Parenchyma : It is the fundamental tissue. 

• Tissue first time evolved in bryophyte. 

• Thin walled cells, oval or spherical in structure. 

• Cell wall mainly composed of cellulose & pectin. 

• Large central vacuole for food & water storage. 

• Primary function is food storage.

• Some parenchyma involved in excretory substance storage are so called as idioblast, storing such as resin, tannin, gums & oils. 

• In typical parenchyma chlorophyll is absent. 

• Chloroplast containing parenchyma tissue are chlorenchyma which perform photosynthesis e.g., mesophyll of leaves. 

• In hydrophytic plants aerenchyma (a type of parenchyma containing air spaces) provides buoyancy. 

• Parenchyma provides turgidity to cells.

Collenchyma : It is the living mechanical tissue.

• Elongated cells with thick corners. 

• Localized cellulose & pectin thickening. 

• Provides flexibility to plant parts & easy bending of various parts of plant. 

• Present only in herbaceous dicot stem. 

• Present at thin margin of leaves. 

• Few chloroplasts may be present. 

• Gives mechanical strength & elasticity to the growing stems.

Sclerenchyma : (Scleras – hard) Strengthening tissue.

• Composed of extremely thick walled cells with little or no protoplasm. 

• Cells are dead & possess very thick lignified walls. 

• Lignin is water-proof material. 

• Intercellular spaces are absent.