- INTRODUCTION
The root is the below surface part of the plant, the part of the plant that grows towards gravity.
THE ROOTS HAVE TWO FUNCTIONS
They serve to anchor the plant to the ground and prevent it being moved by air and water currents and secondly the roots absorb water and minerals which the plants need to survive and grow.
- THE STRUCTURE OF ROOTS
Like stems, roots must possess an outer protective covering which in young dicotyledenous roots and monocotyledonous roots is called the epidermis. However, in older dicotylendenous roots this is superseded by the cork. Roots also contain storage and packing tissue which is called parenchyma and forms the bulk of the Cortex.
Some roots like carrots are storage organs. Conducting tissues will be found in all roots as xylem and phloem. The phloem occurs in circular bundles situated between the points of the star. In dicotyledons, secondary thickening often occurs and the phloem is again pushed to the periphery.
Surrounding the central core of the root there are two rings of cells, the outer ring is called the endodermis and this regulates the flow of water and mineral into the xylem and phloem. The inner ring of cells is called the pericycle and it is from these cells that all branch roots arise.
Near the root tip are numerous root hairs. These root hairs are actually part of individual epidermis cells which are extensions of these cells. Root hairs serve to penetrate the surrounding soil and absorb water and minerals. They have a very large surface area when compared to their volume and have a high surface area to volume ratio. The amount of water absorbed is proportional to the surface area of the absorbing surface.
Root hairs therefore enable the root to have a large effective surface area with a minimum volume of root. Figure 1 shows an old dicot. root and the lower portion of a young dicot. root or monocot. root.
It is important to note that the root hairs do not occur along the full length of the root but only near the tip.
Figure 1: Tip of a growing root
- DEVELOPMENT OF THE ROOT
THERE ARE THREE STAGES TO THE DEVELOPMENT OF THE ROOT:
- The apical meristem;
- Region of enlargement; and
- Region of maturation.
THE APICAL MERISTEM
This root is at the tip and is the region of cell division. It is protected by a layer of cells called the root cap. As the root tip passes through the soil it would be severely damaged if there was no protective cap.
THE REGION OF ENLARGEMENT
Cells behind the apical meristem enlarge and increase in size and called the region of cell enlargement.
THE REGION OF MATURATION
| Girth: the measurement around the middle of something. |
As the cells increase in size they differentiate into the various specialised cells of the root. Secondary thickening occurs in most dicots and girth of the root increases. The secondary xylem and secondary phloem originate from the cambium. See Figure 1.
4. ROOT SYSTEMS
Dicotyledons possess one main long root that is derived from the radical during germination. This is called the tap root. Roots branch out from the tap root and form the root system. As was mentioned earlier, the branch roots originate from the pericycle.
One of the cells in the pericycle begins to divide and form a new root tip which pushes its way through the cortex out into the soil. Such a system is called the Tap Root System. The tap root of carrots is also a storage organ. See Figure 3 and 4.
In monocotyledons the tap root does not develop into the root system of the plants. This includes all grasses such as wheat, maize and millet. In these plants the bulk of the root system originates from the base of the stem. Roots derived from the stem are called adventitious roots. The resulting root system is termed a diffuse root system and is the main root system of monocotyledons.
Figure 2: A cross section of a Root.
NOTE: Monocotyledons have a tap root but this does not develop into the plant’s main root system. Some Dicotyledons will have adventitious roots but do not form the bulk of the root system.
THE EFFECTS OF STRESS
If there is a lack of water the process of root growth is inhibited. Cell division is reduced which includes cell enlargement which is more affected by the lack of water than the process of root growth. Contrary to popular belief roots will not penetrate hard soil in an attempt to find water.
