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Fibrous Roots
Many, but not all, dicots have fibrous root systems composed of numerous thin, branching roots. Because most roots are approximately equal in diameter, this root system has a very large surface area, giving the plant many points of contact with the soil through which it can take up water and nutrients. Fibrous root systems also hold well in the soil and enable plants to root in areas with high rates of soil erosion.
Tissue Arrangement
Dicots have vascular tissues within their roots that conduct food and water up and down through the root tissues. The root of dicot plants contains vascular tissue in a small zone at the center of the root. In the heart of the root is an X-shaped arrangement of xylem, the cells that conduct water and dissolved minerals from the soil up through the roots to all cells of the plant. Outside of the xylem is a circular area filled with phloem cells, which move food down from where it is manufactured in the leaves to feed or be stored in the cells of the plant's root. Surrounding the xylem and phloem is a circular layer of pericycle cells, a group of relatively undifferentiated cells from which branch roots arise. The pericycle, xylem and phloem together are called the stele. Outside the stele is a protective, waterproof layer of cells called the endodermis. The endodermis separates the stele from the cortex, a thick layer of cells that often function as food storage. On the very outside of the root is the epidermis, another protective layer of cells, many of which form long, thin root hairs that further increase the surface area of the root.
Embryonic Development
Most dicots grow from a seed that contains a plant embryo. The root of the plant develops from the radicle, a region in the lower end of the embryo. At the end of this radicle forms an apical meristem, a region of tissue that will divide and become the root system of the young plant.
Growth
The ends of the roots of dicots are composed of four types of tissue. At the far end is a root cap, a protective layer of cells that is constantly being eroded away and regenerated as the root pushes deeper into the soil. Behind the root cap is the zone of cell division, in which cells are dividing rapidly. Behind this is the zone of cell elongation, in which cells formed in the zone of cell division become larger. The zone of cell differentiation lies behind the zone of elongation. In this zone, cells differentiate into the various types of tissues that compose a dicot root.