Meristematic Tissue | Definition, Characteristics, Types & Examples
Meristematic tissue is the plant tissue that has the capability to divide actively throughout its life.
A meristematic tissue is a group of immature cells that are preparing to divide or are in continuous state of division. The basic function of these tissues is to contribute in the growth of the plants. These tissues are found in the growing regions of plants. These are responsible for the formation of primary plant body and then its growth year by year.
For understanding the meristematic tissue, we should have the basic knowledge of growth in plants. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. And this capability of plants is due to the presence of continuously dividing meristematic tissue.
Meristems are the specialised areas in the plant body that possess the meristematic tissue (Gk. meristos divided). These are the areas of active cell division as the cells present in them keep on dividing to form other cells. The cells produced by division of meristematic cells soon lose the capacity to divide and such cells make the plant body.
Characteristics of Meristematic Tissue
- The cells of these tissues are commonly called meristems.
- The meristematic tissue is living and thin-walled.
- The protoplasm of the cells is very dense.
- They exhibit a very high metabolic activity.
- The meristematic tissues heal the wounds of an injured plant.
- The cells of the meristematic tissue are young and immature.
- The meristematic tissue has the quality of self-renewal.
- They have very small and few vacuoles.
- They possess a single, large and prominent nucleus.
- They do not store food.
Classification of Meristems
The meristems can be divided on the basis of
- Origin or order of appearance in the life of a plant.
- Their position in the body of a plant.
1. On the basis of origin in life of a plant : On this basis, the meristems can be distinguished into
(a) Primary meristems: The meristematic cells that appear early in the life of a plant and contribute to the formation of primary plant body are called primary meristems. These cells are always in active state of division. They form primary permanent tissues.
(b) Secondary meristems: The meristems that appear later than primary meristems in life of a plant and are responsible for producing the secondary tissues, particularly the woody areas, are called secondary meristems. They are not present from the very beginning of the formation of an organ but develop at a later stage to give rise to other tissues.
2. On the basis of position in the plant body: The meristems occupy different areas in the body of plants and depending upon this, they can be divided into
(a) Apical meristem : As the name signifies, these tissues are found at the apices or tips of stem, root or branch. These are further of two types, i.e., root apical meristem and shoot apical meristem. Root apical meristem occupies the tip of a root and shoot apical meristem occupies the tip of a shoot.
Function of apical meristem: The apical meristems produce the primary tissues of plant body and are responsible for the primary growth of the plants. They principally contribute to increase in the length, i.e., elongation of the plants along their axis. The growth of roots and stems in length with the help of apical meristems is called primary growth.
Formation of axillary bud: Plant bud is an under developed shoot. We know that the cells of shoot apical meristem continuously divide to increase their number to elongate the stem. While the stem is elongating and leaves are forming on it, some cells ‘left behind’ from shoot apical meristem in the axil of leaves and constitute the axillary bud. Axil is the angle between the upper surface of a leaf stalk and the stem that bears it. These buds are capable of forming a branch or a flower.
(b) Intercalary meristem: As the name depicts, these meristems are intercalated in between the mature tissues, i.e., the permanent tissues. Hence, these meristems are separated from the apex of the organ by mature tissue. The activity of intercalary meristem also adds to the length of plant or its organs.
Intercalary meristems are found in grasses where they help to regenerate the parts removed by the grazing herbivores. They help in elongation of organs and also allow fallen stems of cereals to become erect.
Note: Both apical meristems and intercalary meristems are the primary meristems because they appear early in the life of a plant and contribute to the formation of primary plant body.
(c) Lateral meristem: These meristems are present along the lateral sides of roots and shoots of many plants. They are found in the mature regions of roots and shoots. These do not occur in all plants. They occur only in those plants that produce the woody axis or show secondary growth. These are generally not present from the very beginning of life of a plant and appear later than the primary meristems, that is why they are called secondary meristems. The lateral meristems are the cylindrical meristems which divide in the radial direction resulting in the increase in girth of the stems and roots. Lateral meristems are responsible for producing the secondary tissues like secondary xylem, secondary phloem, secondary medullary rays, cork, secondary cortex, etc.
The various examples of lateral meristems are:
(I) Fascicular vascular cambium: It is the meristematic tissue which develops within the vascular bundles. It is also called intrafascicular vascular cambium.
(II) Interfascicular cambium : The meristematic tissue which develops between the vascular bundles is called interfascicular cambium.
(III) Cork cambium: This lateral meristem forms the cork, a tough protective material during the secondary growth.
Hence, lateral meristems are secondary meristems on the basis of origin (except intrafascicular cambium) and lateral on the basis of location.