A meristem is a group of undifferentiated cells, which are isodiametric and bring about rapid cell division. Meristems are classified based on position, origin and function.

Based on position;

They are grouped into 3 types

• Apical Meristems: Found at the apices of roots and stems resulting in primary growth. They bring about an increase in the length of roots and stems.
• Intercalary Meristems: These are portions of apical meristems separated by permanent tissues. They occur at the base of internodes and leaf sheaths of monocots & grasses. They bring about elongation of the internode. 
• Lateral Meristems: These occupy lateral positions and bring about an increase in diameter.

Based on Origin:

They are grouped into 3 types

(a) Promeristem:

These occupy the tip of stem and roots and represent a small area. By division, they give rise to primary meristems.

(b) Primary Meristems:

These are derivatives of the premeristem and are found at the apices of shoot and root. They bring, about primary growth resulting in elongation of root & shoots.

(c) Secondary meristems; 

These are produced by dedifferentiation of permanent cells and arise later in the life of a plant. They occupy lateral positions and bring about secondary growth resulting in increase in diameter.

Based on Function: 

They are grouped into 3 types 

Protodprm: Single layered outer region which gives rise to the epidermis.

Ground meristem: Many layered and forms the cortex and pith. 

Procambium: many-layered and forms the vascular cylinder.