The Stem

         The stem is usually an above ground part of the plant.  It typically: serves as a mechanical support for the leaves, flowers and fruits; furnishes a path of conduction between these organs and the roots; and, to a limited extent depending on the plant, acts as a storage organ for water and products of photosynthesis. Stems are characterized by having a highly developed vascular system which is critical in their transport and support functions.  As in the case of the root, the internal anatomy of the stem is well suited to its function.  The single layer of epidermal cells have thin inner walls, while the outer walls, which are exposed to air, are thicker and often covered with a cuticle (a waxy substance to prevent water loss).  The epidermis may also develop spines for protection, glandular extensions to attract insects or hairs to reduce the rate of transpiration.  In addition, stems possessing photosynthetic tissue (i.e. they are green) have stomata in their epidermis.  These are openings which facilitate gaseous exchange and limit water loss.  Stems also possess varying amounts of fundamental tissues (parenchyma, sclerenchyma, etc.) to perform the storage and support functions. 

Examine the prepared  slide of the cross section through the stem of Medicago sp., a dicot, (Slide #11) and identify the following, highlighted structures (Use Figure 4  as a guide):

 

  

Figure 4: Cross section of Medicago sp. (alfalfa) stem, a dicot.

         1)            Epidermis -- a single layer of cells which covers the stem.  Observe the cuticle which is present on the outer extremity of each epidermal cell.

         2)            Cortex and Pith. -- the main storage part of the stem of Medicago sp. (alfalfa) is divided into two distinct regions by the ring of vascular tissue.  The region between the epidermis and the vascular tissue is called the cortex  It is composed of several layers of parenchyma cells, some of which may contain chloroplasts, and thick-walled collenchyma cells.  Collenchyma cells are specialized for structural support and are located as cylinders of tissue beneath the epidermis.  The region internal to the vascular tissue is called the pith and is composed of parenchyma. 

         3)            Vascular Bundle -- an area of conducting tissue.

                        Each vascular bundle is surrounded by sclerenchyma cells for support.  Each bundle possesses both xylem and phloem.  The xylem tissue is always located more internally than the phloem tissue.  Observe the several large xylem cells which are known as xylem vessels.  These cells are dead when mature, lose their end walls, and form lengthy continuous tubes for conducting water and minerals upward in the plant.  The phloem tissue, on the other hand, conducts water and organic nutrients upward and downward in the plant.