SECONDARY GROWTH LAB

OBJECTIVES: Upon completion of this exercise you should be able to:

1. Be able to identify the tissues secondary stems and roots
2. Be able to identify the cell types of the tissues of secondary stems and roots

The increasing girth of many tree trunks or some roots is due to a growth pattern referred to as secondary growth.  Primary growth adds mostly length to a plant.  Secondary growth adds girth. The cells that are responsible for primary growth are derived from the apical meristem. The cells that are responsible for secondary growth are derived by cambia (sing.: cambium).  Cambia are thin layers of cells that are undifferentiated cells that carry out extensive cell division (mitosis).

Not all plants can undergo secondary growth.  Only some dicots can undergo true secondary growth.  This gives them a different pattern of vascular tissue than a primary stem or root.

The typical stem that undergoes extensive secondary growth is a Type I stem.  As a primary stem, this stem has very wide vascular bundles with narrow inter fascicular rays. In the picture to the right is an immature Type I stem.  The red arrow is showing where the vascular cambium will develop, between the primary phloem and primary xylem.  The vascular cambium will develop from parenchyma cells between the two primary vascular tissues. If you "mouse" over the picture to the right, you will see a zoomed in picture of this image. You can adjust zoom with the "mouse wheel".

Some Type II stems can experience limited secondary growth.  In Type II stems the vascular bundles are narrow and the interfascicular rays are wide. The picture to the right is is a cross section of the stem of a sunflower plant. The red arrow is showing where the vascular cambium has developed, between the primary phloem and primary xylem.  The vascular cambium has developed from parenchyma cell between the two primary vascular tissues.  This cambium is also called a fascicular cambium because it is located in the fascicle or vascular bundle.  The blue arrow is showing where an interfascicular cambium has developed.  It has developed from parenchyma cells within the interfascicular region (ray)

Some dicot roots can undergo secondary growth.  Many of these become storage roots for plants to store nutrients for the next growing season i.e. a carrot.  A vascular cambium will develop from parenchyma cells between the primary phloem and primary xylem.  In these secondary roots the cortex and epidermis will be sloughed off.  So these tissues (cortex and epidermis) will not be present in a secondary root.

Besides having a cambium that produces secondary phloem and secondary xylem, we need a cambium that produces a tissue that replaces the epidermal tissue (dermal system) in location and function. As a stem or root undergoes secondary growth it becomes larger in diameter.  Epidermal cells can not undergo cell division (mitosis) so they cannot compensate for the increase girth. So the epidermis needs to be replaced.  The tissues that replaces dermal system are called collectively the periderm.  It is three layers thick.  From the outside to the inside they are the cork(phellum), the cork cambium and the phelloderm.  The cork fills the function of the dermal system. The cork cambium makes new cork cells through cell division (mitosis). The phelloderm is also made by the cork cambium through cell division. If a derivative of the cork cambium gets push to the outside of the cork cambium it will become a cork cell. If a derivative gets pushed to the inside of the cork cambium it becomes a pheloderm cell.  The phelloderm acts as a layer of replacement cells for the cork cambium if the cork cambium is damaged.

The cork cells (C) are filled with wax which stains rusty red in this preparation. The cork cambium is marked as "CC". The cells of the cork cambium are narrow and the cork cambium is only one to two cells thick. The phelloderm is marked as "P". These cells are thicker than the cells of the cork cambium.