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Meristematic Cells


Introduction:

  • Meristematic (Greek word meristos, meaning divisible) cells are responsible for the root and shoot growth of plants.
  • These cells have the ability to divide and increase in number and increase the length as well as the girth of the stem and root.
  • These cells have the ability to produce new plant organs like leaves, flowers etc.
  • Meristematic tissues are made up of cells that are immature and has the capacity to divide and give rise to new cells.
  • These cells are commonly known as meristems.
  • There are two main sites where these tissues are found which is at the root apex and the shoot apex.
  • These cells are round, oval or polygonal with no intercellular spaces.
  • The walls of these cells are made of cellulose and are thin.
  • The cells have dense thick cytoplasm and a prominent nucleus.
  • There are almost no intercellular spaces between these cells.
  • These cells have the ability to divide infinitely without undergoing differentiation.
  • These cells can divide in various planes, i.e. anticlinal, periclinal and irregular.
  • Every time the meristem divides, one cell remains identical to its parent cell, while the rest forms specialized structures. Thus they can self-renew themselves.
  • The plastids in these cells are not matured and are termed proplastids.
  • These cells show active metabolism status but do not store food.
  • The respiration in these cells is very high.

Topics covered:

  • Meristematic tissues on the basis of origin and development:
  • Meristematic tissues on the basis of position in the plant body
  • Meristematic tissues on the basis of function

Meristematic tissues on the basis of origin and development:


Introduction:

  • Meristematic tissues are those tissues that have the ability to divide indefinitely.
  • These tissues can originate from various sources in a plant body.
  • The two major types of tissues that are divided based on their origin are Primary meristematic tissues and Secondary meristematic tissues.

Detailed explanation:


A. Primary meristematic tissue:

  • These tissues originate from the meristematic cells of the embryo.
  • They develop the plant body in its initial growth stages.
  • These tissues are responsible for the formation of the primary plant body.
  • These meristematic zones persist in the plant throughout its life.
  • The primary meristem is located in the apex of root and shoot.
  • Primary meristem is the contributing factor to primary growth.
  • Primary meristem is of 2 types: Apical meristem and Intercalary meristem.

B. Secondary meristematic tissues:

  • Secondary meristematic tissues is a type of meristem that is involved in secondary growth and thus gives rise to the secondary tissues of the plant.
  • These tissues originate from permanent cells that gain the function of division by redifferentiation.
  • This meristem is mainly found within the mature roots and shoots.
  • These meristems are formed in the later part of a plant's life.
  • They are mostly laterally positioned.
  • The lateral meristem is an example of a secondary meristem (e.g. cork cambium).
  • They are responsible for the increase in girth or width of stems and roots.
  • Secondary growth is because of the secondary meristem. These produce the secondary tissues in stems and roots that either replace the primary tissues or add to the overall mass.

Meristematic tissues on the basis of position in the plant body


Introduction:

  • Meristematic tissues are found in a plant body in various locations.
  • These locations are important for the growth of plants in various aspects.
  • There are 3 types of meristematic tissues that are separated based on their position in the plant body, i.e. Apical meristem, Intercalary meristem, Lateral meristem.

Detailed explanation:


A. Apical meristem:

  • This particular meristem is found on the tips or apex of the root stem and branches.
  • Apical meristem is responsible for the increase in length of various regions.
  • This apex is responsible for the growth and formation of new leaves on the stems.
  • This apex gives rise to nodes and internodes from which the branches and leaf primordia generate.
  • The shoot apex transforms into the reproductive shoot apex during the reproductive phase. During this phase, the growth stops and no new leaves are generated. Rather, now the apex gives rise to an inflorescence or a single flower.
  • The growth of the root is as the tip is covered with a root cap.
  • The root branches do not originate near the apex, rather they form much behind from the tip or root apical meristem.


    apical-meristem


B. Intercalary meristem:

  • Intercalary meristem is found between the mature tissues.
  • This type of meristematic tissue allows the plant to grow vertically or intercalarially, hence the name.
  • They arise from the apical meristem, they are later separated by permanent tissue formation.
  • They are also found at leaf bases and help wilted leaves to get erect.

C. Lateral meristem:

  • Lateral meristem originates from the procambial strands present between the xylem and the phloem.
  • They are also found in the cork cambium of roots as well as the stem.
  • These cells have a specific dividing pattern, i.e. they divide sideways. This division leads to an increase in girth or width.
  • Lateral meristem provides growth much later in the lifecycle of the plant and is termed secondary growth.

Meristematic tissues on the basis of function


Introduction:

  • The cells of the meristematic tissues divide rapidly and give rise to many regions within the plant.
  • These meristems are of 3 different types based on their function in the plant body, i.e. Protoderm, Procambium, Ground meristem.

Detailed explanation:

- Haberlandt (1914) proposed a different nomenclature of protoderm, ground meristem and procambium on the basis of function.
 

shoot-root-meristem


A. Protoderm-

  • Protoderm is the outer layer of the apical meristem and is responsible for the formation of the epidermal tissue system which includes- epidermis, stomata, root hair, stem hair/trichomes.
  • It forms the epidermis of the stem and epiblema of the root.

B. Procambium-

  • The cells in the procambium are narrow, elongated and with dense cytoplasm.
  • Procambium is responsible for the formation of a vascular tissue system which includes- xylem and phloem (vascular bundles).

C. Ground meristem-

  • The cells of this region are generally thin-walled and isodiametric.
  • The activity of the ground meristem results in the formation of a ground tissue system which includes- Hypodermis, general cortex, endodermis, pericycle, medullary rays and pith.


Frequently asked questions (FAQ)

Q1. Meristems are responsible for?
Ans:

Meristems give rise to meristematic tissue and are responsible for growth in plants. These cells are present as masses of cells at the root and shoot tip. These cells divide rapidly increasing their length and girth.

Q2. Why does a meristematic cell not store food?
Ans:

Vacuoles are responsible for storing nutrients, excess salts, etc. A vacuole causes a hindrance to cell division as it is filled with cell sap in order to provide the cell with rigidity and turgidity. Hence there are no vacuoles present in these cells.

Q3. Why doesn't meristematic tissue have intercellular spaces?
Ans:

The meristematic cells won’t be able to divide as rapidly as they do if there were intercellular spaces and hence there are no intercellular spaces. These cells also do not have vacuoles which would result in the formation of small intercellular spaces because of their size.

Q4. What are the examples of secondary meristems?
Ans:

Secondary meristems are formed later in the life of a plant. These are present in the mature roots and stem regions.
Examples of secondary meristems are:
I. Cambium of root
II. Cork cambium of root
III. Cork cambium of stem
IV. Interfascicular cambium and fascicular cambium of stem

Q5. Describe the role of protoderm?
Ans:

Protoderm is the outer layer of the apical meristem and is responsible for the formation of the epidermal tissue system which includes- epidermis, stomata, root hair, stem hair/trichomes. It forms the epidermis of the stem and epiblema of the root.

Q6. Which tissues are formed off ground meristem?
Ans:

The activity of the ground meristem results in the formation of a ground tissue system which includes- Hypodermis, general cortex, endodermis, pericycle, medullary rays and pith.

Q7. What growth does lateral meristem provide?
Ans:

These cells have a specific dividing pattern, i.e. they divide sideways. This division leads to an increase in girth or width. Lateral meristem provides growth much later in the lifecycle of the plant and is termed secondary growth.

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