Cell wall thickening in plants

1.Annular or ring-like thickenings: The deposition of lignin takes place in the form of rings on the inner
surface of the cell wall. These lignified rings are placed one above the other like coins leaving sufficient
space in between each other. The gaps of the walls remain unthickened.
Example: Such thickenings are commonly found in the vessels and tracheids-Spiral thickenings:
In such cases the deposition of thickening material (lignin) takes place in the form of complete spiral
bands. The number of such bands may be one or more than one.
Example: This type of thickening is commonly found in the vessels or tracheae of angiosperms.

2. Scalariform or ladder-like thickenings:
In such thickenings of the cell wall the lignin is being deposited in the form of the transverse rods of the
ladder, and thus known as scalariform or ladder-like. The unthickened areas between the successive
thickening layers appear as elongated transverse pits.
Example: This type of thickenings is common in xylem vessela and tracheldh of
protoxylem.

3. Reticulate or net-like thickenings:
In such thickenings of the cell wall the thickening matter or lignin is Deng deposited in the form of a net
or reticulum and thus known as retiCulate or net thickening of the cell wall. In such cases the
unthickened areas of the cell wall irregular in shape.
Example: These thickenings are commonly found in the vessels of the stems, roots and leaves of
angiosperms and in the tracheids of protoxylem

4. Pitted thickenings:
In such thickenings of the cell wall, the whole inner wall is more or less uniformly thickened, leaving
here and there some small unthickened areas, the pits. The pits are formed in pairs lying against each
other on the opposite sides of the wall, and morphologically more correct they are called ‘pit pairs. A pit
pair is structural and directional unit constituted by, two pits lying opposite to each other of contiguous
cells.