Tuesday, 17 September 2013

Meristematic Tissues

Meristematic tissues are cells or group of cells that have the ability to divide. These tissues in a plant consist of small, densely packed cells that can keep dividing to form new cells. Meristems give rise to permanent tissues and have the following characteristics:

  • the cells are small,
  • the cells walls are thin,
  • cells have large nuclei,
  • vacuoles are absent or very small, and
  • there are no intercellular spaces.
Meristematic tissues is found in the following locations:

  • near tips of roots and stems. This is called apical meristems.
  • in the buds and nodes of stems.
  • in the cambium between the xylem and phloem in dicotyledonous trees and shrubs.
  • under the epidermis of dicotyledonous trees and shrubs (cork cambium).
  • in the pericycle of roots, producing branch roots.
Two kinds of meristems are found:

Primary Meristems

A primary meristem arises in the tissue of the embryo and continues to exist in the plant organ in which it rose. The primary meristem found at the tips of stems or roots is called the apical meristem which is responsible for increase in length as it gives rise to the first or primary permanent tissues. Apical meristems are composed of the following:
  • The promeristem: this is one cell, or a number of cells, which, by cell division (mitosis), gives rise to the histogens or regions of active cell division and tissue formation.
  • The histogens: these arise from the promeristem and divide repeatedly to form the primary permanent tissue. Different histogens, which give rise to different permanent tissues, can be distinguished, namely:
    • the protodermis from which the epidermis differentiates,
    • the ground meristem which give rise to the cortex,
    • the procambium from which the pericycle and first vascular bundles develop,
    • the calyptrogen (which is found only in roots) from which the root cap (calyptra) differentiates.
A diagram to illustrate the tissues of the root.

Secondary Meristems

In flowering plants, meristems develop from cells that suspend their ability to divide, and resume this activity later. Such meristems are known as secondary meristems. These cells give rise to permanent secondary tissues.

Meristematic tissues are responsible for the division of new cells... they are zones of actively dividing cells
Before we precede, we should first recognize that growth in plants includes two stages:
  • first the production of new cells and,
  • secondly the expansion of these cells via uptake of water by the vacuole.
Cell division occurs solely in meristematic regions, while expansion may occur anywhere.
Thus in a single plant there are zones of young dividing cells, maturing cells, and mature cells.
We will go into more detail on meristematic regions when we go over the stem and root individually, but for now an introduction is sufficient
Let us recognize that there are 3 meristematic regions in the plant:
I. Apical meristems are located at the apices or tips - at root and shoot tips and are directly involved in their elongation
They create derivatives which form primary growth.

  • the protoderm which forms the outer dermal layer of tissues,
  • the ground meristem which forms the cortical cells and
  • the procambium which forms the vascular tissue.
shailesh kr shukla

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