The Structure of Dicot Roots
The primary dicot root consists of a number of rings of specialised tissue surrounding a central cylinder of vascular (transporting) tissue. The root must accomplish four major tasks in most plants. Most importantly, it must absorb water and mineral nutrients from the environment. It must also be able to move these substances to other parts of the plant which do not have direct access to them. In most plants, the roots must anchor the rest of the plant in the soil, and provide enough support to resist wind, gravity, etc. Many roots also serve as the primary area for storing excess energy as starch in plants.
The micrograph above shows a typical dicot root. On the outside is a thin epidermis (visible in the upper left), which must be permeable to water, at least while the root is very young (only the newly matured tips of growing roots absorb water).
Between the epidermis and the central cylinder is a layer of large, loosely packed storage cells called the cortex. In plants like turnips and carrots, which store large amounts of starch, the cortex is huge. In plants like cactus, which must absorb water very quickly when it becomes available, the cortex is very thin. In this plant, the cortex takes up about 80% of the diameter of the root, and dark grains of starch are clearly visible inside the cells.
The spaces between the cortex cells allow water to diffuse through the root much faster than it would through a solid tissue.
The micrograph below shows more detail of the central cylinder
Surrounding the vascular cylinder is a layer of cells whose side and end walls are cemented together like bricks in a wall. Some of these cells appear to have thicker, pinkish walls in the micrograph. The waxy substance between the cells (called the Casparian strip) prevents water from diffusing into or out of the central cylinder except through the membranes of these cells. This layer, called the endodermis, allows the plant to screen what is coming in from the environment, and to regulate the flow of sugars from the phloem to the storage cells in the cortex. If the endodermis were too permeable, sugars would diffuse rapidly into the outer layers of the root, and would leak into the surrounding soils. Since making sugars is the plant's main task, this would be very wasteful.
Immediately inside the endodermis is a layer of unspecialised meristematic cells called the pericycle. Cells of the pericycle can begin to divide and push out through the endodermis and cortex, forming a secondary, or branching, root. The secondary root will repeat the structure of the primary root, so it too can branch. Repeated branching gives the root system both enough surface area to absorb sufficient water, and a complex network of supports for anchoring the rest of the plant.
The centre of the vascular cylinder is occupied by a star-shaped region of xylem (called the stele). The large, thick-walled vessels of the xylem are hollow, and act as the main water transport system to the rest of the plant. Large quantities of water and dissolved minerals can be moved upward through the xylem with very little expenditure of energy.
In the gaps between the arms of xylem are smaller bundles of phloem tissue. Phloem consists of live cells that carry dissolved sugars around the plant. Sugars are made in the leaves by photosynthesis, but they are required by all other living cells of the plant as an energy source. The cells of the phloem regulate how much sugar is moved to and released in any part of the plant. In most plants, excess sugar is moved to the roots and stored in the cortex, where it is comparatively safe from predators.
Between the xylem and phloem, and usually hard to see, is a layer of very small meristematic cells called the vascular cambium. These cells can divide to produce new xylem and phloem, thickening and strengthening the root and increasing its capacity to transport materials.