Root is the vascular part of the plant that is generally located underground and primarily functions as the anchorage of the plant with the soil and absorbs water, nutrients and minerals from the same source and feeds it to the plant body for its growth and development. In some plants, it is also a storage unit for food and nutrients and an organ for vegetative reproduction. In some plant species, the root system also enters into a symbiotic relationship with certain fungi and mycorrhizae and thereby assisting in the growth of the organism and itself.
The root is different from the stem mainly by lacking leaf scars and buds. The system also has a root cap and has branches that originate from the internal tissue. In this topic, we have an overview of the structure of the root system.
The structure of the root system can be categorised into four distinguishing features. These are:
The root cap region.
Region of cell division.
Region of elongation.
Region of maturation.
It is a cup-shaped, loosely cemented mass of parenchyma cells that covers and protects the root tip. When the existing layer of parenchyma cells are lost amongst the soil particles, new cells are formed and added to the meristem of the cap. A large number of new cells are produced which replace the worn off or lost cells and they help the root tip push through the soil. The movement of the root tip is assisted by a slimy substance called mucigel and its functions are :
Lubricating the root.
It contains materials that are inhibitory to roots of other species.
Influences uptake of ion.
Attract helpful microorganisms.
Acts as an adhesive between the root and soil particles and thereby improving soil-plant contact.
Protects the root cells from drying.
This zone is also the region of meristematic activity in roots. An apical meristem lies under and behind the root cap and similar to the stem apical meristem, the apical meristem produces cells that assist in the development of the primary body of the plant. Unlike stem meristem, the apical meristem is not located at the tip of the root but lies behind the root cap. Between the area of active cell division and the cap is a grey area where the cells divide more slowly. It is called the quiescent centre. Most cell divisions in the root system of a plant take place along the edges of this centre and give rise to columns of cells which are arranged parallel to the root axis. The parenchyma cells of the meristem are small, cuboidal, with dense protoplasts devoid of the vacuole. These cells also have a relatively large nucleus.
The apical meristem of the root organises to form the three primary sections of the meristematic region meristems:
The Protoderm: Gives rise to the epidermis.
The Procambium: Produces xylem and phloem tissues.
The Ground Meri - Stem: Forms the cortex.
Pith is a part of the vascular system which is present in most stems is also produced from the ground meristem. This sector is absent in most dicot roots but can be found in many monocot roots.
The cells in this region of elongation stretch and lengthen as small vacuoles within the cytoplasm of the cells coalesce and fill themselves with water. One or a pair of large vacuoles occupy almost all of the cell volume in fully elongated cells. The apical tip and the root cap of the root cap move forward through the soil due to cellular expansion in this region. The entire region is around 4-8 mm long.
The cells of the region of elongation complete their differentiation into the tissues of the primary body in this region or zone. It can be easily recognised due to the presence of many root hairs that extend through the soil as outgrowths from a single epidermal cell.
The root hairs increase the absorptive surface of roots during the growth period of the plant; During this time large quantities of water and nutrients are needed and the root hairs assist in that. The lifetime of an individual root hair is only a day or two. However, new ones are continuously formed near the tip as old ones die in the upper part of the zone.
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The root system has various primary and secondary functions which are integral for a plant body’s growth.
Absorption: One of the primary functions of the root is to absorb water and nutrients from the soil and supply it to the stem. It is an important part of photosynthesis.
Anchorage: The root system anchors the whole plant body with the soil and ensures that it stands erect. Exceptions are climbers.
Storage: Many types of roots act as a primary storage unit for the plant body. They store food in the form of starch which can be used for various body activities. Some examples are radish, carrots and beetroots.
Reproduction: In many plants, roots also act as a reproductive organ for vegetative propagation. Example: Sweet potato.
Prevention of Soil Erosion: Roots hold soil particles firmly for the prevention of soil erosion.
Q 1: What are the Types of Roots?
A: There are three types of roots observed in plants:
Tap Roots: The main root grows down vertically and the numerous lateral roots arise from them. The most common example is the tap roots of dandelion; their tap roots are broken off when weeds are pulled from the ground, so broken roots regrow again from another shoot or a nearby root.
Fibrous Roots: These roots are located closer to the soil surface and there it forms a dense network of roots which also prevents soil erosion. Examples: Roots of wheat, corn, rice etc.
Adventitious Roots: These roots form from any other part of the plant body other than the root system.
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Q 2: What is a Mycorrhiza?
A: A mycorrhiza can be defined as a symbiotic association between a plant and a fungus. The plant body produces organic molecules such as carbohydrates by photosynthesis and supplies them to the fungus. In turn, the fungus supplies the plant with water and mineral nutrients, such as phosphorus, which is absorbed from the soil.
Mycorrhizal associations are found in the roots of vascular plants, but similar associations also occur in bryophytes. There is also fossil evidence that early land plants which lacked proper roots systems formed arbuscular mycorrhizal associations.
This is the most common mycorrhizal association that can be observed in 70% of plant species, including several crop plants such as rice and wheat.
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