Cell wall also includes structures like plasmodesmata and pits, apart from the three main structures namely middle lamellae, primary wall and secondary wall.
The cell wall is interrupted by narrow pores carrying fine bridges of cytoplasm called as plasmodesmata. They form a protoplasmic band 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 serve as a passage for many substances and they also have a role in the relay of stimuli.
Pits and their types
Pits are the areas on the cell wall where secondary wall is not deposited. The pits of adjacent cells are opposite to each other. Each pit consists of 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 it is permeable.
Pits are of two types namely, simple pits and bordered pits.
Simple pits: In simple pits, usually the pit cavities are uniformly wide on all sides. The pit cavity is not enclosed by secondary cell wall.
Bordered pits: In bordered pit, the secondary wall bends like an arch over the pit, this gives a border like appearance and hence the name. The portion of the middle lamellum crossing the pits become thickened and is known as torus.
The Pits help in the translocation of substances between two adjacent cells. Generally pits develop in pairs. In other words, when a pit is present on the secondary wall of one cell, a similar or dissimilar pit will be present on the opposite side of the adjacent cell. Such type of pits form morphological and functional units called pit pairs.
- Describe the importance of plasmodesmata.
- What are pits. Explain types of pits.
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