Short notes on the wall thickenings in xylem

Thickening of the cell wall:

The secondary wall acquires different types of thickenings. This is specially seen in cells which are to differentiate into tracheary elements i.e., vessels and tracheids. In these cells, the thickening is due to the deposition of lignin. The deposition of lignin is not uniform hence we find different types of thickenings. Chiefly, the following types of thickening are seen in xylem.

Annular:

Lignin deposition is in the form of annuli or rings. These rings are unconnected. The space between the rings is unthickened.

Spiral:

Lignifications occurs in the form of a spiral band.

Scalariform (ladder like):

Wall thickening is in the form of transverse rods very much resembling the rungs of a ladder.

Reticulate:

Lignin is deposited in the form of network or reticulum. The unthickened areas of the cell wall are irregular in shape.

Pitted:

Lignin deposition is almost uniform over the wall, except for small pores here and there. These pores are called pits.

Structure of Pit:

Pits are formed on either side of the wall. In any area there will be two pits lying opposite to each other. These are usually termed as “pit pairs”. There is a space inside each pit called pit cavity or pit chamber. There is a thin membrane which separates the pit cavities of the opposite pits. It is called pit membrane. The opening to a pit is called the pit aperture. A pit membrane is common to both the pits. It has a primary wall on either side separated by middle lamalla.

There are two types of pits viz. simple pits and bordered pits. A simple pit is just an opening in the wall. Whereas in a bordered pit, the pit aperture is overarched by the projection of the secondary wall. In a pit pair both may be simple, both may be bordered or one may be simple and the other one bor­dered. The lost mentioned one is generally called a half bordered pit pair. A pit that occurs opposite to an intercellular space will not have its comple­mentary and hence known as a blind pit.

Simple pits generally occur in parenchyma cells, medullary rays, phloem fibres, companion cells, and also in tracheids, pit cavity is uniform in its diameter and the pit membrane is simple. Simple pits may vary in shape (oval, Polygonal, irregular etc.)

In a bordered pit, the pit membrane becomes thick at the centre. The thick­ening is known as the torus. The pit membrane may also change its position. The torus instead of remaining at the centre may shift towards a pit aper­ture. In bordered pits the aperture is narrow (pit canal) and it widens to­wards the interior (pit chamber). Bordered pit occurs in the vessels of Angiosperms and in the tracheids of many Conifers.