Anatomy of Roots

Introduction:

• Roots are the underground plant organs that are embedded within the soil.
• The roots develop from the radicle of the embryo.
• They help in the anchorage of the plant in the soil and conduction of water and minerals from the soil to the aerial parts of the plant.
• The anatomical study of the root shall show us various tissues and types of cells that are involved in the conduction of water and minerals.
• It shall also highlight their structural identity and the function each of these plays in order to maintain the proper water transport in the plants.
• Root anatomy varies between dicots and monocots.

Topics covered:

• Anatomy of Dicot root
• Anatomy of Monocot root
• Comparative study of dicot and monocot root

Anatomy of Dicot root
 

Introduction:

• Taproot is a characteristic feature of dicotyledonous plants.
• The primary roots and its branches constitute the tap root system.
• In the tap root system, a thick primary root arises directly from the radicle and grows inside the soil which bears lateral branches of several orders that are referred to as secondary, tertiary roots etc.
• Internal structure of a dicotyledon root consists of :
  1. Epiblema (Rhizodermis or Piliferous layer)
  2. Cortex
  3. Endodermis
  4. Pericycle
  5. Vascular Bundles
  6. Pith

Detailed explanation:

i) Epiblema (Rhizodermis or Piliferous layer):

• The upper or outermost layer is the epidermis and is composed of parenchymatous cells.
• Cuticle and stomata are absent.
• Unicellular root hairs are formed by the elongation of some of its cells in the maturation zone of the root.
• These hairs perform absorption of water from the soil.

ii) Cortex

• Cortex is a layer composed of homogeneous parenchyma cells.
• It exists in between the epidermis and the endodermis layer.
• Its cells are circular or polygonal with intercellular spaces.
• Due to thin walls and sufficient space between cells, water flows rather easily through this layer.

iii) Endodermis

• It is the innermost single layer of cortex, without intercellular spaces that lies between pericycle and cortex.
• This layer has barrel-shaped cells which possess special thickenings of suberin on radial and tangential walls also called Casparian strips (discovered by Caspari).
• Suberin deposition in the casparian strips, makes it impermeable to water.
• Casparian strips are absent in those cells of endodermis that lie at the front of protoxylem.
• These are called passage cells or transfusion cells that help in the passage of water from cortex to the pericycle.

iv) Pericycle

• It can be one to more layered.
• It consists of parenchyma (prosenchyma).
• Lateral roots originate endogenously from the pericycle.
• The cells of the pericycle play an important role at the time of secondary growth in dicot roots.

v) Vascular bundles

• Vascular bundles are radial and exarch.
• The number of xylem bundles are two to six (diarch to hexarch) [exception– Ficus (Banyan tree) root is polyarch].
• Parenchyma that lies between xylem and phloem is called Conjuctive tissue.
• The latter takes part in the formation of vascular cambium during secondary growth.

vi) Pith

• It is composed of parenchymatous cells with intercellular spaces located at the very centre.
• The pith is inconspicuous ( Not well developed) in dicot roots.

Anatomy of Monocot root
 

Introduction:

• Monocots have short-lived primary roots which are later replaced by fibrous roots; these kinds of roots originate from the base of the stem.

Detailed explanation:

i) Epiblema (Rhizodermis or Piliferous layer):

• The outermost layer is the epiblema which is composed of parenchymatous cells.
• Root hairs originate due to the elongation of the epidermal cells which participate in the absorption of water and mineral salts .
• Cuticle and stomata are absent.

ii) Cortex

• It is multi-layered and homogeneous.
• It consists of parenchyma cells that encloses intercellular spaces for exchange of gases.
• The cells store food.
• The outermost layer of the cortex produces protective exodermis in the older roots.

iii) Endodermis

• It is the innermost layer of the cortex.
• It consists of barrel shaped cells having casparian strips.
• Passage cells are found next to the vascular strands within the endodermis, these are thin suberized cells.

iv) Pericycle

• It is quite distinct.
• It can be single layered (Maize) or multilayered ( Smilax).
• There is no cambium in the case of monocot roots and hence the pericycle is the site of lateral root formation only.

v) Vascular bundles

• The vascular bundles are arranged in a radial manner with alternate xylem and phloem bands.
• The xylem is of exarch type.
• Vascular bundles are of closed type due to the absence of cambium.
• There are much more than just 6 xylem bundles(polyarch condition) in monocots.
• Phloem possess sieve tubes, companion cells and phloem fibers.
• Phloem parenchyma is absent in monocots.
• Conjunctive tissue is present in between the patches of xylem and phloem.

vi) Pith

• The pith is large and well developed in monocot roots.

Comparative study of dicot and monocot root
 

Detailed explanation:

• The dicot and monocot roots have very minimal differences. The differences are arranged in a tabular form.?
   

Frequently asked questions (FAQ)

Q1. The cross-section of a plant material showed the following features when viewed under the microscope.
      a. The vascular bundles were radially arranged.
      b. Four xylem strands with exarch condition of protoxylem.
         To which organ should it be assigned?
Ans: The cross-section is of a dicot root as the radial vascular bundle and four xylem strands with exarch condition of protoxylem are the characteristic of the dicot root.

Q2. How are root hairs formed?
Ans: The epidermal cells of the root surface elongate and form the root hairs with thin walls which increase the rate and area absorption for the plants.

Q3. What are conjunctive tissues?
Ans: It is the parenchymatous tissue that is found in between the xylem and phloem bundles in the monocot and dicot root.

Q4. Which layer does not let water pass through and why?
Ans: The endodermis layer is impervious to water, i.e. water molecules are unable to penetrate it. It happens due to the presence of a waxy substance called suberin within the layer.

Q5. How many xylem bundles can be found in dicot roots?
Ans: Xylem bundles can range from two to six in dicot roots, i.e. diarch to hexarch condition.

Q6. From where do the lateral roots originate in monocots?
Ans: The pericycle cells give rise to the lateral roots and play an important role at the time of secondary growth in dicot roots. But as there is no cambium in the case of monocot roots hence the pericycle is the site of lateral root formation only.

Q7. State the features of vascular bundles of monocot root.
Ans: i. Radial vascular bundles
        ii. Vascular cambium is absent
       iii. No secondary growth is exhibited
       iv. Polyarch condition of xylem elements
       v. Phloem has all the elements except the phloem parenchyma.
      vi. Presence of well-developed pith.

Q8. Where does the Casparian strip occur?
Ans: The presence of the Casparian strips is the characteristic feature of the roots.

Q9. What are passage cells?
Ans: Casparian strips are absent in those cells of the endodermis that lie at the front of the protoxylem. These are called passage cells or transfusion cells that help in the passage of water from the cortex to the pericycle.