When analyzing flowering plants, also known as angiosperms, two main types of structures are often distinguished---the dicot root and the monocot root. Though the terms refer to the number of seed leaves that appear at germination (with dicot meaning there are two and monocot meaning there is only one), the root structure of each is also quite different. Thus, it provides an easy way for botanists to distinguish between the two.
At the innermost structure of the dicot root is the xylem. As defined in the American Heritage College Dictionary, Third Edition, the xylem is "the supporting and water-conducting tissue of vascular plants." Its location, in the dicot root, is directly in the center of the structure, where it forms a distinctive "X" pattern. Without this giving the plant support, the plant would not be able to hold its form.
Moving out from the xylem, the next major region of the dicot root is known as the phloem. This is the portion of the plant directly responsible for drawing and transporting nutrients to the other plant cells. The phloem tissue in a dicot root is located between the arms of the "X" formed by the xylem. The two share very similar functions in that both are responsible for transporting vital substances.
Surrounding the xylem and the phloem in the plant structure of the dicot root is the pericycle. The pericycle enables the roots to branch out, an important function as the plant grows and requires more stability and nutrients. It also represents the outer layer of the vascular portion of the dicot root, which is better known as the stele.
The layer after the pericycle is known as the endodermis, which is tissue that protects the rest stele by forming a sheath around it. The endodermis helps the plant by regulating and screening the various substances coming into the plant from the outside environment.
This is the layer of the plant root that is responsible for storage. Those plants that need a larger capacity for storage of starches will have a much larger cortex than those that must quickly absorb such nutrients. Therefore, the size can vary greatly between plants.
This is the outermost layer of the dicot root. The epidermis is a thin layer of cells that covers the flexible portions of the roots. It allows water and other nutrients to be absorbed through it, but also serves as a layer of protection for the other cells.