cells and is the primary photosynthetic area of the plant.  The spongy mesophyll is composed of a loosely knit network of chlorenchyma cells with numerous intercellular spaces between the cells.       

         3.                        Vascular Bundle -- an area of conducting tissue composed of xylem tissue and phloem tissue.  The vascular bundles in the leaves are the terminal ends of the continuous vascular system.  In the more prominent vascular bundles a bundle sheath of parenchyma or sclerenchyma cells is present.