Read this article to learn about the Structure of Root (Region of Mutation) !
The tip of a young root examined in a longitudinal section shows four regions.
1. Root Cap:
It is a mass of dead cells at the apex of the root and serves to protect the delicate growing tip of the root.
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2. Meristem Region:
This region is just behind the root-cap and is made up of actively dividing cells.
Region of Elongation:
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It lies above the Meristematic region. The newly formed cells in this region elongate and increase in length.
Region of maturation:
The cells of this region develop small, hairy outgrowths called rot-hairns. They are chiefly concerned with the absorption of water from soil.
1. Epiblema (Piliferous Layer):
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It is the outermost layer of thin-walled cells, associated with absorption of water. Some of the cells prolong outwards and form unicellular root-hairs.
2. Cortex:
It is a zone of thin-walled parenchymatous cells inner to epiblema. It is many-celled in thickness.
3. Endodermis:
The innermost layer of cortex is called endodermis. The cells are barrel – shaped and do not have intercellular spaces between them.
4. Pericycle:
It is a single layer of thin-walled parenchymatous cells inside the endodermis.
5. Vascular Bundles:
The vascular bundles are separate xylem and phloem bundles. Xylem and phloem alternate with each other in a radial manner.
6. Pith:
It is a central zone of parenchymatous cells. Pith is normally absent in dicot roots where it is replaced by xylem bundles. Root-hairs are permeable and consist of pectic substances and cellulose which are strongly hydrophilic (water-loving) in nature. Root-hairs contain vacuoles filled with cell-sap. Mechanism of water absorption is of two types:
Absorption of Water
In higher plants, water is absorbed through root-hairs which are in contact with soil water and form a root- hair zone, a little behind the root tips. The walls of root-hairs are permeable and consist of pectic substances and cellulose which are strongly hydrophilic in nature. Root-hairs contain vacuoles filled with cell-sap.
1. Active Absorption of Water:
In this process, the root cells play an active role in the absorption of water and metabolic energy released through respiration is consumed. Active absorption may be of two kinds:
(a) Osmotic Absorption:
In osmotic absorption, water is absorbed from the soil into the xylem of the root according to the osmotic gradient.
(b) Non-Osmotic Absorption:
In non-osmotic absorption, water is absorbed against the osmotic gradient.
2. Passive Absorption of Water:
Passive absorption is mainly due to transpiration. The root cells do not play any active role and remain passive.
Water Holding Capacity of Soil:
After heavy rainfall or irrigation of the soil, some water is drained off along the slopes while the rest percolates down the soil. Some amount of this water gradually reaches the water layer beneath the soil under the force of gravity while the rest is retained by the soil.
This amount of water retained by the soil after the drainage of gravitational water is called field capacity or the water holding capacity of the soil.
Velamen:
Many epiphytic orchids develop special aerial adventitious roots which can absorb moisture from the atmosphere. For this purpose, a special water absorbing tissue is present around the cortex of such roots which is called velamen.
It consists of thin- walled parenchymatous cells and the moisture absorbed by it is transferred to the root xylem through exodermis, cortex, endodermis and the pericycle.