Initially, the description of the life forms around us was made only through the study of external structures made by naked eye. Later as the technology progressed magnifying lenses and microscopes were developed. So the study of internal structures was made possible. And hence, the life forms were studied in more detail.
External structure or morphology is a very important characteristic of plants as it forms the basis for classification. Plants can be classified based on the morphological characters. Any given plant in general as roots, stems, leaves, flowers and fruits.
Root and the root system
When a seed germinates, the radicle of the embryo comes out. The radicle of the embryo develops into first root or primary root. The primary root gives rise to other roots called secondary roots or lateral roots. Finally lateral roots give rise to tertiary roots.
The arrangement of the secondary roots on the primary roots and the arrangement of the tertiary roots on the secondary roots are in acropetal manner. This elaborate organization of all the roots of the plant is called as root system. The root system covers large volume of the soil.
Root is the underground main axis of the plant body which generally grows downwards into the soil. It initially fixes the seedling into the ground. Originally in majority of the dicotyledonous plants primary root is the direct elongation the radicle of the embryo.
Characteristics of root system
- Roots grow underground in positively geotropic or negatively phototrophic manner with exception of few plants (Pneumatophores, Cycas corolloid roots).
- Roots are not differentiated into nodes and internodes
- Leaves and normal buds are absent on the roots
- Roots are generally non-chlorophylls and non-green or brown in color
- The lateral roots are formed endogenously (from inner region) in an acropetal succession from pericycle.
- Root apex is protected by a special structure called as root cap.
- The hairs present on the roots are unicellular. They develop acropetally and are short lived. Root hairs are formed exogenously (from outer region) from epiblem or rhizodermis or periferous layer.
Root cap vs root pocket
|Present in all non-aquatic plants
||Present only in aquatic plants
|It is a tightly fitting structure
||It is aloosely fitting structure
|It can be regenerated
||It cannot be regenerated
Types of root system
Mainly there are two types of roots in angiosperms namely, Tap root system and fibrous root system. Let us discuss each of them.
Tap root system
- This is the most common type of root system found in dicotyledonous plants and gymnosperms.
- In this type of root system, the roots develop from the radicle of the germinating seed.
- In tap root system the radicle is long lived.
- Here the primary root grows into a strong root called as tap root.
- Tap root is in continuation with the main stem.
- The tap root grows vertically into the soil and produces lateral branches endogenously and obliquely.
- The tap root along with its branches is called as tap root system.
- Generally tap roots are deep rooted and provide mechanical support.
Fibrous root system
- This is the most common type of root system found in monocotyledonous plants and pteridophytes.
- In this type of root system, the radicle is short lived and gets degenerated after some time. Later a number of fiber-like new roots develop from the base of the stem.
- As these roots develop from parts of the plant other than radicle, this is called adventitious root system.
- A bunch of uniform roots arise from the base of the stem.
- These roots are slender, shallow and develop equally.
- All the roots look like fibers and hence the name fibrous roots.
Important point !
The seed starts the germination and develops into young plant. Radicle, plumule and cotyledons are the three basic parts of a young seed.
1. Radicle is the embryonic root.
2. Plumule is the embryonic shoot.
3. Colyledons are embryonic leaves.
Based on the number of cotyledons angiosperms can be of two types namely, Dicots or Monocots. Dicots have two cotyledons while monocots have a single cotyledon.
The main differences between plumule and radicle are,
|Plumule is the embryonic shoot.
||Radicle is the embryonic root.
|Plumule grows after the growth of radicle.
||Radicle comes out first from the seed.
|Plumule develops into the future shoot, which is the stem and the leaves of the plant.
||Radicle develops into the future root of the plant.
|Plumule grows upwards out of the soil.
||Radicle grows downwards into the soil.
|Plumule is less white than the radicle.
||Radicle is more white than the plumule.
|Plumule is capable of photosynthesizing in order to produce food for the growing embryonic plant.
||Radicle is capable of absorbing water from the soil, which is required for the development of the embryonic plant.
**Roots developing from the radicle are called as normal roots and the roots developing from any other part of the plant except radicle are called adventitious roots.
Functions of root system
Generally any normal root will show the following functions:
Absorption: Roots absorb the water and minerals from the soil. Absorption is the physiological function.
Fixation: The roots fix the plants firmly into the ground. They provide a proper anchorage to the plant parts. Fixation is morphological function.
Storage: The roots also help in storing reserve food material. Storage is physiological function.
Synthesis: Roots help in the synthesis of plant growth regulators. This is also a physiological function
Conduction: The water and the minerals which are absorbed from the soil are conducted up to the shoot with the help of roots. Conduction is physiological function.
Important point !
In some aquatic plants such as Hydrilla, the roots are not well developed because the plants can absorb water directly through the epidermis.
Some hydrophytes like Wolffia, Ceratophyllum and Utricularia are completely without roots. In these plants the leaves are modified into root like structures. Here true roots are absent.
Regions of the root
A typical young dicotyledonous root shows the following regions.
Root cap region
- The tip of the root is covered by a thimble-like special structure called the root cap.
- Root cap is also called as Calyptra.
- The roots of aquatic plants do not have root caps, instead they have root pockets.
- Root cap develops from a meristem called calyptrogen.
- Root cap protects the tender apex of the root from injury as it makes its way through the soil.
- The outer cells of the root cap become mucilaginous after absorbing water.
- This helps in easy penetration into the soil.
Region of meristematic activity
- Just a few millimeters above the root cap is the region of meristematic activity.
- This sub terminal region is also called as apical meristem.
- The cells of meristematic region are very small, thin-walled and with dense protoplasm.
- The cells present in this region are active and repeatedly undergo mitotic divisions to produce daughter cells.
Region of cell elongation
- The region just above the meristematic region is called the region of elongation.
- The cells of this region undergo rapid elongation and enlargement.
- The length of the cells increases due to the cells of this region.
- This region absorbs a very little amount of water but large amounts of minerals.
Region of maturation
- The cells of the elongation zone gradually differentiate and mature. Hence, the region just above the elongation region is called the region of maturation.
- This is the oldest region of the root.
- This region gives rise to lateral roots.
- The region of maturation helps the plant in fixation and conduction of water and minerals.
- Tissue differentiation takes place at this region.
- Secondary growth occurs in this region.
Region of root hairs
- At the region of maturation, the epidermal cells give out very fine and delicate thread-like structures called root hairs.
- The root hairs are unbranched, tubular and unicellular.
- These root hairs absorb water and minerals from the soil.
- Root hair region is also called as piliferous zone.
- Root hairs are short lived in mesophytes where as they are long lived in xerophytes.
- Many of the modified roots do not show root hairs.
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