The cell wall is not totally complete around the cell. It is interrupted by narrow pores carrying fine strands of cytoplasm, which interlink the contents of the cells. They are called plasmodesmata. They form a protoplasmic continuum called symplast. It consists of a canal, lined by plasma membrane. It has a simple or branched tubule known as desmotubule. Desmotubule is an extension of endoplasmic reticulum. Plasmodesmata serves as a passage for many substances to pass through. It is also believed that they have a role in the relay of stimuli.
Pits are the areas on the cell wall on which the secondary wall is not laid down. The pits of adjacent cells are opposite to each other. Each pit has a pit chamber and a pit membrane. The pit membrane consists of middle lamella and primary wall. Pit membrane has many minute pores and thus they are permeable.
Pits are of two types 1.Simple pits 2.Bordered pits. In simple pits the width of the pit chamber is uniform. There is no secondary wall in the simple pit. In bordered pit the secondary wall partly overhangs the pit. Pits help in the translocation of substances between two adjacent cells. Generally each pit has a complementary pit lying exactly opposite to it in the wall of the neighbouring cell. Such pits form a morphological and functional unit called the pit pair.
It gives definite shape to the cell.
It protects the internal protoplasm against injury.
It gives rigidity to the cell
It prevents the bursting of plant cells due to endosmosis.
The walls of xylem vessels, tracheids and sieve tubes are specialized for long distance transport.
In many cases, the cell wall takes part in offense and defense.
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