The following points highlight the five main parts of a typical root. The parts are: 1. Root Cap 2. Growing Point or Meristematic Zone 3. Region or Zone of Elongation 4. Root Flair Zone 5. Region or Zone of Mature Cells.
Typical Root: Part # 1. Root Cap:
It is a thimble-shaped or cap-like parenchymatous multicellular structure which covers the root meristem. The cells of the root cap secrete mucilage. The latter lubricates the passage of root through the soil. Without it, the tender root would be unable to penetrate the hard soil. Cells of the root cap also possess starch grams. The latter are believed to take part in graviperception.
The role of root cap in perception of gravity was found out by Darwin and Darwin in 1880. Another function of the root cap is the protection of the root meristem from friction of the soil particles. In the process, its outer cells are continuously peeled off and replaced by new cells formed from root meristem.
The aerial stilt roots of Pandanus (Screw Pine) possess multiple root caps with a number of sheaths formed successfully one after the other because they are not peeled off in the absence of soil friction.
In floating aquatic plants the root apices possess loose finger glove-like coverings called root pockets, e.g., Lemna, Pistia, Eichhornia. Root pockets function as balancers. They are structurally similar to root caps but differ from them in the fact that the damaged or lost root pockets are not regenerated.
Typical Root: Part # 2. Growing Point or Meristematic Zone:
It is about 1 mm in length. The growing point of the root is sub-terminal and lies protected below the root cap. It is made up of compactly or closely arranged small thin walled isodiametric and meristematic cells which have dense protoplasm.
They undergo repeated divisions (Gk. meristos— divided). The meristematic region produces new cells for the root cap and the basal region of the root. Therefore, it is essential for the growth of the root.
Typical Root: Part # 3. Region or Zone of Elongation:
It is about 4-8 mm in length. It lies behind the growing point. The cells of this region are newly formed cells which lose the power of division. They elongate rapidly. This increases length of the root. The external cells possess the power of absorption of water and mineral salts from the soil.
Typical Root: Part # 4. Root Flair Zone:
It also represents the zone of differentiation or maturation because different types of primary tissues differentiate or mature in this region (viz., xylem, phloem, pericycle, endodermis, cortex, epiblema, etc.). Root hair zone is 1-6 cm in length. Most of the water absorption occurs in this region. Some of the outer cells of this zone give rise to lateral tubular outgrowths called root hairs (Fig. 5.3).
The root hairs increase the exposed surface of the root for absorption. From the surface, the root hairs appear as white cottony fibers. As the cells of root hair zone become mature, the root hairs shrivel and become non-functional.
However, new root hairs are formed in the older part of the zone of elongation so that the root hairs appear in the newer parts of the soil from where water has not yet been absorbed. This ensures continued water supply to the plant.
Typical Root: Part # 5. Region or Zone of Mature Cells:
It forms the bulk of the root. The cells of this region do not undergo any further change. The outermost layer of this region has thick walled or impermeable cells. So this region cannot help the root in water absorption. Its only function is to anchor the plant firmly in the soil. Lateral roots also arise from the interior of this region.
There are three types of root systems— tap, fibrous and adventitious. Tap root system is more common in dicots. Fibrous root system is prevalent in monocots. Adventitious root system is found in both dicots (e.g., Banyan) and monocots (e.g., Asparagus).
In tap root system, the primary root formed from radicle persists. In fibrous root system, the primary root is short lived. It is replaced by a number of fine fibrous roots developing from the base of the stem (Fig. 5.4).
Adventitious roots are those which develop from any part of the plant other than radicle. The three types of root systems also show modifications. Root modifications are changes in form, structure and physiology of roots to perform functions other than absorption and conduction of water and minerals.