The Cell Wall Structure of a Softwood Tracheid

The middle lamella is the intercellular region. It contains material that holds adjacent cells together. Structurally, it is not part of the cell wall. While the cells are enlarging it is largely pectic and only later does the middle lamella become highly lignified. There are no cellulose microfibrils in the middle lamella. 

Cells that have been formed recently at the vascular cambium have only a very thin primary cell wall. Even in the fully lignified cell the primary wall is very thin (0.1 j,m) and can be hard to distinguish from or isolate from the middle lamella many studies analyse the two together (ML+P) and relate results to the compound middle lamella (CML), a term which embraces both middle lamella and primary wall. The primary wall displays both elasticity and plasticity (permanent extension) during early cell growth and extension - at this stage in tracheid cell development 

Finally a thin warty layer may be deposited on the cell wall, which appears to be composed of proteinaceous or lignin-like material. Where present the warty layer completely lines the cell lumen and pit cavities. It is widely found in softwood tracheid elements. 

---