We have a definite growth pattern and growth stops at certain stage or age. But plant growth is indefinite. All the time, the plant body possess certain cells that are actively dividing and adding new cells thereby effecting growth. These cells are called as meristems.
Definition: Meristems are actively dividing immature cells that form new cells by cell division.
I. Classification based on origin
1. Promeristem or Primordial meristem
-Consists of meristamatic cells representing the earlieast stage of a growing organ
Location: extreme tip of stem and root
Function: forms primary meristem and later the primary structure of the plant body
2. Primary meristem
- Consists of meristamatic cells formed from promeristem
Location: Seen just below promeristem
Function: forms the primary structure of the plant body
3. Secondary meristem
- Consists of meristamatic cells formed from primary permanent tissues. Some of the primary permanent tissues regains the merisitamatic activity and becomes secondary meristem
Location: Laterally placed in stem and root
Function: responsible for secondary growth of the plant
Example: Inter fascicular cambium and cork cambium of stem, cambium of root.
II: Based on position
1. Apical Meristem
Meristem found at the apex of roots and stem. Also called as growing points.
Function: Growth in length of the axis
It includes promeristem and primary meristem.
2. Intercalary Meristem
Meristem found near the nodes in stem (at the base of internode in Grasses, below the node in Mint)
Function: Growth in length of internode
3. Lateral Meristem
Meristem found laterally placed along the axis in dicots and gymnosperms
Function: Growth in thickness of the plant body. Eg: vascular cambium and cork cambium
III: Based on plane of division
In mass Meristem, cells divide in all planes forming mass of tissues.
Eg: Development of embryo, endosperm and sporangia etc
In Plate Meristem, cells divide in two planes forming a plate like structure.
Eg: Single layered epidermis
In Rib Meristem, cells divide in only one plane and forms rows or columns of cells.
Eg: Responsible for the increase in length of organs